HR: 13:55h
AN: P52B-02    [PDF]
TI: Simulation of Energetic Neutral Atom Images at Venus
AU: * Gunell, H
EM: herbert.gunell@physics.org
AF: Swedish Institute of Space Physics, P. O. Box 812, Kiruna, SE-981 28 Sweden
AU: Holmstr\"om, M
AF: Swedish Institute of Space Physics, P. O. Box 812, Kiruna, SE-981 28 Sweden
AU: Biernat, H K
AF: Space Research Institute, Austrian Academy of Sciences Schmiedlstrasse 6, Graz, A-8042 Austria
AU: Erkaev, N V
AF: Institute of Computational Modelling, Russian Academy of Sciences, 660036 Krasnoyarsk, 36 Russian Federation
AU: Lammer, H
AF: Space Research Institute, Austrian Academy of Sciences Schmiedlstrasse 6, Graz, A-8042 Austria
AU: Lichtenegger, H
AF: Space Research Institute, Austrian Academy of Sciences Schmiedlstrasse 6, Graz, A-8042 Austria
AU: Penz, T
AF: Space Research Institute, Austrian Academy of Sciences Schmiedlstrasse 6, Graz, A-8042 Austria
AB: We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus. The plasma flow around Venus is modelled by a semi-analytical MHD simulation that includes mass-loading ({\em Biernat et al.,} J. Geophys. Res., vol. 104, 12617--12626, 1999; {\em Biernat,et al.,} Adv. Space Res., {\bf 28}, 2001). These results are compared with the results that are obtained when the Spreiter-Stahara flow model ({\em Spreiter and Stahara,} Adv Space Res., {\bf 14}, 5--19, 1994) is used. The ENA images are calculated by combining the proton bulk flow and temperature results of the MHD model with a model of the neutral atmosphere using the energy dependent cross sections for the charge exchange collisions. The ENA production rate is integrated along lines of sight to a virtual instrument, thus simulating what could be measured by a space-craft-carried ENA instrument. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The main contribution to the ENA flux observed in the ENA images stems from a region of space between the ionopause and the bow shock on the dayside of the planet. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars ({\em Holmstr\"om et al.,} J. Geophys. Res., {\bf 107}, 1277, doi: 10.1029/2001JA000325, 2002). The reason for the lower ENA flux at Venus is thought to be the smaller extent of Venus' exosphere. The steeper falloff of the neutral gas density with altitude in the exosphere of Venus is caused by Venus' mass, which is 7.5 times greater than the mass of Mars. The dependence of the ENA flux on the altitude of the ionopause is studied numerically, and it is found that the ENA flux decreases as the ionopause altitude is increased.
UR: http://www.irf.se/\~{}herbert/venus/
DE: 5421 Interactions with particles and fields
DE: 6295 Venus
DE: 7837 Neutral particles
DE: 7843 Numerical simulation studies
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting