Publications (102)32.36 Total impact
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Article: Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times.
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ABSTRACT: An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earth's high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.Nature Communications 02/2013; 4:1466. · 7.40 Impact Factor -
Article: Cluster Observations of the Magnetospheric Low-Latitude Boundary Layer and Cusp during Extreme Solar Wind and Interplanetary Magnetic Field Conditions: II. 7 November 2004 ICME and Statistical Survey
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ABSTRACT: We present a study of the plasma properties inside and dynamics of the low-latitude boundary layer (LLBL)/cusp during the ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predominantly strongly northward, up to 50 nT, with some short-duration rotations. The observed LLBL/cusp is very thick (∼6 – 7° invariant latitude (ILAT)) and migrates equatorward with rates of 0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming from multiple reconnection sites. During a period of extremely northward IMF, signatures of pulsed dual reconnection inside the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been suggested previously. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on 28 – 29 October 2003 and 7 – 10 November 2004 is performed. During extreme conditions the LLBL/cusp position is offset by −7° ILAT from the location under normal conditions, which might be explained by the influence of the high solar wind dynamic pressure. The LLBL/cusp moves equatorward with increasing southward and northward IMF. However, the LLBL/cusp position under strong southward IMF is more poleward than expected from previous studies, which could indicate some saturation in the dayside reconnection process or enhancement of the nightside reconnection rate. The LLBL/cusp position under strong northward IMF is extremely low and does not agree with the location predicted in previous studies. For the events with solar wind dynamic pressure >10nPa, the LLBL/cusp position does not depend on the solar wind dynamic pressure. This might indicate some saturation in the mechanism of how the LLBL/cusp location depends on the solar wind dynamic pressure.Solar Physics 04/2012; 244(1):233-261. · 2.78 Impact Factor -
Article: Multi-Spacecraft Study of the 21 January 2005 ICME
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ABSTRACT: We examine the near-Earth Interplanetary Coronal Mass Ejection (ICME) apparently related to the intense Solar Energetic Particle (SEP) event of 20 January 2005. Our purpose is to contribute to the understanding of the macroscopic structure, evolution and dynamics of the solar corona and heliosphere. Using Cluster, ACE and Wind data in the solar wind, and Geotail data in the magnetosheath, we perform a multi-spacecraft analysis of the ICME-driven shock, post-shock magnetic discontinuities and ejecta. Traversals by the well-separated near-Earth spacecraft provide a coherent picture of the ICME geometry. Following the shock, the ICME sequence starts with a hot pileup, i.e., a sheath, followed by a fast ejecta characterised by a non-compressive density enhancement (NCDE), which is caused essentially by an enrichment in helium. The plasma and magnetic observations of the ejecta are consistent with the outskirts of a structure in strong expansion, consisting of nested magnetic loops still connected to the Sun. Within the leading edge of the ejecta, we establish the presence of a tilted current sheet substructure. An analysis of the observations suggests that the tilted current sheet is draped within the overlying cloud canopy, ahead of a magnetic cloud-like structure. The flux rope interpretation of this structure near L1, confirmed by observations of the corresponding magnetic cloud, provided by Ulysses at 5.3 AU and away from the Sun – Earth line, indicates that the bulk of the cloud is in the northwest sector as seen from the Earth, with its axis nearly perpendicular to the ecliptic. This is consistent with the primary direction of travel of the fast halo CME observed at the Sun. Moreover, the NCDE and helium enrichment are consistent with the position near the streamer belt of the flaring active region NOAA 10720 associated with the CME. However, differences between interplanetary and solar observations indicate a large rotation of the erupting filament and overlying arcade, which can be attributed to the flux rope being subject to the helical kink instability.Solar Physics 04/2012; 244(1):139-165. · 2.78 Impact Factor -
Article: The Apparent Layered Structure of the Heliospheric Current Sheet: Multi-Spacecraft Observations
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ABSTRACT: Multiple current sheet crossings are ubiquitous features of the solar wind associated with high-beta plasma sheets, notably during the passage of the heliospheric current sheet (HCS). As the HCS is being convected past near-Earth, we attempt to resolve spatial scales and temporal variations of the apparent layered structure of the HCS, including adjacent large scale field reversals. We use several spacecraft for good spatial and cross-scale coverage, spanning 550RE across and 900RE along the Sun – Earth line: STEREO, ACE and Cluster. The multi-spacecraft magnetic and plasma observations within the leading edge of the sector boundary are consistent with i)abroad multi-layered structure; ii)occasional non-planar structures and Alfvénic fluctuations; iii)various stages of transient outflowing loops formed by interchange reconnection. By comparison of the observations at each spacecraft, we obtain a synthesis of the evolution between the patterns of loops, and hence of the transient outflow evolution along the sector boundary. In particular, we present circumstantial evidence that a heat flux dropout, traditionally signalling disconnection, can arise from interchange reconnection and scattering. Moreover, the inter-spacecraft comparison eliminates ambiguities between interpretations of electron counterstreaming. Overall, the sector boundary layer remains, locally, a steady structure as it is convected in the solar wind across a radial heliospheric distance of 560 – 580RE. However, non-planar structures on the Cluster spatial scale, as well as the variations in angular changes and transition durations on the broader scale, indicate that we are not following the evolution of single loops but more likely a bunch of loops with variable properties.Solar Physics 04/2012; 259(1):389-416. · 2.78 Impact Factor -
Article: Extended magnetic reconnection across the dayside magnetopause.
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ABSTRACT: The extent of where magnetic reconnection (MR), the dominant process responsible for energy and plasma transport into the magnetosphere, operates across Earth's dayside magnetopause has previously been only indirectly shown by observations. We report the first direct evidence of X-line structure resulting from the operation of MR at each of two widely separated locations along the tilted, subsolar line of maximum current on Earth's magnetopause, confirming the operation of MR at two or more sites across the extended region where MR is expected to occur. The evidence results from in-situ observations of the associated ion and electron plasma distributions, present within each magnetic X-line structure, taken by two spacecraft passing through the active MR regions simultaneously.Physical Review Letters 07/2011; 107(2):025004. · 7.37 Impact Factor -
Article: AXIOM: Advanced X-ray Imaging Of the Magnetosphere
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ABSTRACT: Planetary plasma and magnetic field environments can be studied by in situ measurements or by remote sensing. While the former provide precise information about plasma behaviour, instabilities and dynamics on local scales, the latter offers the global view necessary to understand the overall interaction of the magnetospheric plasma with the solar wind. Here we propose a novel and more elegant approach employing remote X-ray imaging techniques, which are now possible thanks to the relatively recent discovery of solar wind charge exchange X-ray emissions in the vicinity of the Earth's magnetosphere. We describe how an appropriately designed and located X-ray telescope, supported by simultaneous in situ measurements of the solar wind, can be used to image the dayside magnetosphere, magnetosheath and bow shock, with a temporal and spatial resolution sufficient to address several key outstanding questions concerning how the solar wind interacts with the Earth's magnetosphere on a global level. Our studies have led us to propose 'AXIOM: Advanced X-ray Imaging Of the Magnetosphere', a concept mission using a Vega launcher with a LISA Pathfinder-type Propulsion Module to place the spacecraft in a Lissajous orbit around the Earth - Moon L1 point. The model payload consists of an X-ray Wide Field Imager and an in situ plasma and magnetic field measurement package. This package comprises sensors designed to measure the bulk properties of the solar wind and to characterise its minor ion populations which cause charge exchange emission, and a magnetometer designed to measure the strength and direction of the solar wind magnetic field. We show simulations that demonstrate how the proposed X-ray telescope design is capable of imaging the predicted emission from the dayside magnetosphere with the sensitivity and cadence required to achieve the science goals of the mission.07/2011; -
Article: Magnetopause reconnection across wide local time
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ABSTRACT: During April to July 2007 a combination of 10 spacecraft provided simultaneous monitoring of the dayside magnetopause across a wide range of local times. The ar-ray of four Cluster spacecraft, separated at large distances (10 000 km), were traversing the dawn-side magnetopause at high and low latitudes; the five THEMIS spacecraft were of-ten in a 4 + 1 grouped configuration, traversing the low lat-itude, dusk-side magnetosphere, and the Double star, TC-1 spacecraft was in an equatorial orbit between the local times of the THEMIS and Cluster orbits. We show here a number of near simultaneous conjunctions of all 10 spacecraft at the magnetopause. One conjunction identifies an extended mag-netic reconnection X-line, tilted in the low latitude, sub-solar region, which exists together with active anti-parallel recon-nection sites extending to locations on the dawn-side flank. Oppositely moving FTE's are observed on all spacecraft, consistent with the initially strong IMF B y conditions and the comparative locations of the spacecraft both dusk-ward and dawn-ward of noon. Comparison with other conjunc-tions of magnetopause crossings, which are also distributed Correspondence to: M. W. Dunlop (m.w.dunlop@rl.ac.uk) over wide local times, supports the result that reconnection activity may occur at many sites simultaneously across the sub-solar and flank magnetopause, but linked to the large scale (extended) configuration of the merging line; broadly depending on IMF orientation. The occurrence of MR there-fore inherently follows a "component" driven scenario irre-spective of the guide field conditions. Some conjunctions al-low the global magnetopause response to IMF changes to be observed and the distribution of spacecraft can directly con-firm its shape, motion and deformation at local noon, dawn and dusk-side, simultaneously.01/2011; 29:1683-1697. -
Article: Midnight sector observations of auroral omega bands
Journal of Geophysical Research - Space Physics. 01/2011; 116(A00I30). -
Article: Interplanetary magnetic field rotations followed from L1 to the ground
Annales Geophysicae 01/2011; 29(9):1549-1569. · 1.84 Impact Factor -
Article: Evidence of the origin of the Hall magnetic field for reconnection: Hall MHD reconstruction results from Cluster observations
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ABSTRACT: Using reconstruction results from Cluster observations in the magnetotail, we present observational evidence for the electron- current loops responsible for generating the out- of- plane Hall magnetic field during reconnection. This reconstruction is the first successful application to actual spacecraft data of a newly developed reconstruction technique (Sonnerup and Teh, 2009) based on Hall MHD equations in a steady state, two-dimensional (2-D) geometry. Maps from our reconstructions show that the regions of the Hall magnetic field are surrounded by electron-current loops, which is confirmed by direct observations from Cluster 2 of the electrons that flow in and out of the reconnection site within the Hall field region. From the observational perspective our results show that the Hall magnetic field is generated by the Hall electron-current loop as predicted by reconnection theory. Comparison of the reconstructed magnetic field maps from Cluster 1 and Cluster 3 shows that the current sheet is being temporarily distorted, indicating a wavy motion along the current sheet during the event, as suggested by Volwerk et al. (2003). Also, the reconstruction maps help us interpret the observations in more detail. The proper frame for the reconstruction is obtained by optimizing the correlation coefficient between predictions from the map on the basis of Cluster 1 and measurements from the other three spacecraft not otherwise used in the reconstruction. The resulting frame velocity is consistent with that obtained from the timing analysis method for single X line reconnection configuration. In addition, we use Grad-Shafranov reconstruction for force-free conditions together with higher-resolution magnetic field data to derive a 2-D magnetic field map of a secondary magnetic island structure seen by Cluster 3 within the Hall field region and first identified by Eastwood et al. (2007).Journal of Geophysical Research-Space Physics. 01/2011; 116. -
Article: 10 years of the Cluster mission
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ABSTRACT: Andrew Walsh and UK Cluster specialists mark 10 years of innovation and discovery by ESA's multi-spacecraft magnetosphere mission.Astronomy & Geophysics 09/2010; 51(5):5.33 - 5.36. · 0.61 Impact Factor -
Article: Comprehensive ground‐based and in situ observations of substorm expansion phase onset
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ABSTRACT: 1] In this paper, we present comprehensive ground‐based and space‐based in situ geosynchronous observations of a substorm expansion phase onset on 1 October 2005. The Double Star TC‐2 and GOES‐12 spacecraft were both located within the substorm current wedge during the substorm expansion phase onset, which occurred over the Canadian sector. We find that an onset of ULF waves in space was observed after onset on the ground by extending the AWESOME timing algorithm into space. Furthermore, a population of low‐energy field‐aligned electrons was detected by the TC‐2 PEACE instrument contemporaneous with the ULF waves in space. These electrons appear to be associated with an enhancement of field‐aligned Poynting flux into the ionosphere which is large enough to power visible auroral displays. The observations are most consistent with a near‐Earth initiation of substorm expansion phase onset, such as the Near‐Geosynchronous Onset (NGO) substorm scenario. A lack of data from further downtail, however, means other mechanisms cannot be ruled out.Journal of Geophysical Research 01/2010; 115. · 3.02 Impact Factor -
Article: Simultaneous FAST and Double Star TC1 observations of broadband electrons during a storm time substorm
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ABSTRACT: 1] Broadband electrons (BBEs) exhibit remarkable electron flux enhancements over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. Here, we report a BBE event observed by the Fast Auroral Snapshot (FAST) satellite at 1355–1359 UT, ∼61°–66° invariant latitudes, ∼0600 magnetic local time (MLT), and ∼3800 km altitude during a storm on 25 July 2004. The Double Star (DS) TC1 satellite was located near the magnetic equator at L = 5.7, close to the same local time as FAST. We investigate the acceleration process of BBEs from the inner magnetosphere to near the ionosphere by comparing electron data obtained by FAST and DS TC1. We also investigate both plasma and field variations in the inner magnetosphere associated with substorm onset using DS TC1 data to examine the relationship between the BBEs and the storm time substorm. Ground geomagnetic field data show a positive H‐bay at ∼1349 UT at ∼0600 MLT, indicating that a storm time substorm started just before the appearance of the BBEs. At ∼1350 UT, a tailward ion flow was observed by DS TC1. Then, DS TC1 observed a local dipolarization and a drastic ion density enhancement at ∼1351 UT, indicating that particle heating associated with the substorm was occurring in the inner magnetosphere. From ∼1352 UT, electron fluxes were isotropically enhanced at energies above ∼0.5 keV as observed by DS TC1. On the other hand, the pitch angle distribution of BBEs at the FAST altitude showed field‐aligned lower‐energy electrons below ∼0.5 keV and isotropic higher‐energy electrons above ∼0.5 keV. From these data, it was inferred that the BBEs might consist of two energy components due to the acceleration or heating of electrons at different altitudes in association with the storm time substorm.J. Geophys. Res. 01/2010; 115. -
Chapter: Mixed Azimuthal Scales of Flux Transfer Events
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ABSTRACT: Previous observations have allowed the scale size of flux transfer events (FTEs) to be determined both normal to the magnetopause and in the direction of motion of the FTE, but a key difference between some different models of FTE structure is their azimuthal scale size. Previous ground-based observations of the ionospheric signatures of FTEs indicated that magnetic reconnection can occur coherently over large extents of the magnetopause, but in situ determination of the azimuthal scale size of FTEs has not been possible until recent Cluster magnetopause crossing seasons when the separation of the spacecraft was ∼10, 000 km. In this paper, we present Cluster observations of flux transfer events from the 27th March 2007, along with observations of the conjugate ionospheric signatures. We highlight two magnetospheric FTEs which were consistent with long X-line FTE models, but note also several FTEs with considerably smaller azimuthal scale.12/2009: pages 389-398; -
Chapter: Cluster-PEACE In-flight Calibration Status
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ABSTRACT: We briefly summarise key aspects of our on-going in-flight calibration work for the Cluster Plasma Electron And Current Experiment (PEACE) instruments, and demonstrate the quality of moments which may be achieved, by comparisons with measurements from other Cluster instruments. As improved calibrations are generated, data in scientific units which have been produced for the Cluster Active Archive will be systematically updated. This article is not intended as a detailed description of our calibration studies, but rather as a snapshot of the calibration status at the time of writing, which will give the researcher using the CAA an indication of the levels of accuracy that can be achieved at this time.12/2009: pages 281-299; -
Chapter: Electron Density Estimation in the Magnetotail: a Multi-Instrument Approach
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ABSTRACT: Electron density is a key physical quantity to characterize any plasma medium. Its measurement is thus essential to understand the physical processes occurring in the environment of a magnetized planet, both macroscopic and microscopic. Since 2000, the four satellites of the European Space Agency (ESA) Cluster mission have been orbiting the Earth from 4 RE to 20 RE and probing the density with several types of instruments. In the magnetotail, this rare combination of experiments is particularly useful since the electron density and the temperature fluctuate over several decades. Two of these experiments, a relaxation sounder and a high-time resolution wide-band receiver, have rarely been flown together in the far tail. Such wave data can be used as a means to estimate the electron density via the identification of triggered resonances or the cutoffs of natural wave emissions, typically with an accuracy of a few percent. For the first time in the magnetotail ( ∼20 RE), the Z-mode is proposed as the theoretical interpretation of the cutoff observed on spectrograms of wave measurements when the plasma frequency is greater than the electron gyrofrequency. We present examples found in the main regions of the magnetotail, comparing simultaneous density estimation from active and passive wave measurements with a particle instrument and calibrated spacecraft-to-probe potential difference data. With these examples, we illustrate the benefit of a multi-instrument approach for the estimation of the electron density in the magnetotail and the care that should be taken when determining the electron density from wave data.12/2009: pages 261-279; -
Chapter: PEACE Data in the Cluster Active Archive
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ABSTRACT: We describe the data products of the Cluster Plasma Electron and Current Experiment (PEACE) instruments which have been produced for the Cluster Active Archive. This article is intended to introduce the PEACE instruments, the measured data and the data products provided through the CAA. We aim to help the researcher to choose the data products most suited to their needs and to avoid inadvertent misuse of the data.12/2009: pages 129-144; -
Article: Multipoint observations of plasma distributions around an X line
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ABSTRACT: Using electron and magnetic field data obtained from the Cluster satellites, we identify the spatial distribution of highly accelerated electron distributions up to 10 keV. They are generally isotropic and form flat‐top distributions in the phase space. These distributions are observed in the vicinity of the X line associated with the quadrupole‐like magnetic field and energetic ions, throughout the plasma sheet. In some cases, these distributions are quasi‐stable, continuously observed for a few minutes with a stable Bz polarity and low current density in the center of the plasma sheet.AIP Conference Proceedings. 06/2009; 1144(1):40-43. -
Article: Simultaneous observations of flux transfer events by THEMIS, Cluster, Double Star, and SuperDARN: Acceleration of FTEs
J. Geophys. Res. 01/2009; 114. -
Article: Multipoint observations of plasma distributions around an X line
Future Perspectives of Space Plasma and Particle Instrumentation and International Collaborations. 01/2009; 1144:40-43.
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2006–2012
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University College London
London, ENG, United Kingdom
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