G. Guymer

Aberystwyth University, Aberystwyth, Wales, United Kingdom

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Publications (14)0 Total impact

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    ABSTRACT: The current work is an investigation of the characteristics of the bow shock, magnetic pile-up boundary, and ion composition boundary at Venus. The aim is to provide better resolution for the boundaries detected from ion data. The data from the Venus Express Ion Mass Analyser (IMA), magnetometer (MAG), and Electrostatic Analyser (ELS) are used. These were gathered over a period of 14 months Due to fluctuations in the solar wind at Venus, which cause boundary motions which are rapid compared to the spacecraft velocity, we need a procedure to identify the position of the spacecraft relative to the boundary. Using electron measurements of dayside boundary crossings a transition parameter is defined. This is then used to reorder the sparse ion data.
    European Planetary Science Congress 2012; 09/2012
  • 09/2010;
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    ABSTRACT: Previous work by Whittaker et. al. combined remote sensing observations from Interplanetary Scintillation observations (IPS) with in-situ ion data collected by the Ion Mass Analyser (IMA) onboard Venus Express to study solar wind interactions with Venus. In-situ magnetic field observations from the Venus Express magnetometer will now be applied as a complimentary tool in identifying different cases of these interactions. Three case studies are undertaken; a co-rotating interaction region (CIR) arriving on ~30th April 2007, a coronal mass ejection (CME), arriving on ~25th and 26th May 2007 and mass loading of the solar wind coinciding with Venus moving into the tail disconnection event of comet 2P/Encke on ~22nd April 2007.We are interested both in making in situ measurements of heliospheric structures previously remote sensed by IPS, and also in understanding the response of the Venus magnetosphere to their passage. Ref: I.C. Whittaker, G.D. Dorrian, A. Breen, M. Grande. In-Situ observations of a co-rotating interaction region at Venus identified by IPS and STEREO ( submitted to Solar Physics topical issue: Remote sensing of the heliosphere 2009)
    04/2010; 12:5430.
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    ABSTRACT: The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) instrument on Venus Express is used to determine bow shock position at Venus using ion data alone, using data recorded during a solar minimum from the Ion Mass Analyzer (IMA) which is part of the ASPERA-4 package. Previous models constructed from solar minimum data using Venus Express, Pioneer Venus Orbiter (PVO) and Venera 9 and 10 are also compared to the current fit. An important feature of this new fit is a statistical accuracy introduced in the form of a probability weighting function for the data points, based on the time spent in particular locations. The bow shock curve is then compared to two-dimensional ion maps. These verify the accuracy of this and previous solar minimum fit curves based on PVO and Venus Express magnetic data. Comparing all bow shock models to the 2D ion maps shows that a combination of models produces the best fit. Since all the fitted curves show differences in position they are investigated relative to the solar conditions pertaining at the times when the individual data sets were measured. The sub solar point and terminator distance were thus found to vary linearly with sunspot number and hence with solar activity. This relationship, which was already known to exist between solar maximum and solar minimum, is now shown to exist between different solar minima and even within the same minimum. This indicates a need for the mechanisms for bow shock maintenance and variance to be more closely modeled.
    Journal of Geophysical Research, v.115 (2010). 01/2010;
  • I. Whittaker, M. Grande, G. Guymer
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    ABSTRACT: The ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms) instrument onboard Venus Express is used to investigate Bow Shock and Ionopause positions. The movement of these layers during Fast-Slow stream Solar Wind interaction is looked at as well as during Solar events. The methods of determining Solar events from the data are shown as well as a brief discussion on how the data has been specifically cleaned up.
    04/2009;
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    ABSTRACT: Using the ASPERA-4 instrument onboard Venus Express the change in the atmospheric boundary layers are investigated. Using the instrument when it is in the apoapsis period of the polar orbit allows pure Solar Wind measurements to be taken. Composition ratios are determined thus identifying periods of fast and slow solar wind. This is then applied to ion maps of the Venusian atmosphere to determine the response after noise calibration methods. The Species reactions to the differing Solar Wind are also used to investigate the inner boundary movements.
    AGU Fall Meeting Abstracts. 12/2008;
  • G Guymer, M Grande, I Whittaker
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    ABSTRACT: Abstract Venus has a negligible intrinsic magnetic moment with an upper limit a factor 10-5 of earth's [1]. This entails that the ionosphere is vulnerable to scavenging by the solar wind. However, magnetic fields may be induced in the ionosphere by interaction with the interplanetary magnetic field frozen-in to the solar wind. The presence of small scale magnetic structures in the dayside ionosphere of the planet Venus has been long established and were first observed in Pioneer Venus Orbiter (PVO) data in 1979 [2] during the run up to solar maximum. These ionospheric `flux ropes' were observed in over 70% of passes in which the orbit of PVO intersected the dayside ionosphere [3]. Magnetic flux ropes are identified as brief, discrete disturbances from any background magnetic field, lasting a few seconds with a magnitude of up to many 10's of nano-Teslas in strength [3, 4]. Flux ropes have a strong central, axial field, that is wrapped with field lines of weakening strength and increased helical angle with distance from the central field lines [4]. Due to this particular structure, flux ropes present a specific signature in the three variance projections (also known as a hodogram) when minimum variance analysis is applied to the magnetic data set [2]. With Venus Express now in operational orbit around the planet, flux ropes are being observed in the data retrieved by the magnetometers (MAG [5]) onboard. The magnetic data used in this analysis is the 1Hz data set provided by H. Wei (of UCLA). Variance projections have been produced for several structures in 2006, revealing them to be flux ropes (see figure 1). Using the Ion Mass Analyser (IMA; part of the ASPERA-4 package [6]) and MAG, the ion composition within the ropes and the effect of such magnetic structures upon ionospheric erosion is being studied. Where flux ropes have been evident in the magnetic data, ion spectra have been produced in an attempt to deduce any compositional differences between a flux rope free ionosphere and one in which flux ropes have been detected. The spectra have been constructed to show protons, alpha particles, singly ionised helium and singly ionised oxygen, displaying a mix of solar wind and atmospheric particles. ELS (electron spectrometer [6]) data sets have been inspected to ascertain if the ropes occur deep in the ionosphere or at its fringes. The case that is primarily being studied occurs on 2006-07-11, DOY 192. The spacecraft travels along a near-terminator orbit from midnight to midday. H+ and He++ spectra show double energy populations in the region of the flux rope. These double populations show strongly in the count rate. A double population can also be weakly observed in the O+ spectra. The energies of these populations are approximately 1KeV and 20eV, indicating a mix of solar wind and planetary ions. The electron data does not appear to show any signature of the ionosphere, but is suspected to be in a position at the fringe of the night-side ionopause (see figure 2). In order to determine the pressure balance across the ropes the average plasma and magnetic pressures both across the ropes and during a three minute period either side of the ropes has been calculated. Flux ropes with a force-free nature have increased magnetic pressure but do not have a decreased thermal pressure. Indicating that particle density within the flux rope will not be reduced [7]. For ions to be eroded from the atmosphere by flux ropes, the ropes would need to be force-free, i.e., non-density depleted.
    European Planetary Science Congress 2008; 09/2008
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    ABSTRACT: Introduction We examine ion populations at Venus. Previous models use magnetic crossing points to derive the bow shock position. The current work uses data from the ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms) [1] instrument to measure ion populations and derive a bow shock position at Venus. Instrumentation The ASPERA-4 instrument flies onboard Venus Express (VEX) and is comprised of five different detectors (Barabash et al 2006 [1]). A neutral particle detector and analyser, an electron spectrometer and the Ion Mass Analyser (IMA) (ref). This paper uses the IMA instrument for all its data and an explanation of the specifications is required. The instrument is a top hat electrostatic analyser; it runs through voltages to scan look angles and also acceptance energies. In one look direction it scans through 96 different energy values before changing to the next. A full scan of all look directions and energies takes 192 seconds. Data Collection All data is weighed dependant upon its probability of the spacecraft measuring at a particular point and when fitted produces a value of 1.24 RV, somewhat closer distance for the sub solar point than previous authors - see figure 1. We separate the data according to slow or fast solar wind and not the similarities and differences in the results derived. The inbound and outbound bow shock crossings were taken by inspection of 106 orbits between November 2006 and February 2007. Any orbits where the crossing point was not clear or with data missing were ignored. The occupational probability is found from orbital mechanics. By setting up a grid and deriving the amount of time it takes to cross each square the probability as a whole can then be determined. Ion distribution plots Two dimensional maps of the ions are produced and the bow shock model overplotted to verify its accuracy, as shown in figure 3. The test of the bow shock is to place it upon real data and examine the fit. To do this ion distribution plots are created and have the bow shock model placed upon them. The maps are shown in fig 8 and comprise 6 months of data from VEX in 2007. The count rates of all spectra of every orbit were stored and binned into a grid system. Each box in the grid being averaged from all values placed into it. The results were then smoothed and the maps created for individual species and plot types. Fig 3 shows maps for hydrogen ions in the x-y plane and in cylindrical coordinates signed with y. This plot is the same as a standard cylindrical plot but the r value is positive if y is positive and negative if y is negative. Effect of Coronal mass Ejections The HI imager on STEREO is able to image Coronal mass Ejections (CME) in the inner Solar System. In a recent paper, Roullard et al 2008 [2] have considered a CME observed to impact Venus, and used in situ measurements to examine the response of the magnetosphere. The plots in figure4 show the measured ion response to this and an earlier CME. We will examine the ion signatures in detail. Acknowledgements We acknowledge the contributions of the entire Aspera 4 team: S. Barabash, R. Lundin, H. Andersson, K. Brinkfeld, A. Grigoriev, H. Gunell, M. Holmström, M. Yamauchi, K. Asamura, P. Bochsler, P. Wurz, R. Cerulli-Irelli, A. Mura, A. Milillo, M. Maggi, S. Orsini, A. J. Coates, D. R. Linder, D. O. Kataria, C. C. Curtis, K. C. Hsieh, B. R. Sandel, R. A. Frahm, J. R. Sharber, J. D. Winningham, M. Grande, E. Kallio, H. Koskinen, P. Riihelä, W. Schmidt, T. Säles, J. U. Kozyra,N. Krupp, J. Woch,.S. Livi, J. G., Luhmann, S. McKenna-Lawlor, E. C. Roelof, D. J. Williams, J.-A. Sauvaud, A. Fedorov, and J.-J. Thocaven. References [1] S. Barabash, R. Lundin, H. Andersson, K. Brinkfeld, A. Grigoriev, H. Gunell, M. Holmström, M. Yamauchi, K. Asamura, P. Bochsler, P. Wurz, R. Cerulli-Irelli, A. Mura, A. Milillo, M. Maggi, S. Orsini, A. J. Coates, D. R. Linder, D. O. Kataria, C. C. Curtis, K. C. Hsieh, B. R. Sandel, R. A. Frahm, J. R. Sharber, J. D. Winningham, M. Grande, E. Kallio, H. Koskinen, P. Riihelä, W. Schmidt, T. Säles, J. U. Kozyra,N. Krupp, J. Woch,.S. Livi, J. G., Luhmann, S. McKenna-Lawlor, E. C. Roelof, D. J. Williams, J.-A. Sauvaud, A. Fedorov, and J.-J. Thocaven The Analyser of Space Plasmas and Energetic Atoms (ASPERA-3) for the Mars Express Mission. SPACE SCIENCE REVIEWS, 126 (1-4): 113-164 OCT 2006 [2] P. Rouillard et al 2008 submitted Science.
    European Planetary Science Congress 2008; 09/2008
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    ABSTRACT: Flux ropes are created in the ionosphere of Venus during the interaction of Venus ionosphere with the solar wind and also at Titan from its interaction with the Saturnian magnetospheric plasma flow. They were first detected by Pioneer Venus Orbiter during solar maximum and now by Venus Express during solar minimum. The formation of a flux rope is thought to occur as follows: a flux tube in the magnetic barrier at the ionopause first twists (possibly due to the shear-velocity gradient along the flux tube) and forms a helical structure near the boundary between the magnetic barrier and the ionosphere; later it sinks into the field-free ionosphere and becomes further twisted. The well-formed ropes and the ropes during formation are studied and compared in this paper. With the knowledge of flux rope structure in the Venus ionosphere, we study the twisted fields in the lower ionosphere of Titan from Cassini observations. These twisted field line structures resemble the flux ropes near the Venus ionopause which appear to be in the act of formation. The comparison of the flux rope structures in the two ionospheres can improve our knowledge of flux rope formation and what determines their observed properties.
    01/2008;
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    ABSTRACT: We will investigate the position and angular distribution of oxygen ions in the Venusian magnetosphere and ionosphere. These exhibit a complex pattern, and penetration depths from solar wind oxygen often greatly exceed the gyroradius.We will present an analysis of the populations of heavy ions ordered with particular respect to the boundary layers. The data is being taken from the ASPERA-4 IMA instrument onboard Venus Express and being analysed using the CL software.
    European Planetary Science Congress 2007; 08/2007
  • G. Guymer, M. Grande, I. Whittaker
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    ABSTRACT: Ion data received by the IMA (Ion Mass Analyser) onboard Venus Express has been examined for features concurrent to magnetic flux ropes detected in magnetometers data within the Venus ionosphere. The interactions of these events are being examined as possible contributors to atmospheric erosion.
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    ABSTRACT: An investigation into the global scale dynamics of individual ion species at Venus is carried out using IMA mass spectrometer onboard Venus-Express ASPERA. We examine the location of different populations relative to boundaries. We note that at solar minimum the structure of solar wind streams is important for the behaviour. We will also consider the evolution of theses streams as sampled at both Venus and Earth. Maps of average cases will be presented, and CME and CIR events compared to this baseline.