Department of Space and Climate Physics

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Recent PublicationsView all

• Article: Progressively weighted affine adaptive correlation matching for quasi-dense 3D reconstruction
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ABSTRACT: Correlation matching has been widely accepted as a rudimentary similarity measure to obtain dense 3D reconstruction from a stereo pair. In particular, given a large overlapping area between images with minimal scale differences, the correlation results followed by a geometrically constrained global optimisation delivers adequately dense and accurate reconstruction results. In order to achieve greater reliability, however, correlation matching should correctly account for the geometrical distortion introduced by the different viewing angles of the stereo or multi-view sensors. Conventional adaptive least squares correlation (ALSC) matching addresses this by modifying the shape of a matching window iteratively, assuming that the distortion can be approximated by an affine transform. Nevertheless, since an image captured from different viewing angle is often not practically identical due to scene occlusions, the matching confidence normally deteriorates. Subsequently, it affects the density of the reconstruction results from ALSC-based stereo region growing algorithms. To address this, we propose an advanced ALSC matching method that can progressively update matching weight for each pixel in an aggregating window using a relaxation labelling technique. The experimental results show that the proposed method can improve matching performance, which consequently enhances the quality of stereo reconstruction. Also, the results demonstrate its ability to refine a scale invariant conjugate point pair to an affine and scale invariant point pair.
Pattern Recognition 10/2012; 45(10):3795–3809. DOI:10.1016/j.patcog.2012.03.023
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Article: J.-P.: An explicit growth model of the stereo region growing algorithm for parallel processing
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ABSTRACT: Commission V, WG V/4 ABSTRACT: GOTCHA is a well-tried and tested stereo region growing algorithm, which iteratively applies Adaptive Least Square Correlation (ALSC) matching to the adjacent neighbours of a seed point in order to achieve a dense reconstruction with sub-pixel precision. It is, however, a computationally expensive algorithm as every seed point collected by the ALSC matching produces quadrants or octants of new matching candidates. Accordingly, the computational complexity increases exponentially as the stereo matching region grows. To expedite the matching process of a traditional GOTCHA, this paper proposes a parallelised stereo region growing algorithm called a MT-GOTCHA. To achieve data parallelism, the proposed method initially divides a stereo image from arbitrary distributed seed points, which are able to employ multiple GOTCHA's. In addition, since it estimates a cluster of neighbours using a non-linear diffusion equation and performs multiple ALSC processes in parallel to verify local matching candidates, more tiepoints are obtained within less processing time. Experimental results demonstrate the proposed method can reduce the processing time of a dense reconstruction at a reasonable cost of memory consumption.
• Article: Solar Cycle Effects on the Dynamics of Jupiter’s and Saturn’s Magnetospheres
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ABSTRACT: The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn.
Solar Physics 12/2011; 274(1):481-502. DOI:10.1007/s11207-011-9748-z
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Article: The Sodium Tails of Near-Sun Comets
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ABSTRACT: We present results of our survey of the sodium tails of several comets observed by the Solar and Heliospheric Observatory spacecraft, SOHO, using its LASCO coronagraph. We report on the morphology and brightness of these comets' sodium tails, using photometric analysis to estimate their relative sodium production rates. In addition, we attempt to simulate the observed tails using a Monte Carlo model. Simulation of the tail's morphology and appearance is not straightforward; the anti-sunward acceleration of sodium atoms is a strong function of the atoms' radial velocity, due to the dependence of the acceleration on the strength of the Doppler-shifted Fraunhofer sodium absorption lines in the solar spectrum in the atoms' frame of reference. We discuss the implications our results for our understanding of near-Sun comets' composition and origins.
10/2011;
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Article: Testing the blast wave model with Swift GRBs
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ABSTRACT: The complex structure of the light curves of Swift GRBs (e.g. superimposed flares and shallow decay) has made their interpretation and that of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to constrain the blast wave parameters: electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multi-wavelength light curves and X-ray spectra of a Swift sample to the predictions of the blast wave model.We can successfully interpret all of the bursts in our multi-wavelength sample of 10, except two, within the framework of the blast wave model, and we can estimate with confidence the electron energy distribution index for 6 of the sample. Furthermore we identify jet breaks in almost half of the bursts. The values of k suggest that the circumburst density profiles are not drawn from only one of the constant density or wind-like media populations. A statistical analysis of the distribution of p reveals that, even in the most conservative case of least scatter, the values are not consistent with a single, universal value. This is in agreement with our results for a larger sample of X-ray only afterglows which we summarise here.
Advances in Space Research 04/2011; 47(8-47):1362-1366. DOI:10.1016/j.asr.2010.08.005
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Article: Modeling the broadband persistent emission of magnetars
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ABSTRACT: In this paper, we discuss our first attempts to model the broadband persistent emission of magnetars within a self-consistent, physical scenario. We present the predictions of a synthetic model that we calculated with a new Monte Carlo 3D radiative code. The basic idea is that soft thermal photons (e.g. emitted by the star surface) can experience resonant cyclotron upscattering by a population of relativistic electrons treated in the twisted magnetosphere. Our code is specifically tailored to work in the ultra-magnetized regime; polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. We discuss the predicted spectral properties in the 0.1–1000 keV range, the polarization properties, and we present the model application to a sample of magnetars soft X-ray spectra.
Advances in Space Research 04/2011; 47(8-47):1298-1304. DOI:10.1016/j.asr.2010.08.003
• Thesis: Landscape evolution as evidence for climate change on Mars from orbital stereo DTMs

Department of Space and Climate Physics, University College London, 01/2011, Degree: Masters, Supervisor: Jan-Peter Muller
• Thesis: Exploring the Early Solar System Chariklo Fly-by Mission Concept

Department of Space and Climate Physics, University College London, 01/2011, Degree: Masters (group), Supervisor: Dr. David Williams
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Article: Uranus Pathfinder: Exploring the origins and evolution of Ice Giant planets
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ABSTRACT: The “Ice Giants” Uranus and Neptune are a different class of planet compared to Jupiter and Saturn. Studying these objects is important for furthering our understanding of the formation and evolution of the planets, and unravelling the fundamental physical and chemical processes in the Solar System. The importance of filling these gaps in our knowledge of the Solar System is particularly acute when trying to apply our understanding to the numerous planetary systems that have been discovered around other stars. The Uranus Pathfinder (UP) mission thus represents the quintessential aspects of the objectives of the European planetary community as expressed in ESA’s Cosmic Vision 2015–2025. UP was proposed to the European Space Agency’s M3 call for medium-class missions in 2010 and proposed to be the first orbiter of an Ice Giant planet. As the most accessible Ice Giant within the M-class mission envelope Uranus was identified as the mission target. Although not selected for this call the UP mission concept provides a baseline framework for the exploration of Uranus with existing low-cost platforms and underlines the need to develop power sources suitable for the outer Solar System. The UP science case is based around exploring the origins, evolution, and processes at work in Ice Giant planetary systems. Three broad themes were identified: (1) Uranus as an Ice Giant, (2) An Ice Giant planetary system, and (3) An asymmetric magnetosphere. Due to the long interplanetary transfer from Earth to Uranus a significant cruise-phase science theme was also developed. The UP mission concept calls for the use of a Mars Express/Rosetta-type platform to launch on a Soyuz–Fregat in 2021 and entering into an eccentric polar orbit around Uranus in the 2036–2037 timeframe. The science payload has a strong heritage in Europe and beyond and requires no significant technology developments.
Experimental Astronomy 01/2011; 33(2):753-791. DOI:10.1007/s10686-011-9251-4
• Article: Modulation of the jovian ring current and magnetodisc due to impulsive volcanic activity on Io
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ABSTRACT: Orbit-to-orbit changes in the ring/magnetodisc current system at Jupiter were inferred from Galileo magnetometer data by Russell et al. (2001) and indicated modulations of around 5 nT which lasted for less than one Galileo orbit. These observations showed both positive magnetic field perturbations, associated with compression of the magnetosphere by the solar wind, and negative perturbations associated with an increase in the mechanical stresses involved in force balance with the jxB force (Leisner et al., 2007). In this study we examine the role that impulsive volcanism on Io might play in driving the latter type of ring current modulations. Using recent UV observations of the Io plasma torus (Yoneda et al., 2010) we use an Euler potential model of the jovian magnetodisc (Achilleos et al., 2010) to quantitatively investigate changes in the ring current associated with impulsive volcanic eruptions on Io. We find (negative) magnetic field perturbations with an amplitude of around 5 nT that persist for around 10 - 15 days which is consistent with the magnetic field observations presented by Russell et al. (2001). We conclude by commenting on the possibility of impulsive events on Enceladus driving the saturnian ring current perturbations reported by Leisner et al. (2007).
• Article: Mapping Medusae Fossae Formation materials in the southern highlands of Mars
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ABSTRACT: The Medusae Fossae Formation (MFF) is an extensive deposit (2.2 × 106 km2, Bradley, B.A., Sakimoto, S.E.H., Frey, H., Zimbelman, J.R. [2002]. J. Geophys. Res. 107, 5058) of wind-eroded material of widely debated origin, which unconformably overlies a considerable area of the crustal dichotomy boundary on Mars. The MFF shows a variety of layering patterns, erosional styles and channel-like forms and has been mapped into five main outcrops and three geological members according to exposure and stratigraphy (Scott, D.H., Tanaka, K.L., 1986. USGS Map I-1802-A; Greeley, R., Guest, J.E., 1987. Map I-1802-B; Zimbelman, J.R., Crown, D., Jenson, D., 1996. Lunar Planet. Sci. XXVII. Abstract #1748.). Away from the three main lobes are numerous outliers of MFF materials. These have mainly been reported in the northern lowlands regions (Keszthelyi, L., Jaeger, W.L., and HiRISE team, 2008. Lunar Planet. Sci. XXXIX. Abstract #2420.) but few studies have examined the possibility of MFF outliers on high ground south of the dichotomy boundary. We have searched Mars Orbiter Camera Narrow Angle (MOC NA) images for outliers in this region. Our observations show that there are many MFF outliers on the southern highlands. The characteristics of the outliers indicate materials which overlie the underlying terrain for they appear widely in dips, craters and topographic lows. The surfaces are typified by yardang fields and have a similar patchy and discontinuous nature to materials of the upper member of the MFF. Most have consistent lineation orientations across the wider area which match the dominant orientation of yardangs in the main MFF outcrops. Furthermore, elevation data shows that the maximum, minimum and mean elevations of these newly discovered outliers are closest to those of the upper member of the MFF. We therefore conclude that these deposits are MFF outliers and that they probably represent remnant upper member material. We suggest that there might be two possible explanations for these outliers: (1) the MFF had a much greater pre-erosional extent than previously estimated, or (2) materials from the main outcrops were eroded and then blown south to accrue in the highland areas, where they were subsequently reworked. We suggest that the topography of the region favors the first option. We outline an “overflowing” layer-cake deposition model, in which layers of sediment stacked up against the dichotomy boundary until they reached the topographic level of the highlands. Further materials (that went onto become upper-member MFF material and outliers) were then deposited across a wider area, including south of the dichotomy boundary. Severe erosion subsequently removed much of this material.
Icarus 10/2010; 209(2-209):405-415. DOI:10.1016/j.icarus.2010.04.016
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Article: Polytropic dark halos of elliptical galaxies
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ABSTRACT: The kinematics of stars and planetary nebulae in early-type galaxies provide vital clues to the enigmatic physics of their dark matter haloes. We fit published data for 14 such galaxies using a spherical, self-gravitating model with two components: (i) a Sérsic stellar profile fixed according to photometric parameters, and (ii) a polytropic dark matter halo that conforms consistently to the shared gravitational potential. The polytropic equation of state can describe extended theories of dark matter involving self-interaction, non-extensive thermostatistics or boson condensation (in a classical limit). In such models, the flat-cored mass profiles widely observed in disc galaxies are due to innate dark physics, regardless of any baryonic agitation. One of the natural parameters of this scenario is the number of effective thermal degrees of freedom of dark matter (Fd) which is proportional to the dark heat capacity. By default, we assume a cosmic ratio of baryonic and dark mass. Non-Sérsic kinematic ideosyncrasies and possible non-sphericity thwart fitting in some cases. In all 14 galaxies, the fit with a polytropic dark halo improves or at least gives similar fits to the velocity dispersion profile, compared to a stars-only model. The good halo fits usually prefer Fd values from six to eight. This range complements the recently inferred limit of 7 < Fd < 10, derived from constraints on galaxy cluster core radii and black hole masses. However, a degeneracy remains: radial orbital anisotropy or a depleted dark mass fraction could shift our models' preference towards lower Fd; whereas a loss of baryons would favour higher Fd.
Monthly Notices of the Royal Astronomical Society 06/2010; 405(1):77 - 90. DOI:10.1111/j.1365-2966.2010.16448.x
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Article: Late-stage water eruptions from Ascraeus Mons volcano, Mars: Implications for its structure and history
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ABSTRACT: Ascraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera (HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a higher resolution than previously, and details of its lava flows, sinuous rilles, flank vents and tectonic features indicate an unexpected origin for some of these features. We establish the time-stratigraphic sequence for these features, and use a numerical model on HRSC stereo DTMs of the sinuous rilles, and conclude that they were formed by water erosion. Terrestrial analogues for such features are found at Réunion Island and other volcanoes. We then examine the overall structure of the volcano, which is dissimilar to that of large terrestrial volcanoes in important respects, and perform laboratory analogue experiments of its deformation, concluding that the tectonic features were formed by sinking of the volcano into a substratum that was much weaker than the volcanic edifice. An ice-rich substratum melted by a combination of pressure melting and magmatic heating seems the most likely mechanism. Analogous water-escape structures in a similar volcanic situation have been identified at Mt Haddington in the Antarctic. The possible role of a hydrological cycle and a hydrothermal system within the volcano are discussed. Based on field evidence, we propose that much of the broad aprons of lobate flows issuing from the NE and SSW foot of Ascraeus Mons are composed of mudflows rather than lava flows. These different approaches are linked into a coherent history of this volcano. The similarity of Ascraeus Mons to Pavonis Mons and Arsia Mons (though Ascraeus is younger) suggests that some of our conclusions may apply to these volcanoes too.
Earth and Planetary Science Letters 06/2010; 294(3-294):479-491. DOI:10.1016/j.epsl.2009.06.020
• Article: Energetic particles in Saturn's magnetosphere during the Cassini nominal mission (July 2004–July 2008)
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ABSTRACT: In July 2004 the Cassini spacecraft began its orbital tour in the Saturnian system and performed 74 orbits during the nominal mission (July 2004–July 2008) providing data from nearly all local times at various distances and latitudes relative to the planet. The particles and field instruments onboard the spacecraft were essentially operating continuously offering the possibility to study the global configuration and the dynamics of the second largest magnetosphere in our solar system extensively. One of those instruments aboard Cassini is the Low Energy Magnetospheric Measurement System (LEMMS), one of three particle detectors of the Magnetospheric Imaging Instrument (MIMI). MIMI/LEMMS measures the intensity, energy spectra and pitch angle distributions of energetic ions and electrons separately. The measured energetic particle distributions together with the measured magnetic field provide a very powerful tool to investigate the Saturnian magnetosphere in those regions covered by the Cassini orbits. This paper will give an overview of the energetic particle measurements of the MIMI/LEMMS sensor in the Saturnian system. In the first part of the paper synoptic maps will be shown where all the data are presented as a function of various trajectory parameters of the spacecraft. Secondly bi-directional electron distributions along the magnetic field direction will be described as a feature in the Saturnian system. Thirdly the particle parameters in the inner magnetosphere with absorption signatures of the various moons are presented. Fourthly it will be shown that the region around about 15RS seems to be a characteristic region where depletion signatures in energetic particle distributions are very often observed. At the end of this work a 60 min intensity periodicity in the MIMI/LEMMS data is discussed.
Planetary and Space Science 12/2009; 57(14-15-57):1754-1768. DOI:10.1016/j.pss.2009.06.010
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Article: On the amount of heavy molecular ions in Titan's ionosphere
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ABSTRACT: We present observational evidence that the ionosphere of Titan below an altitude of 1150 km is a significant source of heavy (>100 amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950–1300 km had densities in the range 900–3000 cm−3. Below these peaks the number densities of heavy positively charged ions reached 100–2000 cm−3 and approached 50–70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50–150 cm−3. These results imply that ~105–106 heavy positively charged ions/m3/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1–1 Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan's ionosphere is dominated by heavy (>100 amu) molecular ions below 950 km.
Planetary and Space Science 12/2009; 57(14-15-57):1857-1865. DOI:10.1016/j.pss.2009.07.014
• Article: Heavy negative ions in Titan's ionosphere: Altitude and latitude dependence
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ABSTRACT: One of the unexpected results of the Cassini mission was the discovery of negative ions at altitudes between 950 and 1400 km in Titan's ionosphere with masses up to 10,000 amu/q [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978; Waite Jr., J.H., Young, D. T., Coates, A. J., Crary, F. J., Magee, B. A., Mandt, K. E., Westlake, J. H., 2008. The Source of Heavy Organics and Aerosols in Titan's Atmosphere, submitted to Organic Matter in Space, Proceedings IAU Symposium no. 251]. These ions are detected at low altitudes during Cassini's closest Titan encounters by the Cassini plasma spectrometer (CAPS) electron spectrometer. This result is important as it is indicative of complex hydrocarbon and nitrile chemical processes at work in Titan's high atmosphere. They may play a role in haze formation and ultimately in the formation of heavy particles (tholins), which fall through Titan's atmosphere and build up on the surface. During Cassini's prime mission negative ions were observed on 23 Titan encounters, including 7 in addition to those reported by Coates et al. [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978]. Here, we also examine the altitude and latitude dependence of the high-mass negative ions observed in Titan's ionosphere, and we examine the implications of these results. We find that the maximum negative ion mass is higher at low altitude and at high latitudes. We also find a weaker dependence of the maximum mass on solar zenith angle.
Planetary and Space Science 12/2009; 57(14-15-57):1866-1871. DOI:10.1016/j.pss.2009.05.009
• Article: Surface waves on Saturn's dawn flank magnetopause driven by the Kelvin–Helmholtz instability
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ABSTRACT: Crossings of Saturn's magnetopause made by the Cassini spacecraft on 12, 13 and 17 March 2006 are analysed. During this period Cassini's trajectory was approximately parallel to the magnetopause boundary given by a model of the surface. Magnetic field and electron data are used to identify excursions into the magnetosheath bounded by crossings of the magnetopause current layer. Minimum variance analysis of the magnetic field vector measurements is used to determine the normal to the boundary for each crossing. The normals corresponding to the crossings oscillate about an average orientation that is consistent with the unperturbed normal predicted by the surface model. This reveals the presence of regular boundary waves with a direction of propagation found to be within 24° of Saturn's rotational equator. Two categories of boundary wave are identified: the first with a period of the order of hours, and the second with a period of 45±9 min. Based on the propagation direction and a comparison of magnetospheric and magnetosheath magnetic fields, we conclude that both types of wave were driven by the Kelvin–Helmholtz instability. The observed boundary perturbations are consistent with a superposition of different types of surface wave activity.
Planetary and Space Science 12/2009; 57(14-15-57):1769-1778. DOI:10.1016/j.pss.2009.02.010
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Article: Plasma electrons in Saturn's magnetotail: Structure, distribution and energisation
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ABSTRACT: In this paper Saturn's nightside and pre-dawn electron (0.5 eV–28 keV) plasma sheet is studied using Cassini plasma electron and magnetic field data from 2006. Case studies are presented which exemplify the typical and atypical states of the plasma sheet, and are complemented by a statistical study of the plasma sheet. It will be shown that Saturn's nightside and pre-dawn electron plasma sheet exists in two states: a quiescent state with a steady electron temperature of and where the electron distribution functions are best characterised by Kappa distributions, and a disturbed state where the electrons are hot and often seen in alternating layers between warm and hot populations. Evidence is also presented for bimodal cold/warm (both quiet and disturbed states) and warm/hot distributions (disturbed states). The disturbed states are qualitatively similar to electron distributions from Earth's magnetotail during intervals of reconnection and we argue that these disturbed states also result from periods of tail reconnection. We present statistics of electron number density, temperature, partial electron beta, and pressure, and show that large values of partial beta are necessary but not sufficient to uniquely identify the central plasma sheet. Finally the thermodynamic properties of the electron plasma sheet are studied and we show that the electrons behave isothermally. These results are important for modelling and theoretical analyses, and for use in studies which examine dynamics in Saturn's magnetosphere.
Planetary and Space Science 12/2009; 57(14-15-57):2032-2047. DOI:10.1016/j.pss.2009.09.007
• Article: Multi-resolution topographic data extraction from Martian stereo imagery
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ABSTRACT: The acquisition of high resolution topographic data such as Digital Terrain Models (DTMs) and orthoimages from various Martian stereo imagery is now readily available. The very successful European Space Agency (ESA) Mars Express mission includes the High Resolution Stereo Camera (HRSC) which has provided, since March 2004, an increasingly global set of along-track stereo coverage at relatively high spatial resolution (<100 m, mainly 12.5–25 m) over the Martian surface. Previous DTM generation was only accomplished with planned or serendipitous Viking Orbiter and more recently from Mars Orbiter Camera Narrow Angle (MOC-NA) stereo-pairs. Neither system was designed for stereo photogrammetric DTM generation. Recently, the NASA Mars Reconnaissance Orbiter (MRO) deployed two major optical pushbroom CCD cameras which are capable of across-track stereo targeting. Both Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) provide very high resolution stereo imagery at 6 m and submetre, respectively. A stereo processing chain has been developed which uses a non-rigorous sensor model with geodetic control derived from a reference stereo data source (HRSC) and is here shown to be successfully applied to CTX and HiRISE stereo imagery for 3 test areas. This processing chain is here demonstrated to generate excellent quality DTMs (up to a maximum grid spacing of 0.7 m with HiRISE and 10 m with CTX) and associated ortho images. The photogrammetric quality of these products is here verified using inter-comparisons with HRSC and Mars Orbiter Laser Altimeter (MOLA) data sets and shows good agreement.
Planetary and Space Science 12/2009; 57(14-15):2095–2112. DOI:10.1016/j.pss.2009.09.024
• Article: Self-consistent Euler potential modelling of the jovian magnetodisc
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ABSTRACT: Caudal (1986) presented an elegant Euler potential model of the jovian magnetodisc where the magnetospheric mechanical and magnetic stresses were balanced. The modelling methodology results in a magnetic structure that is in stress balance with the energetic particle and plasma properties in the magnetosphere. These plasma properties are introduced into the model via a set of boundary and input conditions. Recently, Achilleos et al. (submitted manuscript) have implemented this model for both Jupiter and Saturn and have used this model to investigate magnetospheric stress balance at Saturn using Cassini-era plasma data. In this paper, the Achilleos et al. implementation of Caudal's model is used to build a new model stress balanced model for the jovian magnetodisc using up-to-date plasma parameters from Voyager, Ulysses and Galileo. The calculated magnetic field profile and plasma sheet structure will be compared with observations and previous models. As part of ongoing work we will investigate and discuss local time effects, dayside magnetodisc disruption, and magnetodisc stress balance in the jovian magnetosphere.
• Article: Addressing Science Use Cases with HELIO
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ABSTRACT: The Heliophysics Integrated Observatory (HELIO) is a new VO project funded under the EC's Seventh Framework Programme (FP7). It includes thirteen partners scattered over six countries and is led by University College London. HELIO is designed to support the heliophysics community and is based on a Service Oriented Architecture. The services developed by and integrated into HELIO can be used to address a wide range of science problems; they can be used individually or as part of a work-flow driven search engine that can use a propagation (or other) model to help locate obervations that describe interesting phenomena. We will describe and discuss how the components of HELIO could be used to address science use cases, particularly how a user can adapt the work flow to their own science interests. Networking is one of the three Activities of the HELIO Integrated Infrastructure Initiatives (I3) project. Within this activity we plan to involve the community in all aspects of the design and testing of the HELIO system, including determining which data and metadata should be included, how the quality and content of metadata can be included, etc. We are investigating ways of making HELIO "domain-aware" so that researchers who are specialists in one of the communities that constitute heliophysics can easily identify, access and use data they need from the other communities. We will discuss how the community can help us develop this capability.
• Article: A refined chronology of catastrophic outflow events in Ares Vallis, Mars
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ABSTRACT: We investigate the geomorphology and chronology of catastrophic flooding in a major martian outflow channel, Ares Vallis. We use recently acquired stereo and colour images and derived topographic data with grid-spacing of 50 m from the High Resolution Stereo Camera (HRSC) onboard Mars Express to constrain the detailed flood geomorphology of proximal Ares Vallis. We identify morphologic evidence for 6 distinct erosion surfaces that were likely formed by multiple episodes of catastrophic flooding. These surfaces occur at distinct topographic levels, exhibit multiple sets of flood grooves, and contain distinct post-flood crater populations. Crater count statistics on the flood-eroded surfaces from MRO Context Camera (CTX) and Thermal Emission Imaging System (THEMIS) visible images indicate that flooding was initiated during the Late Noachian/Early Hesperian at ∼ 3.6 Ga. The morphology data confirm that the oldest flooding events occurred on the topographically highest erosion surfaces while subsequent floods abandoned the upper surfaces and continued to modify craters in the topographically lowest, main channel of Ares Vallis into the Late Hesperian/Early Amazonian (∼ 2.5 Ga). Our results suggest that release of water to Ares Vallis likely occurred at repeated intervals over a long period of geological time, thus supporting recent numerical modelling studies that suggest a recharging water source.
Earth and Planetary Science Letters 10/2009; 288(1-288):58-69. DOI:10.1016/j.epsl.2009.09.008
• Article: Plasma environment of Jupiter family comets
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ABSTRACT: As any comet nears the Sun, gas sublimes from the nucleus taking dust with it. Jupiter family comets are no exception. The neutral gas becomes ionized, and the interaction of a comet with the solar wind starts with ion pickup. This key process is also important in other solar system contexts wherever neutral particles become ionized and injected into a flowing plasma such as at Mars, Venus, Io, Titan and interstellar neutrals in the solar wind. At comets, ion pickup removes momentum and energy from the solar wind and puts it into cometary particles, which are then thermalised via plasma waves. Here we review what comets have shown us about how this process operates, and briefly look at how this can be applied in other contexts. We review the processes of pitch angle and energy scattering of the pickup ions, and the boundaries and regions in the comet–solar wind interaction. We use in-situ measurements from the four comets visited to date by spacecraft carrying plasma instrumentation: 21P/Giacobini-Zinner, 1P/Halley, 26P/Grigg-Skjellerup and 19P/Borrelly, to illustrate the process in action. While, of these, comet Halley is not a Jupiter class comet, it has told us the most about cometary plasma environments. The other comets, which are from the Jupiter family, give an interesting comparison as they have lower gas production rates and less-developed interactions. We examine the prospects for Rosetta at comet Churyumov-Gerasimenko, another Jupiter family comet where a wide range of gas production rates will be studied.
Planetary and Space Science 08/2009; 57(10-57):1175-1191. DOI:10.1016/j.pss.2009.04.009
• Article: The effect of spacecraft radiation sources on electron moments from the Cassini CAPS electron spectrometer
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ABSTRACT: Data from the Cassini plasma spectrometer (CAPS) electron spectrometer (ELS) have been found to be contaminated with an energy-independent background count rate which has been associated with radiation sources on Cassini. In this paper we describe this background radiation and quantitatively assess its impact on numerically integrated electron moments. The general properties of such a background and its effects on numerical moments are derived. The properties of the ELS background are described and a model for the background presented. A model to generate synthetic ELS spectra is presented and used to evaluate the density and temperature of pure noise and then extended to include ambient distributions. It is shown that the presence of noise produces a saturation of the electron density and temperature at quasi-constant values when the instrument is at background, but that these noise level moments are dependent on the floating spacecraft potential and the orientation of the ELS instrument with respect to the spacecraft. When the ambient distribution has a poor signal-to-noise ratio (SNR) the noise determines the density and temperature; however, as the SNR increases (increasing primarily with density) the density and temperature tend to those of the ambient distribution. It is also shown that these noise effects produce highly artificial density–temperature inverse correlations. A method to subtract this noise is presented and shown to correct for the presence of the noise. Simulated error estimates for the density and temperature are also presented. The analysis described in this paper not only applies to weak background noise, but also to more significant penetrating backgrounds such as those in radiation belt regions of planetary magnetospheres.
Planetary and Space Science 06/2009; 57(7-57):854-869. DOI:10.1016/j.pss.2009.02.011
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Article: Testing the blast wave model with Swift GRBs
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ABSTRACT: The complex structure of the light curves of Swift Gamma-Ray Bursts (GRBs) has made the identification of breaks, and the interpretation of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to identify breaks, which are possibly hidden, and to constrain the blast wave parameters; electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multiwavelength light curves and X-ray spectra of our sample to the predictions of the blast wave model. We can successfully interpret all of the bursts in our sample of 10, except two, within this framework and we can estimate, with confidence, the electron energy distribution index for 6 of the sample. Furthermore, we identify jet breaks in a number of the bursts. A statistical analysis of the distribution of p reveals that, even in the most conservative case of least scatter, the values are not consistent with a single, universal value. The values of k suggest that the circumburst density profiles are not drawn from only one of the constant density or wind-like media populations.
Monthly Notices of the Royal Astronomical Society 04/2009; 395(1):580 - 592. DOI:10.1111/j.1365-2966.2009.14578.x
• Article: Kelvin-Helmholtz Instability at the Dawn Flank of Saturn's Magnetopause
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ABSTRACT: Crossings of Saturn's magnetopause made by the Cassini spacecraft between 12 and 17 March 2006 are analysed. Magnetic field and plasma data are used to identify excursions into the magnetosheath bounded by crossings of the magnetopause current layer. During most of this period Cassini's trajectory was approximately parallel to the magnetopause boundary given by a model of the surface. Minimum variance analysis (MVA) of the magnetic field vector measurements is used to determine the normal to the boundary for each crossing of the current layer. The normals corresponding to the crossings made on 12, 13 and 17 March oscillate about the normal predicted by the surface model. This suggests the presence of regular boundary waves with a direction of propagation found to be close to parallel to Saturn's rotational equator, and not coincident with the expected solar wind flow direction in the local magnetosheath. Based on this propagation direction and the magnetospheric and magnetosheath magnetic fields we propose that these waves were generated by the Kelvin-Helmholtz instability. In addition we discuss the possibility that on 15 and 16 March nonlinear Kelvin-Helmholtz waves produced a strongly perturbed magnetopause boundary that may have led to local magnetic reconnection.
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Article: Multi-scale reconnections in a complex CME
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ABSTRACT: A series of three flares of GOES class M, M and C, and a CME were observed on 20 January 2004 occurring in close succession in NOAA 10540. Types II, III, and N radio bursts were associated. We use the combined observations from TRACE, EIT, Hα images from Kwasan Observatory, MDI magnetograms, GOES, and radio observations from Culgoora and Wind/ WAVES to understand the complex development of this event. We reach three main conclusions. First, we link the first two impulsive flares to tether-cutting reconnections and the launch of the CME. This complex observation shows that impulsive quadrupolar flares can be eruptive. Second, we relate the last of the flares, an LDE, to the relaxation phase following forced reconnections between the erupting flux rope and neighbouring magnetic field lines, when reconnection reverses and restores some of the pre-eruption magnetic connectivities. Finally, we show that reconnection with the magnetic structure of a previous CME launched about 8 h earlier injects electrons into open field lines having a local dip and apex (located at about six solar radii height). This is observed as an N-burst at decametre radio wavelengths. The dipped shape of these field lines is due to large-scale magnetic reconnection between expanding magnetic loops and open field lines of a neighbouring streamer. This particular situation explains why this is the first N-burst ever observed at long radio wavelengths.
Advances in Space Research 09/2008; 42(5-42):858-865. DOI:10.1016/j.asr.2007.04.065
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Article: Large contribution of sea surface warming to recent increase in Atlantic hurricane activity
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ABSTRACT: Atlantic hurricane activity has increased significantly since 1995 (refs 1-4), but the underlying causes of this increase remain uncertain. It is widely thought that rising Atlantic sea surface temperatures have had a role in this, but the magnitude of this contribution is not known. Here we quantify this contribution for storms that formed in the tropical North Atlantic, Caribbean Sea and Gulf of Mexico; these regions together account for most of the hurricanes that make landfall in the United States. We show that a statistical model based on two environmental variables--local sea surface temperature and an atmospheric wind field--can replicate a large proportion of the variance in tropical Atlantic hurricane frequency and activity between 1965 and 2005. We then remove the influence of the atmospheric wind field to assess the contribution of sea surface temperature. Our results indicate that the sensitivity of tropical Atlantic hurricane activity to August-September sea surface temperature over the period we consider is such that a 0.5 degrees C increase in sea surface temperature is associated with a approximately 40% increase in hurricane frequency and activity. The results also indicate that local sea surface warming was responsible for approximately 40% of the increase in hurricane activity relative to the 1950-2000 average between 1996 and 2005. Our analysis does not identify whether warming induced by greenhouse gases contributed to the increase in hurricane activity, but the ability of climate models to reproduce the observed relationship between hurricanes and sea surface temperature will serve as a useful means of assessing whether they are likely to provide reliable projections of future changes in Atlantic hurricane activity.
Nature 02/2008; 451(7178):557-60. DOI:10.1038/nature06422
• Article: A resonant cyclotron scattering model for the soft-ray spectra of magnetar candidates
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ABSTRACT: Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are peculiar classes of X-ray sources which share a number of common properties. Among others are: a sporadic bursting activity,slow rotation periods in a narrow range (a few seconds), high secular spindown and a X-ray luminosity of ˜ 1034 - 1036 erg/s. Over the last few years, observations provided increasing evidence that SGRs and AXPs are magnetars: ultra-magnetized neutron stars with surface field in excess of 1014 G, i.e. well above the threshold at which QED effects become relevant. Spectral analysis is an important tool in magnetar astrophysics since it can provide key information on the emission mechanisms. The first attempts at modelling the soft X-ray (< 10 keV) spectra of AXPs proved that a model consisting of a blackbody (kT ˜ 0.3 - 0.6 keV) plus a power-law (photon index ˜ 2 - 4) could successfully reproduce the observed emission. However, despite the blackbody plus power-law spectral model has been largely applied to the X-ray spectra of magnetar candidates, a convincing physical interpretation is still missing. In this talk, I will present the application of synthetic model spectra, that we calculated with a new magnetic montecarlo radiative code. Our code accounts for resonant cyclotron upscattering of soft thermal photons (emitted by the star surface), by a population of relativistic electrons threated in the magnetosphere. Polarization and QED effects are consistently accounted for. Further applications to the modeling of the hard X-ray magnetar emission are discussed.
• Article: Periodic crossings of Saturn's magnetospheric current/plasma sheet observed by Cassini CAPS/ELS and MAG
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ABSTRACT: Cassini observations in Saturn's magnetotail during 2006 show evidence of periodic encounters with the plasma and current sheet. In this talk we discuss these encounters as observed by the Cassini magnetometer (MAG) and electron spectrometer (ELS). We emphasise that the tail observations can simply be interpreted in terms of periodic vertical oscillations of the sheet, obviating the need for a sub-corotating plume structure as advocated by some authors. Furthermore, many encounters with the sheet are "double-peaked" which can also be simply explained by the vertical oscillation paradigm. We explore these vertical oscillations, consider simple structural models to fit the observations, and compare our results with data from other instruments (e.g., Carbary et al. 2007).
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Article: Effect of Strong External Turbulence on a Wall Jet Boundary Layer
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ABSTRACT: The initial stage of the development of a wall jet under the influence of strong external turbulence has been studied in a novel shear-flow mixing-box experiment. A fully developed channel flow of depth h (40mm) enters along the top wall of a cuboidal box of height 11h in which a combination of oscillatory and turbulent velocity fluctuations are generated by a vertical oscillating grid at the midplane 5h below the wall. When the ratio of the rms grid-generated velocity fluctuations, u¢g u^{\prime }_{g} , to the local mean velocity inside the wall jet layer, u, is greater than about 0.1, significant changes are observed in the mean shear profile and in the eddy structure of the wall jet. The wall jet thickness increases by approximately 25% but the maximum velocity decreases by less than 10% compared to the case without the external turbulence. Fluctuations of the streamwise velocity component increase as expected in the outer part of the wall jet, but the most significant result is the increase by 70% of the fluctuations in the boundary layer close to the wall. CFD simulations using the k-ɛ RNG of the FLUENT CFD Code do not properly model the effect of the large scale external turbulence in this experiment. However, an artificial method, which introduces a series of small inlet/outlet jets to represent external turbulence, approximately simulates the overall effects of the oscillating grid on the wall jet, but does not simulate the amplification of the near wall turbulence.
Flow Turbulence and Combustion 08/2007; 79(2):155-174. DOI:10.1007/s10494-007-9078-2
• Article: 3D processing of HiRISE stereo pair using a non-rigorous sensor model and epi-polarity
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ABSTRACT: The accuracy and spatial resolutions of modern planetary mapping missions are being rapidly improved by more sophisticated sensors and tracking systems. Especially for Mars, orbital cameras such as MOC, THEMIS and HRSC are providing optical imagery up-to 10-20 metres or even a few metres' resolutions. Moreover the capability of stereo imaging of HRSC and MOC-NA make it feasible to extract a relatively high resolution DEM (Digital Elevation Model) for geological and geo-morphological research. However, even with HRSC which has 12.5 m spatial resolution, the quality of reconstructed 3D Martian surfaces is still not suitable for several purposes such as landing site survey, local roughness extraction and small structure observations. The successful deployment of the NASA MRO with the 30cm HiRISE has dramatically upgraded the resolution limit of Martian surface observation for the targets, many of which have never been observed by other optical sensors before. The difficulty in fully exploiting the potential of HiRISE is that the tracking information which is indispensable for sensor modeling and stereo 3D data extraction, is not yet available at a sufficiently high accuracy. Therefore photogrammetrically well controlled stereo or monocular mapping by HiRISE has only been performed by a few teams who can fully access and update all sensor and navigation data (Heyd et al., 2007 LPSC, Kirk et al., 2007 ISPRS WG IV/7 Workshop). We have developed a simple generic mapping method for HIRISE stereo imagery and applied this to a stereo pair in the Eberswalde Delta which is publicly available. Without any detailed tracking information and sensor specification, it produce what appears to be a very reasonable quality set of mapping products including a 2.5m resolution stereo DTM and a 30cm orthoimage which is controlled up to 30-40m horizontal accuracy when compared with a bundle-adjusted HRSC image. The theory of this method is based on a non-rigorous sensor model employing HRSC intersection points as the control information [3] (Kim et al., 2007) which also exploits epi-polarity. For more reliable horizontal and vertical control, the quality of the HRSC intersection point was optimized by employing a sophisticated image matching and bundle adjusted orientation information for HRSC which was kindly provided by U of Hannover. By creating epi-polar rectified images of HiRISE and then applying a non-rigorous sensor model, the degrees of freedom of the geospatial coordinates were reduced and the relative coordinate was easily converted into a Mars mapping coordinate system. In addition, using image matching with epi-polar rectified image pair provided more reliable disparity values. The constructed DTM is well correlated with the geo-morphological features, which is observed in the optical image. However, a current unknown with this method is that it is still not clear whether stereo intersections of HRSC image can provided sufficient vertical accuracy to project relative disparity into ellipsoidal heights. We suggest that we could use this method as a temporary mapping aid to an eventual rigorous mapping of HiRISE. Over the areas which have been acquired using HiRISE stereo scenes, this method is being tested and evaluations perform to compare against HSRC products.
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Article: Giant flares in soft γ-ray repeaters and short GRBs
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ABSTRACT: Soft gamma-ray repeaters (SGRs) are a peculiar family of bursting neutron stars that, occasionally, have been observed to emit extremely energetic giant flares (GFs), with energy release up to approximately 10(47) ergs(-1). These are exceptional and rare events. It has been recently proposed that GFs, if emitted by extragalactic SGRs, may appear at Earth as short gamma-ray bursts. Here, I will discuss the properties of the GFs observed in SGRs, with particular emphasis on the spectacular event registered from SGR 1806-20 in December 2004. I will review the current scenario for the production of the flare, within the magnetar model, and the observational implications.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 06/2007; 365(1854):1307-13. DOI:10.1098/rsta.2006.1998
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Article: Shock break-out: How a GRB revealed the beginnings of a supernova
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ABSTRACT: In February 2006, Swift caught a gamma-ray burst (GRB) in the act of turning into a supernova, and made the first ever direct observations of the break-out and early expansion of a supernova shock wave. GRB 060218 began with an exceptionally long burst of non-thermal gamma-rays, lasting over 2000s, as a jet erupted through the surface of the star. While this was in progress, an optically-thick thermal component from the shock wave of the supernova explosion grew to prominence, and we were able to track the mildly relativistic expansion of this shell as the blackbody peak moved from the X-rays into the UV and optical bands. The initial radius of the shock implied that it was a blue supergiant that had exploded, but the lack of hydrogen emission lines in the supernova spectrum indicated a more compact star. The most likely scenario is that the shock ploughed into the massive stellar wind of a Wolf-Rayet progenitor, with the shock breaking-out and becoming visible to us once it reached the radius where the wind became optically-thin. I present the Swift observations of this landmark event, and discuss the new questions and answers it leaves us with.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 06/2007; 365(1854):1263-8. DOI:10.1098/rsta.2006.1968
• Article: Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1
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ABSTRACT: We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29–2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4±0.2)×1032 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190±20 km2.
Icarus 03/2007; 187(1-187):123-131. DOI:10.1016/j.icarus.2006.09.021
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Article: The 2005/06 UK and European winter: the UCL forecast and its assessment against observations

Weather 01/2007; 61(12):347 - 352. DOI:10.1256/wea.183.06
• Article: Study of CME transit speeds for the event of 07-NOV-2004
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ABSTRACT: Several methods for CME speed estimation are discussed. These include velocity derivation based on the frequency drifts observed in metric and decametric radio wave data using a range of coronal density models. Coronagraph height–time plots allow measurement of plane-of-sky and expansion speeds. These in turn can enable propagation speeds to be derived from a range of empirical relations. Simple geometric e.g., cone, models can provide propagation velocity estimates for suitable halo or partial halo events. Interplanetary scintillation observations allow speed estimates at large distances from the Sun detecting in particular the deceleration of the faster CMEs. Related interplanetary shocks and the arrival times and speeds of the associated magnetic clouds at Earth can also be considered. We discuss the application of some of these methods to the transit to Earth of a complex CME that originated earlier than 16:54 U.T. on 07-NOV-2004. The difficulties in making velocity estimates from radio observations, particularly under disturbed coronal conditions, are highlighted.
Advances in Space Research 01/2007; 40(12-40):1807-1814. DOI:10.1016/j.asr.2007.01.005
• Article: Magnetic coupling of the Sun–Earth system – The view from STEREO
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ABSTRACT: The STEREO mission, launched on October 25 2006, will provide the first stereoscopic view of the Sun and its magnetic environment. A suite of identical instruments on two continuously separating spacecraft will monitor the onset of solar eruptive phenomena, and track them as the shocks and ejected material propagate through the interplanetary medium (IPM). The combination of remote sensing and in situ instrumentation will provide new insights into the onset of eruptions, the extent of their effects on the global structure of the low corona, and their subsequent evolution through the IPM. These observations will provide new constraints on the processes involved and allow us to distinguish between competing models of eruptive solar phenomena.
Advances in Space Research 01/2007; 39(12-39):1791-1803. DOI:10.1016/j.asr.2007.02.043
• Article: Anisotropic model for granulated sea ice dynamics
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ABSTRACT: A continuum model describing sea ice as a layer of granulated thick ice, consisting of many rigid, brittle floes, intersected by long and narrow regions of thinner ice, known as leads, is developed. We consider the evolution of mesoscale leads, formed under extension, whose lengths span many floes, so that the surrounding ice is treated as a granular plastic. The leads are sufficiently small with respect to basin scales of sea ice deformation that they may be modelled using a continuum approach. The model includes evolution equations for the orientational distribution of leads, their thickness and width expressed through second-rank tensors and terms requiring closures. The closing assumptions are constructed for the case of negligibly small lead ice thickness and the canonical deformation types of pure and simple shear, pure divergence and pure convergence. We present a new continuum-scale sea ice rheology that depends upon the isotropic, material rheology of sea ice, the orientational distribution of lead properties and the thick ice thickness. A new model of lead and thick ice interaction is presented that successfully describes a number of effects: (i) because of its brittle nature, thick ice does not thin under extension and (ii) the consideration of the thick sea ice as a granular material determines finite lead opening under pure shear, when granular dilation is unimportant.
Journal of the Mechanics and Physics of Solids 06/2006; 54(6-54):1147-1185. DOI:10.1016/j.jmps.2005.12.006
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Article: Rapid discharge connects Antarctic subglacial lakes. Nature
[Hide abstract]
ABSTRACT: The existence of many subglacial lakes provides clear evidence for the widespread presence of water beneath the East Antarctic ice sheet, but the hydrology beneath this ice mass is poorly understood. Such knowledge is critical to understanding ice flow, basal water transfer to the ice margin, glacial landform development and subglacial lake habitats. Here we present ice-sheet surface elevation changes in central East Antarctica that we interpret to represent rapid discharge from a subglacial lake. Our observations indicate that during a period of 16 months, 1.8 km3 of water was transferred over 290 km to at least two other subglacial lakes. While viscous deformation of the ice roof above may moderate discharge, the intrinsic instability of such a system suggests that discharge events are a common mode of basal drainage. If large lakes, such as Lake Vostok or Lake Concordia, are pressurizing, it is possible that substantial discharges could reach the coast. Our observations conflict with expectations that subglacial lakes have long residence times and slow circulations, and we suggest that entire subglacial drainage basins may be flushed periodically. The rapid transfer of water between lakes would result in large-scale solute and microbe relocation, and drainage system contamination from in situ exploration is, therefore, a distinct risk.
Nature 05/2006; 440(7087):1033-6. DOI:10.1038/nature04660
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Article: Quantum properties of a cyclic structure based on tripolar fields
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ABSTRACT: The properties of cyclic structures (toroidal oscillators) based on classical tripolar (colour) fields are discussed, in particular, of a cyclic structure formed of three colour-singlets spinning around a ring-closed axis. It is shown that the helicity and handedness of this structure can be related to the quantum properties of the electron. The symmetry of this structure corresponds to the complete cycle of ${2/3}\pi$-rotations of its constituents, which leads to the exact overlapping of the paths of its three complementary coloured constituents, making the system dynamically colourless. The gyromagnetic ratio of this system is estimated to be g$\approx 2$, which agrees with the Land\'e g-factor for the electron.
Physica D Nonlinear Phenomena 04/2006; 226(2). DOI:10.1016/j.physd.2006.11.009
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Article: XMM–Newton observations of the Seyfert 1 AGN H 0557−385
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ABSTRACT: We present XMM–Newton observations of the Seyfert 1 active galactic nucleus (AGN) H 0557 − 385. We have conducted a study into the warm absorber present in this source, and using high-resolution Reflection Grating Spectrometer (RGS) data we find that the absorption can be characterized by two phases: a phase with log ionization parameter ξ of 0.50 (where ξ is in units of erg cm s−1) and a column of 0.2 × 1021 cm−2, and a phase with log ξ of 1.62 and a column of 1.3 × 1022 cm−2. An iron Kα line is detected. Neutral absorption is also present in the source, and we discuss possible origins for this. On the assumption that the ionized absorbers originate as an outflow from the inner edge of the torus, we use a new method for finding the volume filling factor. Both phases of H 0557 − 385 have small volume filling factors (≤1 per cent). We also derive the volume filling factors for a sample of 23 AGN using this assumption and for the absorbers with log ξ > 0.7, we find reasonable agreement with the filling factors obtained through the alternative method of equating the momentum flow of the absorbers to the momentum loss of the radiation field. By comparing the filling factors obtained by the two methods, we infer that some absorbers with log ξ < 0.7 occur at significantly larger distances from the nucleus than the inner edge of the torus.
Monthly Notices of the Royal Astronomical Society 01/2006; 366(2):521 - 528. DOI:10.1111/j.1365-2966.2005.09869.x
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Article: Tracing magnetic helicity from the solar corona to the interplanetary space
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ABSTRACT: On October 14, 1995, a C1.6 long duration event (LDE) started in active region (AR) NOAA 7912 at approximately 5:00 UT and lasted for about 15 h. On October 18, 1995, the Solar Wind Experiment and the Magnetic Field Instrument (MFI) on board the Wind spacecraft registered a magnetic cloud (MC) at 1 AU, which was followed by a strong geomagnetic storm. We identify the solar source of this phenomenon as AR 7912. We use magnetograms obtained by the Imaging Vector Magnetograph at Mees Solar Observatory, as boundary conditions to the linear force-free model of the coronal field, and, we determine the model in which the field lines best fit the loops observed by the Soft X-ray Telescope on board Yohkoh. The computations are done before and after the ejection accompanying the LDE. We deduce the loss of magnetic helicity from AR 7912. We also estimate the magnetic helicity of the MC from in situ observations and force-free models. We find the same sign of magnetic helicity in the MC and in its solar source. Furthermore, the helicity values turn out to be quite similar considering the large errors that could be present. Our results are a first step towards a quantitative confirmation of the link between solar and interplanetary phenomena through the study of magnetic helicity.
Journal of Atmospheric and Solar-Terrestrial Physics 12/2005; 67(17-18-67):1734-1743. DOI:10.1016/j.jastp.2005.07.003
• Conference Paper: Microchannel plates at high rates: The challenges for future space plasma missions
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ABSTRACT: The rate performance of microchannel plates (MCPs) used in the PEACE instruments of the Cluster and Double Star mission is discussed. MCPs are currently the preferred sensor element for the detection of low energy (∼0 to 30 keV) electrons and ions in space plasma instruments. Low resistance plates have recently been tested for pulse height distribution and an increase in the rate handling by a factor of three have been achieved. However, for future applications, a factor of more than 25 will be required.
Vacuum Nanoelectronics Conference, 2005. IVNC 2005. Technical Digest of the 18th International; 08/2005
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Article: Granular flow in the marginal ice zone
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ABSTRACT: The region of sea ice near the edge of the sea ice pack is known as the marginal ice zone (MIZ), and its dynamics are complicated by ocean wave interaction with the ice cover, strong gradients in the atmosphere and ocean and variations in sea ice rheology. This paper focuses on the role of sea ice rheology in determining the dynamics of the MIZ. Here, sea ice is treated as a granular material with a composite rheology describing collisional ice floe interaction and plastic interaction. The collisional component of sea ice rheology depends upon the granular temperature, a measure of the kinetic energy of flow fluctuations. A simplified model of the MIZ is introduced consisting of the along and across momentum balance of the sea ice and the balance equation of fluctuation kinetic energy. The steady solution of these equations is found to leading order using elementary methods. This reveals a concentrated region of rapid ice flow parallel to the ice edge, which is in accordance with field observations, and previously called the ice jet. Previous explanations of the ice jet relied upon the existence of ocean currents beneath the ice cover. We show that an ice jet results as a natural consequence of the granular nature of sea ice.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 08/2005; 363(1832):1677-700. DOI:10.1098/rsta.2005.1601
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Article: A mathematical model of crystallization in an emulsion
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ABSTRACT: A mathematical model incorporating many of the important processes at work in the crystallization of emulsions is presented. The model describes nucleation within the discontinuous domain of an emulsion, precipitation in the continuous domain, transport of monomers between the two domains, and formation and subsequent growth of crystals in both domains. The model is formulated as an autonomous system of nonlinear, coupled ordinary differential equations. The description of nucleation and precipitation is based upon the Becker-Doring equations of classical nucleation theory. A particular feature of the model is that the number of particles of all species present is explicitly conserved; this differs from work that employs Arrhenius descriptions of nucleation rate. Since the model includes many physical effects, it is analyzed in stages so that the role of each process may be understood. When precipitation occurs in the continuous domain, the concentration of monomers falls below the equilibrium concentration at the surface of the drops of the discontinuous domain. This leads to a transport of monomers from the drops into the continuous domain that are then incorporated into crystals and nuclei. Since the formation of crystals is irreversible and their subsequent growth inevitable, crystals forming in the continuous domain effectively act as a sink for monomers "sucking" monomers from the drops. In this case, numerical calculations are presented which are consistent with experimental observations. In the case in which critical crystal formation does not occur, the stationary solution is found and a linear stability analysis is performed. Bifurcation diagrams describing the loci of stationary solutions, which may be multiple, are numerically calculated.
The Journal of Chemical Physics 06/2005; 122(17):174910. DOI:10.1063/1.1886705

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Article: Saunders, M. A. & Lea, A. S. Seasonal prediction of hurricane activity reaching the coast of the United States. Nature 434, 1005-1008
[Hide abstract]
ABSTRACT: Much of the property damage from natural hazards in the United States is caused by landfalling hurricanes--strong tropical cyclones that reach the coast. For the southeastern Atlantic coast of the US, a statistical method for forecasting the occurrence of landfalling hurricanes for the season ahead has been reported, but the physical mechanisms linking the predictor variables to the frequency of hurricanes remain unclear. Here we present a statistical model that uses July wind anomalies between 1950 and 2003 to predict with significant and useful skill the wind energy of US landfalling hurricanes for the following main hurricane season (August to October). We have identified six regions over North America and over the east Pacific and North Atlantic oceans where July wind anomalies, averaged between heights of 925 and 400 mbar, exhibit a stationary and significant link to the energy of landfalling hurricanes during the subsequent hurricane season. The wind anomalies in these regions are indicative of atmospheric circulation patterns that either favour or hinder evolving hurricanes from reaching US shores.
Nature 05/2005; 434(7036):1005-8. DOI:10.1038/nature03454
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Article: Multiple stationary solutions of an irradiated slab
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ABSTRACT: A mathematical model describing the heat budget of an irradiated medium is introduced. The one-dimensional form of the equations and boundary conditions are presented and analysed. Heat transport at one face of the slab occurs by absorption (and reflection) of an incoming beam of short-wave radiation with a fraction of this radiation penetrating into the body of the slab, a diffusive heat flux in the slab and a prescribed incoming heat flux term. The other face of the slab is immersed in its own melt and is considered to be a free surface. Here, temperature continuity is prescribed and evolution of the surface is determined by a Stefan condition. These boundary conditions are flexible enough to describe a range of situations such as a laser shining on an opaque medium, or the natural environment of polar sea ice or lake ice. A two-stream radiation model is used which replaces the simple Beer's law of radiation attenuation frequently used for semi-infinite domains. The stationary solutions of the governing equations are sought and it is found that there exists two possible stationary solutions for a given set of boundary conditions and a range of parameter choices. It is found that the existence of two stationary solutions is a direct result of the model of radiation absorption, due to its effect on the albedo of the medium. A linear stability analysis and numerical calculations indicate that where two stationary solutions exist, the solution corresponding to a larger thickness is always stable and the solution corresponding to a smaller thickness is unstable. Numerical simulations reveal that when there are two solutions, if the slab is thinner than the smaller stationary thickness it will melt completely, whereas if the slab is thicker than the smaller stationary thickness it will evolve toward the larger stationary thickness. These results indicate that other mechanisms (e.g. wave-induced agglomeration of crystals) are necessary to grow a slab from zero initial thickness in the parameter regime that yields two stationary solutions.
Journal of Crystal Growth 04/2005; 276(3-4):688-697. DOI:10.1016/j.jcrysgro.2004.11.417
• Article: The Solar-B EUV imaging spectrometer and its science goals
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ABSTRACT: The Solar-B mission includes an Extreme-UV Imaging Spectrometer (EIS). It detects photons in the ranges 170–210 and 250–290 Å which include emission lines from several highly ionised species that exist at temperatures log T = 4.7, 5.6, 5.8, 5.9 and 6.0–7.3 K. In this paper, we will describe the design and operation of the instrument and present its performance parameters, e.g., spectral and spatial resolution and sensitivity. Preliminary results of recent calibration measurements will be described. Its role in the Solar-B mission will be illustrated with reference to several key science topics that the EIS is expected to address. The anticipated observing strategy for the first three months of the mission will be outlined.
Advances in Space Research 01/2005; 36(8-36):1494-1502. DOI:10.1016/j.asr.2005.04.106
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Article: The smallest source region of an interplanetary magnetic cloud: A mini-sigmoid
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ABSTRACT: We provide evidence for the smallest sigmoid eruption – CME – interplanetary magnetic cloud event ever observed by combining multi-wavelength remote sensing and in situ observations, as well as computing the coronal and interplanetary magnetic fields. The tiny bipole had 100 times less flux than an average active region (AR). It had a sigmoidal structure in the corona and we detected a very high level of twist in its magnetic field. On 11 May 1998, at about 8 UT, the sigmoid underwent eruption evidenced by expanding elongated EUV loops, dimmings and formation of a cusp. The Wind spacecraft, 4.5 days later, detected one of the smallest magnetic clouds (MC) ever identified (100 times less magnetic flux than an average MC). The link between the EUV bright point eruption and the interplanetary MC is supported by several pieces of evidence: timing, same coronal loop and MC orientation relative to the ecliptic, same magnetic field direction and magnetic helicity sign in the coronal loops and in the MC, comparable magnetic flux measured in the dimming regions and in the interplanetary MC and, most importantly, the pre- to post-event change of magnetic helicity in the solar corona is found to be comparable to the helicity content of the cloud.
Advances in Space Research 01/2005; 36(8-36):1579-1586. DOI:10.1016/j.asr.2005.02.003
• Article: Anisotropic Continuum Model of Granulated Sea Ice
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ABSTRACT: A continuum model describing sea ice as a layer of granulated thick ice intersected by long and narrow regions of thinner ice, leads, is developed. Sea ice is considered to be a two-dimensional granular material, whose deformation occurs through motion of floes generated in ice fracturing under applied stress. We consider dynamics of mesoscale leads generated under tensile stress, whose dimensions is larger than those of floes, so that deformation of the surrounding ice is described through the granular plastic rheology, but still sufficiently smaller than the basin scale, so that they can be modelled using continuum approach. The model consists of the stress expression depending on orientational distribution of lead characteristics, thick ice thickness and rheology. It also includes evolution equations for the orientational distribution of leads, their thickness and width expressed through second-rank tensors. The results of the calculations show that the model produces reasonable behaviour in simple flows. The consideration of the thick ice as granular material elliminates the problem of infinite lead opening under pure shear produced by an earlier model considering thick ice deformation only through ridging.
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Article: A puzzling event during the X-ray emission of the binary system GX 1+4
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ABSTRACT: We report on a long X-ray observation of the the slow-rotating binary pulsar GX 1+4. BeppoSAX observed, in the 0.1–200 keV energy range, an event in which the source flux dropped for almost a day, and then recovered. During this event only the high-energy emission was found to be pulsed and the pulsations were shifted in phase of ∼0.2. The spectrum during the event was well fitted by a Compton-reflection model. A broad iron line at ∼6.55 keV was present outside of the event, where instead two narrow emission lines at ∼6.47 and ∼7.05 keV were detected. The pulse profile was highly variable as a function of both energy and time. We interpret this low-flux event as an occultation of the direct X-ray emission, due to the increase of a torus-like accretion disk; we then discuss similarities between this source and the recently discovered highly absorbed INTEGRAL sources.
Advances in Space Research 10/2004; 38(7-38):1453-1456. DOI:10.1016/j.asr.2005.04.086
• Article: Spectroscopic observations of coronal waves and coronal mass ejections
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ABSTRACT: It is common to use imaging instruments such as EUV and X-ray imagers and coronagraphs to study large-scale phenomena such as coronal mass ejections and coronal waves. Although high resolution spectroscopy is generally limited to a small field of view, its importance in understanding global phenomena should not be under-estimated. I will review current spectroscopic observations of large-scale dynamic phenomena such as global coronal waves and coronal mass ejections. The aim is to determine plasma parameters such as flows, temperatures and densities to obtain a physical understanding of these phenomena.
Advances in Space Research 01/2004; 41(1-41):138-143. DOI:10.1016/j.asr.2007.06.005
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Article: Ultraviolet images of galaxies from the Optical Monitor on XMM-Newton
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ABSTRACT: A large number of galaxies, both normal and active, have been observed in ultraviolet light by the Optical Monitor on XMM-Newton. These are some of the deepest wide-field ultraviolet images of these galaxies yet obtained, and in many cases the first collected in this waveband. We present images of five active galaxies, and discuss the potential uses of the ultraviolet surface brightness distribution and morphology, in association with X-ray data, for Active Galactic Nuclei, star formation and galaxy evolution studies.
Advances in Space Research 01/2004; 34(12-34):2540-2543. DOI:10.1016/j.asr.2003.02.079
• Article: Validation of satellite altimeter range measurements over salar de Uyuni, Bolivia
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ABSTRACT: The salar de Uyuni in the Bolivian altiplano covers approximately 9600 km2, and is the largest dry salt lake in the world. This vast, flat stable surface is ideal for estimation of the range bias on altimeter instruments such as GLAS on board ICESat (launched 12th January 2003), RA-2 on board ENVISAT (launched March 1st 2002) and the ERS-2 radar altimeter (RA, launched 1995), and to compare the measurements from these altimeters. Here we describe a kinematic GPS survey of the salar de Uyuni that was carried out in August/September 2002 and was designed to calibrate GLAS and RA-2. The eastern part of the salar was surveyed with 8 grids 22.5 km x 13.5 km at 2.25 km spacing, and with 2 grids which straddled one ascending and one descending ERS-2/ENVISAT orbit across this part of the salar, one 44.5 x 9 km, the other 18 x 13.5 km. Comparison of GPS heights from one GPS grid to the next and crossover analysis at intersections suggests that RMS accuracy of the GPS measurement is around 2 cm. We retracked the altimeter waveforms by fitting the system point target response to retrieve the altimeter surface elevation. We fitted a gaussian-smoothed surface to the GPS heights collected around the ERS-2/ENVISAT grids and interpolated these surfaces to the locations of altimeter footprints to obtain an estimate of the range bias for each instruments (GLAS, RA-2 and the ERS-2 RA). We also compare our results with data from one cycle of TOPEX data, which was collected during an orbit manoeuvre phase in September 2002. Ground tracks from this cycle serendipitously crossed our survey area at the same time we were on the ground.
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Article: On the Effect of Planetary Rotation on Shear and Density Layers in Stratified Flows
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ABSTRACT: The effect of the Coriolis forces on the dynamicsof shear and density layers instratified flows is investigated, an effectthat has not been taken into account inmost previous studies of turbulence-meanfield or wave-mean field interactions.For instance, recent studies have shownthat shear and density layers can grow inthe presence of turbulence in a stronglystratified fluid but the effect of planetaryrotation was not taken into account. Toaddress this problem, wave-mean flowinteraction in a stratified fluid is hereinvestigated in the presence of rotation usingdirect numerical simulation. The results showthat the wave-mean flow interactionand the formation of layers is less intensewhen rotation is present because thehorizontal mean motions are deviated by theCoriolis forces, which tends to reducethe distortion of the wave field, and thusthe wave-induced fluxes of buoyancy andmomentum. This effect appears even when therotation rate is weak.
Boundary-Layer Meteorology 01/2003; 108(2):201-219. DOI:10.1023/A:1024198104962
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Article: Explosive events on the Sun
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ABSTRACT: I describe two of the most dynamic and highly energetic phenomena in the Solar System--the explosive flares that can occur when plasma is confined by magnetic fields and the large-scale ejections of material known as 'coronal mass ejections'. These explosive events are poorly understood and yet occur in a variety of contexts in the Universe, ranging from planetary magnetospheres to active galactic nuclei. Understanding why flares and coronal mass ejections occur is a major goal across a wide range of space physics and astrophysics. Although explosive events from the Sun have dramatic effects on Earth, flares in other stars, for example, can be vastly more energetic and have an even more profound effect on their environment. We are now in the unprecedented position of having access to a number of space observatories dedicated to the Sun: the Yohkoh spacecraft, the Solar and Heliospheric Observatory, the Transition Region and Coronal Explorer and the Ramaty High Energy Solar Spectroscopic Imager. These cover a wide wavelength range from white light to gamma rays with both spectroscopy and imaging, and allow huge progress to be made in understanding the processes involved in such large explosions. The high-resolution data show dramatic and complex explosions of material on all spatial scales on the Sun. They have revealed that the Sun is constantly changing everywhere on its surface--something that was never imagined before. One of the mechanisms that has been proposed to account for the large energy release is magnetic reconnection. Recent observations from space increasingly support this view. This article will discuss those observations that support this model and also those that suggest different processes. The current space missions have given us an excellent insight into the actual explosive processes in the Sun. However, they have provided us with only a tantalizing glimpse of what causes the elusive trigger. Future missions such as Solar-B (the follow-on to Yohkoh), the Solar Terrestrial Relations Observatory, the Solar Dynamics Observatory and the Solar Orbiter mission will allow us to probe the trigger in a way that was not dreamt of a decade ago, by providing stereo views, measurements from Sun-grazing orbit, and much higher spatial, temporal and spectral resolution. It is an exciting time for solar physics and everything that we learn about the Sun will improve our ability to understand other magnetic phenomena in the Universe.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2003; 360(1801):2757-71. DOI:10.1098/rsta.2002.1057
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Article: Observations of magnetic helicity
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ABSTRACT: The first observational signature of magnetic helicity in the solar atmosphere (sunspot whirls) was discovered 77 years ago. Since then, the existence of a cycle-invariant hemispheric helicity pattern has been firmly established through current helicity and morphological studies. During the last years, attempts were made to estimate/ measure magnetic helicity from solar and interplanetary observations. Magnetic helicity (unlike current helicity) is one of the few global quantities that is conserved even in resistive magnetohydrodynamics (MHD) on a timescale less than the global diffusion timescale, thus magnetic helicity studies make it possible to trace helicity as it emerges from the sub-photospheric layers to the corona and then is ejected via coronal mass ejections (CMEs) into the interplanetary space reaching the Earth in a magnetic cloud. We give an overview of observational studies on the relative importance of different sources of magnetic helicity, i.e. whether photospheric plasma motions (photospheric differential rotation and localized shearing motions) or the twist of the emerging flux tubes created under the photosphere (presumably by the radial shear in the differential rotation in the tachocline) is the dominant helicity source. We examine the sources of errors present in these early results and try to judge how realistic they are.
Advances in Space Research 01/2003; 32(10-32):1855-1866. DOI:10.1016/S0273-1177(03)90619-3
• Article: Active region helicity evolution and related coronal mass ejection activity
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ABSTRACT: The computation of magnetic helicity has become increasingly important in the studies of solar activity. Observations of helical structures in the solar atmosphere, and their subsequent ejection into the interplanetary medium, have resulted in considerable interest to find the link between the amount of helicity in the coronal magnetic field and the origin of coronal mass ejections (CMEs), which provide a natural method to remove helicity from the corona. Recent works have endeavored to find the source of helicity to explain the observed CME activity in specific cases. The main candidates being differential rotation, shear motions or a transfer of helicity from below the photosphere into the corona. We study an active region for several rotations during 1997 and 1998 to investigate the relative importance of these mechanisms. We find that photospheric differential rotation cannot provide the required magnetic helicity to the corona and the ejected CMEs. Localized photospheric motions can provide a larger helicity flux, though still not sufficient.
Advances in Space Research 01/2003; 32(10-32):1959-1964. DOI:10.1016/S0273-1177(03)90633-8
• Article: Anatomy of a flare and coronal mass ejection
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ABSTRACT: We present observations of an X2.3 flare which was observed on the April 10, 2001. This was subject of a letter by Foley et al. (2001). In this short paper we present further observations obtained with the TRACE spacecraft of the evolution of the EUV flare ribbons and of the flux rope's intensity variation. These are discussed in the content of the Standard Flare model.
COSPAR Colloquia Series 12/2002; 13(13):253-256. DOI:10.1016/S0964-2749(02)80062-2
• Article: Non-thermal velocities in solar flares
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ABSTRACT: The high resolution spectroscopic information from the Bragg Crystal Spectrometer on board Yohkoh has provided us with new and exciting information about flares. In particular, there has been much work on understanding the excess line broadening above the thermal width (known as non-thermal line broadening). We have been able to look for the first time spectroscopically at the preflare stages in X-rays. The timings of the non-thermal velocity relative to the hard X-ray emission has been investigated. Non-thermal velocities have been observed to increase ten minutes before the main flare begins. Progress has been made to locate the region of dominant non-thermal velocity. This is difficult due to the lack of spatial resolution. A discussion will be made on what can be expected from the EUV Imaging Spectrometer on-board Solar-B, which combines both high spatial and spectral resolution simultaneously.
COSPAR Colloquia Series 12/2002; 13(13):261-269. DOI:10.1016/S0964-2749(02)80064-6
• Article: Helicity loading and dissipation: The helicity budget of ar 7978 from the cradle to the grave
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ABSTRACT: Through a multi-wavelength and multi-instrument analysis we evaluate the magnetic helicity budget of an isolated active region (NOAA 7978) from its emergence throughout its decay. Using Yohkoh/ SXT images and linear force-free magnetic extrapolations carried out on SOHO/MDI magnetograms, we compute the relative magnetic helicity in the corona. Based on the observed magnetic field distribution we calculate the magnetic helicity injected by differential rotation. Then, using SOHO/LASCO & EIT and SXT images we identify all the 26 coronal mass ejections (CMEs) which originated from this active region during its lifetime and we estimate the amount of helicity which was shed via CMEs. Comparing these three values we find that the differential rotation can neither provide enough helicity to account for the diagnosed coronal helicity values, nor for the helicity carried away by CMEs. We suggest that the main source of the magnetic helicity must be the inherent twist of the magnetic flux tube forming the active region.
• Article: The Coronal Emission of Photospheric Magnetic Fragments
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ABSTRACT: This paper examines the relationship between magnetic dipoles in the photosphere and X-ray bright points (XBPs) in the corona, using an XBP special campaign dataset obtained by the Yohkoh SXT and the NSO/Kitt Peak magnetograph. We find that for the cases where a simple dipole exists in the photosphere, the condition that they are separated by a distance less than the interaction distance defined by Longcope1998 is favorable for an XBP to be observed. For the cases where the magnetic topology is more complicated due to the addition of an extra fragment, we find that the geometry of the magnetic fragments is a major factor that determines if an XBP is observed. XBPs are more likely to be formed above magnetic fragments arranged in such a way that photospheric motions giving rise to reconnection between any two fragments will also give rise to reconnection with the remaining fragment.
Solar Physics 11/2002; 211(1):125-134. DOI:10.1023/A:1022452308560
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Article: A Drought Climatology for Europe
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ABSTRACT: We present a high spatial resolution, multi-temporal climatology for the incidence of 20th century European drought. The climatology provides, for a given location or region, the time series of drought strength, the number, the mean duration, and the maximum duration of droughts of a given intensity, and the trend in drought incidence. The drought climatology is based on monthly standardized precipitation indices (SPIs) calculated on a 0.5° grid over the European region 35–70 °N and 35 °E–10 °W at time scales of 3, 6, 9, 12, 18, and 24 months for the period 1901–99. The standardized property facilitates the quantitative comparison of drought incidence at different locations and over different time scales. The standardization procedure (probability transformation) has been tested rigorously assuming normal, log–normal, and gamma statistics for precipitation. Near equivalence is demonstrated between the Palmer drought severity index (PDSI) and SPIs on time scales of 9 to 12 months. The mean number and duration by grid cell of extreme European drought events (SPI ≤ −2) on a time scale of 12 months is 6 ± 2 months and 27 ± 8 months respectively. The mean maximum drought duration is 48 ± 17 months. Trends in SPI and PDSI values indicate that the proportion of Europe experiencing extreme and/or moderate drought conditions has changed insignificantly during the 20th century. We hope the climatology will provide a useful resource for assessing both the regional vulnerability to drought and the seasonal predictability of the phenomenon. Copyright © 2002 Royal Meteorological Society.
International Journal of Climatology 11/2002; 22(13):1571 - 1592. DOI:10.1002/joc.846
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Article: X–ray observations of accreting white–dwarf systems
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ABSTRACT: Accretion in white-dwarf binary systems can occur through discs, accretion columns or a combination of these, depending on the magnetic field of the white dwarf. Recent high-quality X-ray observations with the XMM-Newton and Chandra observatories have significantly advanced our understanding of the physics of the accretion process, and place severe tests on our existing models. There have been some surprises, such as the strong dependence of atmospheric heating on accretion rate. However, we believe that we are now confident that we understand in general the physical processes in the accretion region, although some complicating factors, such as absorption, remain. We also discuss new developments in ultra-short-period white-dwarf binary systems.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 10/2002; 360(1798):1951-66. DOI:10.1098/rsta.2002.1046
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Article: Floods in a changing climate: A review
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ABSTRACT: This paper begins with an analysis of flooding as a natural disaster for which the solutions to the environmental, social and economic problems are essentially those of identifying and overcoming hazards and vulnerability, reducing risk and damaging consequences. Long-term solutions to flooding problems, especially in a changing climate, should be sought in the wider context of developing more sustainable social organization, economics and technology. Then, developments are described of how scientific understanding, supported by practical modelling, is leading to predictions of how human-induced changes to climatic and geological conditions are likely to influence flooding over at least the next 300 years, through their influences on evaporation, precipitation, run-off, wind storm and sea-level rise. Some of the outstanding scientific questions raised by these problems are highlighted, such as the statistical and deterministic prediction of extreme events, the understanding and modelling of mechanisms that operate on varying length- and time-scales, and the complex interactions between biological, ecological and physical problems. Some options for reducing the impact of flooding by new technology include both improved prediction and monitoring with computer models, and remote sensing, flexible and focused warning systems, and permanent and temporary flood-reduction systems.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 08/2002; 360(1796):1531-43. DOI:10.1098/rsta.2002.1016
• Article: The Magnetic Helicity Injected by Shearing Motions
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ABSTRACT: Photospheric shearing motions are one of the possible ways to inject magnetic helicity into the corona. We explore their efficiency as a function of their particular properties and those of the magnetic field configuration. Based on the work of M.A.Berger, we separate the helicity injection into two terms: twist and writhe. For shearing motions concentrated between the centers of two magnetic polarities the helicity injected by twist and writhe add up, while for spatially more extended shearing motions, such as differential rotation, twist and writhe helicity have opposite signs and partially cancel. This implies that the amount of injected helicity can change in sign with time even if the shear velocity is time independent. We confirm the amount of helicity injected by differential rotation in a bipole in the two particular cases studied by DeVore (2000), and further explore the parameter space on which this injection depends. For a given latitude, tilt and magnetic flux, the generation of helicity is slightly more efficient in young active regions than in decayed ones (up to a factor2). The helicity injection is mostly affected by the tilt of the AR with respect to the solar equator. The total helicity injected by shearing motions, with both spatial and temporal coherence, is at most equivalent to that of a twisted flux tube having the same magnetic flux and a number of turns of 0.3. In the solar case, where the motions have not such global coherence, the injection of helicity is expected to be much smaller, while for differential rotation this maximum value reduces to 0.2 turns. We conclude that shearing motions are a relatively inefficient way to bring magnetic helicity into the corona (compared to the helicity carried by a significantly twisted flux tube).
Solar Physics 04/2002; 207(1):87-110. DOI:10.1023/A:1015531804337
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Article: Determining the mass of the accreting white dwarf in magnetic cataclysmic variables using RXTE data
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ABSTRACT: We have extracted spectra of 20 magnetic cataclysmic variables (mCVs) from the RXTE archive and best fitted them using the X-ray continuum method of Cropper et al. to determine the mass of the accreting white dwarf in each system. We find evidence that the mass distribution of these mCVs is significantly different from that of non-magnetic isolated white dwarfs, with the white dwarfs in mCVs being biased towards higher masses. It is unclear if this effect is a result of selection or whether this reflects a real difference in the parent populations.
Monthly Notices of the Royal Astronomical Society 04/2002; 314(2):403 - 408. DOI:10.1046/j.1365-8711.2000.03239.x
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Article: Indirect imaging of the accretion stream in eclipsing polars – III. HU Aquarii low state
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ABSTRACT: We apply our technique for indirect imaging of the accretion stream to the polar HU Aqr, using eclipse profiles observed when the system was in a low-accretion state. The eclipse profile is different from that in the high state, and more variable from cycle to cycle. We find that the stream maps are brightest near the white dwarf and there is no significant brightening in the threading region. In the low state the stream threads on to the magnetic field closer to the L1 point than in the high state, with a footpoint of the accreting field line at high latitude. We then produce maps of the accretion region from polarimetry using Stokes imaging. These show that the majority of the accretion occurs near the equator. The difference between the maps may be explained if most of the stream material is not emitting significantly in the low state. If so, neither the stream eclipse mapping nor Doppler tomography techniques will trace the bulk of the accretion flow between the two stars.
Monthly Notices of the Royal Astronomical Society 04/2002; 326(2):788 - 798. DOI:10.1046/j.1365-8711.2001.04691.x
• Article: The magnetic topology of a sigmoid
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ABSTRACT: Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.
Journal of Atmospheric and Solar-Terrestrial Physics 03/2002; 64(5-6-64):497-504. DOI:10.1016/S1364-6826(02)00005-6
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Article: Submillimeter Evidence for the Coeval Growth of Massive Black Holes and Galaxy Bulges
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ABSTRACT: The correlation, found in nearby galaxies, between black hole mass and stellar bulge mass implies that the formation of these two components must be related. Here we report submillimeter photometry of eight x-ray–absorbed active galactic nuclei that have luminosities and redshifts characteristic of the sources that produce the bulk of the accretion luminosity in the universe. The four sources with the highest redshifts are detected at 850 micrometers, with flux densities between 5.9 and 10.1 millijanskies, and hence are ultraluminous infrared galaxies. If the emission is from dust heated by starbursts, then the majority of stars in spheroids were formed at the same time as their central black holes built up most of their mass by accretion. This would account for the observed demography of massive black holes in the local universe. The skewed rate of submillimeter detection with redshift is consistent with a high redshift epoch of star formation in radio-quiet active galactic nuclei, similar to that seen in radio galaxies.
Science 02/2002; 294(5551):2516-8. DOI:10.1126/science.1065880
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Article: Trajectories of Microwave Prominence Eruptions
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ABSTRACT: On the basis of 17 GHz microwave images from the Nobeyama Radioheliograph near solar maximum (1999-2000), we examined trajectories of 50 prominence eruptions in order to address how prominence motions affect or reflect the surrounding coronal structures. We marked the heliocentric latitude of the top of the moving prominences on the white-light synoptic maps, which were constructed at 2.5-4.5 solar radii from the LASCO C2 coronagraph on the SOHO spacecraft. We found that i) 92% of the prominence eruptions were associated with coronal mass ejections (CMEs) and that the remaining 8% show weak mass motions confined to nearby streamers, and ii) coronal mass motions involving prominence eruptions and CMEs are not random but are organized by bundles of streamers. Our findings support the idea of multiple plasma sheets emanating from active regions, arcades, trans-equatorial interconnecting loops, and polar crown filaments, through which coronal mass is transported toward interplanetary space. This study suggests that microwave observations can provide useful information on the activity at the base of such "coronal mass corridors".
Astronomy and Astrophysics 02/2002; 382(2). DOI:10.1051/0004-6361:20011658
• Article: Trajectories of microwave prominence eruptions
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ABSTRACT: The classical coronal mass ejection (CME) scenario describes CMEs as associated with helmet streamer configurations, in which a quiescent prominence over a magnetic neutral line becomes unstable starts to erupt, resulting in a CME e.g. Low 1994). Recently, Hori (2000) studied two microwave prominences that were initially not embedded in a streamer but moved toward the streamer latitude and then erupted or disappeared. These prominences seem to be different from those in the classical CME scenario. Further, such prominence behavior suggests that streamers play the role of “mass corridor” through which coronal mass flows out toward the interplanetary space (Crooker et al. 1993). In order to address how prominence motions affect or reflect the surrounding coronal structures, we examined trajectories of 50 prominence eruptions that were observed with the Nobeyama Radioheliograph (NoRH) at 17 GHz near solar maximum (1999–2000). By comparing the prominence trajectories with the white-light synoptic maps from the Large Angle and Spectrometric Coronagraph (LASCO) C2 instrument on the SOHO spacecraft, we confirmed that coronal mass motions involving eruptive prominences and CMEs are not random but are organized as a bundle of pre-existing streamers. Large scale evolution of coronal features suggests that streamers are a signature of multiple plasma sheets emanating from active regions, arcades, trans-equatorial interconnecting loops, and polar crown filaments, as consistent with Crooker et al. (1993). Our study demonstrates that microwave observations can provide useful information on the activity at the base of such “coronal mass corridors”†
Astronomy and Astrophysics 02/2002; 382(2-13):305-306. DOI:10.1016/S0964-2749(02)80078-6
• Article: Multi-wavelength observations of an X-class flare without a coronal mass ejection
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ABSTRACT: Developments in our knowledge of coronal mass ejections (CMEs) have shown that many of these transients occur in association with solar flares. On the occasions when there is a common occurrence of the eruption and the flare, it is most likely that the flare is of high intensity and/or long-duration (Burkepile, Hundhausen, and Webb, 1994; Munro etal., 1979; Webb and Hundhausen, 1987). A model for the relationship between the long-duration event and eruption has been developed (Carmichael, 1964; Sturrock, 1966; Hirayama, 1974; Kopp and Pneuman, 1976), but not so for the high-intensity flares and eruptions. This work investigates the magnetic topology changes that occur for a X1.2 GOES classification flare which has no associated CME. It is found that the flare is likely to result from the interaction between two pre-existing loops low in the corona, producing a confined flare. Slightly higher in the corona, a loop is observed which exhibits an outward motion as a result of the reconfiguration during reconnection. The objective of this work is to gain insight on the magnetic topology of the event which is critical in order to determine whether a high-intensity flare is likely to be related to a CME or not.
Solar Physics 01/2002; 205(2):325-339. DOI:10.1023/A:1014211528863
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Article: Position sensitive anodes for MCP read-out using induced charge measurement
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ABSTRACT: We investigate the method of an indirect detection of a MCP charge avalanche projected onto a resistive layer (G. Battistoni, et al., Nucl. Instr. and Meth., 202 (1982) 459). If the sheet resistance is favourable one can detect the charge cloud by the capacitive coupling to an anode structure a few millimetres behind the layer. The anode structure can be, for example, a wedge-and-strip electrode pattern (M. Unverzagt, Diplomarbeit, Universit. at Frankfurt 1992, private communication) as it is used for directly collecting the electron avalanche from a MCP. Detection of the induced charge is beneficial in several respects. Firstly, image distortions produced by secondary electron mediated charge redistribution are eliminated. Secondly, the noise component due to quantized charge collection, commonly referred to as partition noise, is not present. In addition, the dielectric substrate can function both as an element of the vacuum enclosure and HV insulator, making the electrical connections easily accessible and the pattern operable at ground potential, independently of detector operating voltages. This technique can be used to simplify the electronic design requirements where varying high voltages are required at the detector input face such as plasma analysers, etc. It also has application in the manufacture of intensifier tubes (J. Barnstedt, M. Grewing, Nucl. Instr. and Meth., these proceedings) where the inclusion of a readout pattern inside the intensifier body with associated electrical feed-throughs can prove problematic. We will present data on the performance of such detection geometries using several types of charge division anode, and discuss the advantages compared with the ''traditional'' charge collecting method. r 2002 Elsevier Science B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2002; 477:256-261. DOI:10.1016/S0168-9002(01)01843-5
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Article: Fast imaging readout and electronics - A novel high-speed imaging system for micro-channel plates
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ABSTRACT: The band-width of charge division readout anodes used with micro-channel plates (MCP) is usually limited by the speed of the acquisition electronics. We present a novel charge division anode that does not require analogue to digital conversion. The Fast Imaging Readout and Electronics is a new concept in high-speed imaging using an MCP detector. The imaging system described comprises an MCP intensifier coupled to a charge division image readout using high-speed, multichannel electronics. It has a projected spatial resolution of up to 128×128 pixels, though the image format is inherently flexible, and the potential for rates up to 100 million events per second with nanosecond timing resolution.The readout pattern has a planar electrode structure and the collected charge from each event is shared amongst all electrodes, grouped in pairs. The unique design of the readout obviates the need for charge measurement, usually the dominant process determining the event-processing deadtime. Instead, high-speed signal comparators, each of which act on the signals from an electrode pair, are used to define a binary code from which the position co-ordinate is directly mapped.We describe a proof of the concept of prototype anode and associated electronics using a novel application of very high-speed digital circuitry. We present preliminary results showing signal waveforms measured using a one-dimensional 16-pixel anode pattern.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2002; 477(1-3):273-278. DOI:10.1016/S0168-9002(01)01850-2
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Article: Imaging achievements with the Vernier readout
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ABSTRACT: We describe the Vernier anode, a high resolution and charge division image readout for microchannel plate detectors. It comprises a planar structure of insulated electrodes deposited on an insulating substrate. The charge cloud from an event is divided amongst all nine electrodes and the charge ratio uniquely determines the two-dimensional position coordinate of the charge centroid. We discuss the design of the anode pattern and describe the advantages offered by this readout. The cyclic variation of the electrode structure allows the image resolution to exceed the charge measurement resolution and enables the entire active area of the readout to be utilized. In addition, fixed pattern noise is greatly reduced. We present results demonstrating the position resolution and image linearity. A position resolution of 10μm FWHM is demonstrated and the overall imaging performance is shown to be limited by the microchannel plate pore spacing. We present measurements of the image distortions and describe techniques used to minimize them.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2002; 477(1):250-255. DOI:10.1016/S0168-9002(01)01840-X
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Article: Seasonal prediction of European spring precipitation from ENSO and local sea surface temperatures
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ABSTRACT: The extent to which European seasonal precipitation is predictable is a topic of scientific and societal importance. Although the potential for seasonal prediction is much less over Europe than in the tropics, it is not negligible. Previous studies suggest that European seasonal precipitation skill may peak in the spring (March–April–May) period, this being the season when El Niño–Southern Oscillation (ENSO) teleconnections to the North Atlantic and European sector are at their strongest. Examination of the correlation significance and temporal stability of contemporaneous and lagged ENSO links to European and North African precipitation over 98 years confirms this to be the case. The strongest ENSO links are found across the central European region (45°N–55°N,35°E–5°W). These links are symmetric with the sign of ENSO. Using a linear statistical model employing temporally stable lagged ENSO and lagged local North Atlantic sea surface temperatures as predictors, we compute the forecast skill and significance of central European spring precipitation over 30 independent years. For early March forecasts our model skill is 14–18% better than climatology, which is significant at the 95% level. Copyright © 2002 Royal Meteorological Society
International Journal of Climatology 01/2002; 22(1):1 - 14. DOI:10.1002/joc.723
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Article: Cyclotron emission from stratified accretion shocks in magnetic cataclysmic variables
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ABSTRACT: We present calculations of cyclotron emission from stratified accretion shocks for different accretion rates (and hence shock heights), magnetic fields and white-dwarf masses. Our calculations aim to provide models to explain photopolarimetric and spectroscopic observations of magnetic cataclysmic variables. We have shown that the continuum and harmonic structures of cyclotron emission from stratified shocks are different from those of models assuming constant temperature and density, and that the spectral properties of the emission depend strongly on the shock structures.
12/2001; 261:165.
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Article: What is the mass ratio of the binary system RW Tri?
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ABSTRACT: The eclipsing disc accreting CV RW Tri is one of the brightest known novalike variables, but the masses of the component stars are still poorly constrained. Results from a study of RW Tri in the near infrared and infrared constrain the secondary star velocity to 200 km/s (accurate to 10%). Combining this with optical and ultraviolet data for the primary star gives a range of mass ratios (M2 / M1) of 0.9--1.5.
12/2001; 261:67.
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Article: Genetically modified fireflies'' -- 3D eclipse mapping for polars
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ABSTRACT: We present a new method for modelling the accretion stream emission in AM Herculis systems based on eclipse profiles. In our approach we model the accretion stream as a collection of light emitting particles (i.e. fireflies''). The shape and location of the stream is found by means of genetic optimization.
12/2001; 261:153.
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Article: Discovery of a new NLQSO: RX J1334.2+3759
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ABSTRACT: A new narrow-line QSO, RX J1334.2+3759, has been identified at a redshift of 0.3858. The QSO has intrinsic soft X-ray luminosity ~ 2.8 x 1044 erg s-1. Soft X-ray emission from the QSO is highly variable. Rapid variability (change in intensity by a factor of ~ 4 within ~ 400 s) has also been found. The soft X-ray spectra of the QSO is well described by a power-law of photon index, ?x ~ 3.8. The optical spectrum of RX J1334.2+3759 is typical of narrow-line Seyfert 1 galaxies. The NLR of RX J1334.2+3759 appears to be significantly different from those of normal Seyfert 1 galaxies.
Bulletin of the Astronomical Society of India 08/2001; 29:417-420.
• Article: Improving the effectiveness of international collaboration in space science
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ABSTRACT: From the start of the 20th century, a strong tradition of collaboration has developed in the physical sciences. World War II and the following period changed this situation with a quickening of the pace of application. Thus, while basic research continues to benefit from collaboration among scientists worldwide, the increasingly complex background in which science evolves, through higher implementation costs and more difficult approval processes, renders collaboration among nations ever more pressing. Space science, with its comparatively high access cost but large fundamental importance, substantial public appeal and outstanding ability to motivate young people, shares this need. This article focuses on a recent ESSC-ESF study undertaken to improve the effectiveness of such cooperative efforts. Related findings and recommendations are presented along with a proposed operational structure for their implementation.
Space Policy 08/2001; 17(3-17):179-186. DOI:10.1016/S0265-9646(01)00028-5
• Article: Inland thinning of Pine Island Glacier, West Antarctica. Science
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ABSTRACT: The Pine Island Glacier (PIG) transports 69 cubic kilometers of ice each year from approximately 10% of the West Antarctic Ice Sheet (WAIS). It is possible that a retreat of the PIG may accelerate ice discharge from the WAIS interior. Satellite altimetry and interferometry show that the grounded PIG thinned by up to 1.6 meters per year between 1992 and 1999, affecting 150 kilometers of the inland glacier. The thinning cannot be explained by short-term variability in accumulation and must result from glacier dynamics.
Science 03/2001; 291(5505):862-4. DOI:10.1126/science.291.5505.862
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Article: Location of the source of soft X-ray non-thermal line broadenings in a solar flare
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ABSTRACT: We determine the location of the source of the non-thermal Soft X-ray line broadenings in an M1.7 two-ribbon solar flare using multi-wavelength observations. Using a combination of the Yohkoh Soft X-ray Telescope (SXT), Hard X-ray Telescope (HXT), Bragg Crystal Spectrometer (BCS) and the Transition Region and Coronal Explorer (TRACE), we find the source of the non-thermal broadenings, at their peak value, to be located in and above the SXR flare loops, not at the flare loop footpoints. After eliminating the footpoints as a potential source we discuss the likelihood that the source of non-thermally broadened emission lines is either evaporating chromospheric plasma or plasma above the Soft X-ray flare loop that is associated with the flare energy release.
Astronomy and Astrophysics 11/2000; 364(2):859-872.
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Article: Changes in the solar magnetic field preceding a Coronal Mass Ejection
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ABSTRACT: The combined observing power of the Yohkoh, SOHO and TRACE spacecrafts, along with the continuing ground-based observations has proved invaluable for the detection of changes in the magnetic morphology preceding coronal mass ejections (CMEs). A wide range of activity from small scale dimmings to large scale eruptions covering half the solar disk have been observed. The relationship between flares and CMEs has also become clearer. Rather than one event causing the other it would seem that it is a global change in the magnetic field which causes both. Recently there has been a lot of interest in the sigmoid (S-shaped) structures seen in soft X-rays. The likelihood of a CME occuring appears to increase if there is a sigmoidal structure observed. This has formed the basis of more extensive studies into predicting the time and location of a CME from the changes in behaviour of features on the solar disk. Key words: Solar;Eruption;Magnetic field 1 Introduction We are now in the unpreceden...
Journal of Atmospheric and Solar-Terrestrial Physics 09/2000; 64(5). DOI:10.1016/S1364-6826(02)00006-8
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Article: The Energy Supply to X-ray Bright Points
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ABSTRACT: X-ray bright points (XBPs) are usually assumed to be isolated structures in the solar atmosphere that are powered exclusively by magnetic reconnection. We analyse a large XBP that does not satisfy this assumption. The XBP is observed to be connected to an active region approximately 280000 km away by a magnetic loop. We find that the soft X-ray intensity and thermal energy of the XBP are very sensitive to the existence of the magnetic loop. Both the intensity and energy decrease significantly at the times when the loop disappears, indicating that the loop is a medium for energy transfer from the active region to the XBP. We deduce that the mechanism for the energy transfer is most likely to be Alfvn or fast-mode magnetoacoustic waves created by photospheric motions in the active region. These waves can dissipate energy at the density gradient between the XBP and the loop via phase mixing or resonant absorption.
Solar Physics 08/2000; 196(1):137-156. DOI:10.1023/A:1005218932680
• Article: High resolution optical spectrograph (HROS): A summary of progress
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ABSTRACT: A design for HROS has now been developed which is able to meet the science requirements as specified by the community. This design calls for a Cassegrain-mounted Âchelle spectrograph with a resolving power of R=50,000. Efficiency is maximised both by the location at the Cassegrain focus, and by the adoption of prism (as opposed to grating) cross-dispersion. All of the transmitting optical elements will be made of fused silica, to maximise to efficiency in the UV. The design relies on the concept of an 'immersed' Âchelle, in which the ruled surface of the Âchelle grating is 'immersed' in a fused silica prism. This paper outlines the working of the HROS instrument and summarises the progress that has been made on the project, on a modular level. The reader is also refered to 3 additional paper in these proceedings. Paper 4008-14 discusses the flexural performance of this instrument, paper 4008-15 discusses the optical design of the instrument and paper 4008-98 describes the Active Flexure Compensation (AFC) that is to be used on the HROS instrument.
Proceedings of SPIE - The International Society for Optical Engineering 08/2000; DOI:10.1117/12.395515
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Article: Emerging flux as a driver for homologous flares
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ABSTRACT: We present multi-wavelength observations of 2 M-class solar flares observed by SoHO and Yohkoh, which appear to be homologous. By examination of the flare loop morphology and footpoints we propose a schematic reconnection scenario of a two loop interaction in a quadrupolar magnetic configuration, for both flares (Machado ?; Mandrini ?). After the first flare, the combination of chance emergence of new flux at an opportune location and a subsequent flare, of the type described by Heyvaerts et al. (?), form a new quadrupolar configuration in which the second flare occurred. Therefore though the two M-class flares are homologous by definition, they appear to conform to a scenario in which the preflare conditions are reformed after the first flare by emerging flux, rather than models which involve the continual shearing of a single magnetic structure.
Astronomy and Astrophysics 07/2000; 360:1163-1169.
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Chapter: Magnetic Refrigeration

Wiley Encyclopedia of Electrical and Electronics Engineering, 12/1999; , ISBN: 9780471346081
• Article: Nonthermal Electron Energy Deposition in the Chromosphere and the Accompanying Soft x-ray Flare Emission
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ABSTRACT: We analyse four solar flares which have energetic hard X-ray emissions, but unusually low soft X-ray flux and GOES class (C1.0–C5.5). These are compared with two other flares that have soft and hard X-ray emission consistent with a generally observed correlation that shows increasing hard X-ray accompanied by increasing soft X-ray flux. We find that in the four small flares only a small percentage of the nonthermal electron beam energy is deposited in a location where the heating rate of the electron beam exceeds the radiative cooling rate of the ambient plasma. Most of the beam energy is subsequently radiated away into the cool chromosphere and so cannot power chromospheric evaporation thus reducing the soft X-ray emission. We also demonstrate that in the four small flares the nonthermal electron beam energy is insufficient to power the soft X-ray emitting plasma. We deduce that an additional energy source is required, and this could be provided by a DC-electric field (where quasi-static electric field channels in the coronal loops accelerate electrons, and those electrons with velocity below a critical velocity will heat the ambient plasma via Joule heating) in preference to a loop-top thermal source (where heat flux deposited in the corona is conducted along magnetic field lines to the chromosphere, heating the coronal plasma and giving rise to further chromospheric evaporation).
Solar Physics 03/1999; 185(2):323-350. DOI:10.1023/A:1005176932415
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Article: Flare loop geometry
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ABSTRACT: In selected flares that occurred in AR7260, we have studied the geometry of the brightest soft X-ray loop by tracing it on an image. Even under the assumption that the loop is contained in a plane, it is clear that a single image does not permit us to determine the full geometry. It only provides possible loop shapes as a function of the inclination angle of the loop plane with respect to the vertical. However, all the loops that reproduce the observed appearance give the same direction of increasing height as projected on to the image plane. This direction is compared with two relevant observations. Based on 2-D reconnection models that involve a cusp configuration, it is expected that the soft X-ray loop top source moves upward with time and that a higher temperature region exists above the loop top. Several flares are found to contradict these predictions, presumably implying the inadequacy of the models. Lastly we discuss a possibility of constraining the inclination angle (and hence the loop shape) with spatially-unresolved soft X-ray line spectra which are Doppler-shifted due to plasma upflows.
Solar Physics 01/1999; 189(1):181-198. DOI:10.1023/A:1005290426454
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Article: Properties of transition region and coronal loops
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ABSTRACT: The magnetic eld plays a vital role in governing the size, shape and dynamics of loops in the solar atmo-sphere and as such it seems reasonable to ask whether diierences in the spatial distribution of these struc-tures are indicative of diierences in the form of the heating. Using observations from the Coronal Di-agnostic Spectrometer (CDS) on SoHO and the Soft X-ray Telescope (SXT) on Yohkoh we investigate how the spatial distribution of EUV and X-ray emission in active regions varies with temperature. We employ Fourier methods to obtain the two dimensional power spectrum of the intensity distribution for a number of lines at diierent temperatures. Integrating this over polar angle we nd isotropic power-law behaviour at all temperatures in a number of topologically diier-ent active regions, with a tendency for atter spectra at lower temperatures. The existence of power-law spectra indicates that there is no preferred length scale within the regions. Flatter spectra at lower temperatures are consistent with emission predomi-nantly from smaller scale structures such as low-lying loops or footpoints.
01/1999;
• Article: Comet Hyakutake Gas Arcs: First Observational Evidence of Standing Shock Waves in a Cometary Coma
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ABSTRACT: We present the discovery and observations throughout the period March 25–April 5, 1996, ofC2andCNarc structures centered on the anti-sunward axis of comet Hyakutake (C/1996 B2). We interpret them as the signatures of the interaction between two supersonic rarefied gas jets: H2O vapor emanating from the comet nucleus on one hand and gas flowing from a secondary source centered on the anti-sunward direction on the other. An upgraded version of the Gasdynamic code described in Crifoet al.(1995, 1997a) is used to model the interaction. To accommodate low gas densities, Navier–Stokes equations are used instead of Euler equations, and photochemical effects are introduced to compute the water-group secondary molecules' distributions. A double-shock H2O structure, characteristic of the interaction between two opposite supersonic flows, is found. Associated with it is an arc-shaped distribution of OH. In view of the dominance of the collisions between and with H2O molecules, the distribution of any other primary molecules with lifetimes comparable to that of H2O (e.g., HCN) will be identical to that of H2O. The spatial distribution of their daughter products (e.g., CN) will be similar to that of OH, if their lifetimes are comparable to that of OH. We show that, given the limitations of the observations and of the modeling method, it is not possible to derive a unique solution in terms of secondary source properties and of near-nucleus night-side production. We show in particular that the solution proposed by Harriset al.(1997) for the companion OH arcs is only one of the possible solutions, and that, in conflict with what those authors find, it does result in a standing shock structure between the nucleus and the source. The successful simulations of the arcs presented here constitute the first observational evidence for the formation of shock waves in neutral cometary atmospheres, originally predicted by Kitamura (1990) and subsequently advocated as an essential process in the formation of the circumnuclear coma by Crifoet al.(1995, 1997a, b).
Icarus 12/1998; 136(2-136):232-267. DOI:10.1006/icar.1998.6010
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Article: On the role of beam driven return current instabilities in white-light flares
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ABSTRACT: It has been shown previously that the low ionization levels in the deep chromosphere of solar flares can cause the return current driven by a thick target electron beam to be unstable to ion aoustic wave generation, contrary to conventional wisdom. We investigate, using Yohkoh data, the possibility that anomalous heating as a result of this instability is capable of producing sufficient heating, in the right places at the right times, to account for the enhanced continuum emission actually observed in white-light flares. The Soft X-ray Telescope (SXT) onboard \it Yohkoh incorporates an aspect camera which, prior to its failure in 1992, provided white-light images at 431 nm with a bandpass of 3 nm. A number of white-light flares were observed during its operational lifetime for which there was coincident hard X-ray data from the Hard X-ray Telescope (HXT), providing suitable candidates for study. Four such events are discussed, and the model found to be viable.
Astronomy and Astrophysics 11/1998; 340(1):277-286.
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Article: The GOSSIP on the MCV V347 Pavonis
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ABSTRACT: Modelling of the polarized cyclotron emission from magnetic cataclysmic variables (MCVs) has been a powerful technique for determining the structure of the accretion zones on the white dwarf. Until now, this has been achieved by constructing emission regions (for example arcs and spots) put in by hand, in order to recover the polarized emission. These models were all inferred indirectly from arguments based on polarization and X-ray light curves. Potter, Hakala & Cropper (1998) presented a technique (Stokes imaging) which objectively and analytically models the polarized emission to recover the structure of the cyclotron emission region(s) in MCVs. We demonstrate this technique with the aid of a test case, then we apply the technique to polarimetric observations of the AM Her system V347 Pav. As the system parameters of V347 Pav (for example its inclination) have not been well determined, we describe an extension to the Stokes imaging technique which also searches the system parameter space (GOSSIP).
10/1998; 157:253.
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Conference Paper: Geophysical signatures from precise altimetric height measurements in the Arctic Ocean
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ABSTRACT: The use of altimeter data in the polar regions has previously been limited by the presence of permanent and seasonal ice cover. Changes in the radar echo shape received by the altimeter over sea ice, as compared with the open ocean, cause problems in the on-board estimates of surface height, making the data unusable. The majority of noise on the signal can be reduced by retracking the full waveform data set (WAP). Careful quality control is applied to ensure that only those return echoes from which accurate height measurements can be obtained are retained. Consideration of possible backscattering mechanisms from ice and water, and comparisons with imagery, suggest that the specular waveforms typically found in altimeter data over sea ice originate from regions of open water or new thin ice exposed within the altimeter footprint. However, diffuse waveforms similar to those found in ice free seas have been observed in areas of consolidated ice, and may be used to measure ice freeboard. Until recently, even retracked heights contained substantial residual errors due to the interaction of the on-board tracking system with the complex return echoes over sea ice. Software simulation of the tracking system has led to the development of new ground processing algorithms, which further reduce the short wavelength (~26 km) noise, from 30-50 cm to around 7 cm. This provides, for the first time in ice covered seas, the capability for accurate mean sea surface generation, measurement of tidal and oceanographic signals and determination of sea ice freeboard. The authors present the results of comparisons of sea surface height variability from ERS-2 radar altimetry in the Arctic with the output from a high resolution Arctic Ocean circulation model
Geoscience and Remote Sensing Symposium Proceedings, 1998. IGARSS '98. 1998 IEEE International; 08/1998
• Article: The hard X‐ray spectrum of soft X‐ray‐selected AGN
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ABSTRACT: I use ASCA data to investigate the 2–10 keV X-ray emission of active galactic nuclei (AGN) taken from the ROSAT International X-ray Optical Survey (RIXOS). I find that the integrated spectrum of these faint, soft X-ray-selected AGN in the 2–10 keV band is harder (best-fitting = 0.8 ± 0.1) than the slope measured with ROSAT between 0.1 and 2 keV, but softer than the 2–10 keV X-ray background, and consistent with the average 2–10 keV spectrum of bright, nearby Seyfert galaxies. With this spectral slope and using measurements of the AGN contribution to the 1–2 keV X-ray background, I estimate that the AGN percentage contribution to the 2–10 keV background is 0.60 +0.19−0.14 times the AGN percentage contribution to the 1–2 keV background. Hence AGN produce between 12 and 32 per cent of the 2–10 keV X-ray background. This is only the contribution from the types of AGN which are found in soft X-ray surveys; a population of absorbed AGN could represent an additional component of the 2–10 keV X-ray background.
Monthly Notices of the Royal Astronomical Society 07/1998; 298(2):537 - 542. DOI:10.1046/j.1365-8711.1998.01629.x
• Article: The Natural Fluctuations of Firn Densification and Their Effect on the Geodetic Determination of Ice Sheet Mass Balance
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ABSTRACT: A one-dimensional, numerical model of time-evolving firn densification was used to simulate the response of the density profile through an ice sheet to changes in the temperature, density and accumulation rate at the surface. The equilibrium response of the model was compared with ice-core density profiles from Byrd, Antarctica and Site 2, Greenland, and the model predicted the density to within 10% of both cores. The response of the model to step-wise changes and random fluctuations in the surface boundary conditions was investigated. The standard deviation of elevation changes as a function of observation interval was computed. These changes were found to be small in comparison with the magnitude of present uncertainties in the mass balances of the Antarctic and Greenland Ice Sheets. It was concluded that, in the dry snow zones, natural variability in the densification will not prevent the geodetic determination of ice sheet mass balance from improving upon current estimates. Uncertainty in the constitutive equation for snow and firn is the dominant source of error in the calculations.
Climatic Change 01/1998; 40(3):605-624. DOI:10.1023/A:1005320713306
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