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Communication Dans Un Congrès Année : 2018

Warm trapped fossils? Electron structure and dynamics on the nightside of Mars

Résumé

The nightside ionosphere of Mars is known to be highly variable, with densities varying substantially with ion species, solar zenith angle, solar wind conditions and geographic location. The MAVEN mission has been the first to systematically sample the density, temperature and composition of the nightside ionosphere. In this study we use measurements of suprathermal electron fluxes, thermal electron densities and temperatures, and neutral and ion densities between 120 and 300 km on the nightside of Mars, to study the complex system of ionization, heating, cooling, magnetic exclusion/mirroring and diffusion, that govern electron densities and temperatures. At non-collisional altitudes (>170 km, <10-7 Pa), thermal electrons have relatively uniform temperatures around 1400 K, regardless of magnetic field geometry/topology. For strongly collisional altitudes (<135 km, > 5 x 10-6 Pa), electron-neutral cooling dominates and electrons under all magnetic conditions are 800 K. But as intermediate atmospheric pressures (2 x 10-7 to 3 x 10-6 Pa), we find electron temperatures are significantly higher in plasma voids (where suprathermal electrons can't cause ionization) compared with open field lines (where they can and do). Thermal electrons in these voids remain at steady, low densities throughout the night side because recombination reactions become increasingly rare as more ions recombine. Since neutral density, and therefore cooling efficiency, is the same between open and closed field regions, we hypothesize that these could be warm "fossil" photoelectrons, left over from the dayside. In contrast, electrons on open field lines on the deep nightside are more likely to be created by electron impact ionization and therefore should have a different expected temperature. We will use 1-D and global ionospheric circulation models to calculate electron production, heating and cooling rates in an attempt to explain this phenomenon. Figure 1 shows electron impact ionization frequency, thermal electron density and temperature as a function of atmospheric pressure and magnetic elevation angle and field strength.
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insu-04409943 , version 1 (22-01-2024)

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R. J. Lillis, Jean-Yves Chaufray, D. J. Pawlowski, D. L. Mitchell, M. Benna, et al.. Warm trapped fossils? Electron structure and dynamics on the nightside of Mars. American Geophysical Union, Fall Meeting 2018, Dec 2018, washington, United States. pp.abstract #P43K-3907. ⟨insu-04409943⟩
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