M. O. Fillingim1, L. M. Peticolas1, R. J. Lillis1, D. A. Brain1, J. S. Halekas1, D. Lummerzheim2, and S. W. Bougher3
1Space Sciences Laboratory, University of California, Berkeley
2Geophysical Institute, University of Alaska, Fairbanks
3Department of Atmospheric, Oceanic and Space Sciences,
University of Michigan, Ann Arbor
Presented at the AGU Chapman Conference on the Solar Wind Interaction with Mars, San Diego, CA, 22 - 25 January 2008
Mars lacks a global magnetic field, but it does have intense and localized crustal fields yielding a complex magnetic topology. Where the crustal field has a nearly radial orientation, there is a tendency for the field lines to connect with the IMF, forming cusps that provide a conduit for ionospheric plasma to escape and for solar wind plasma to precipitate into the atmosphere. On the nightside one expects ionization due to solar wind electron precipitation in regions of open (radial) field lines at cusps and an absence of ionization in closed (horizontal) field regions. Recently observed accelerated electrons, which appear to be associated with cusps surrounding the strongest custal fields, will also create very localized regions of enhanced ionization.
Using an electron transport model, we calculate the electron density of the nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements including an interval when accelerated electrons were observed. Precipitating accelerated electrons increase the maximum ionospheric number density by a factor of 3 over that produced by typical tail electrons. These regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the ionospheric electron density on the night side of Mars can reach ~4000 per cc over 200 km or ~20 per cc per km. Even sharper gradients occur near plasma voids; the electron density can go from effectively 0 per cc to ~5000 per cc over a few km.
Such strong gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field and will generate horizontal currents and electric fields which will in turn lead to localized Joule heating. Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a funcion of local time, can generate horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. Closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes.
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Matt Fillingim
matt at ssl dot berkeley dot edu
University of California, Berkeley
Space Sciences Laboratory # 7450
7 Gauss Way
Berkeley, CA 94720-7450