The Near and Post-Terminator Ionosphere of Mars

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We model the near- and beyond-terminator ionosphere of Mars. Preliminary models of the ionospheres in the 60-90° solar zenith angle (SZA) range based on a SZA invariant background neutral model have shown that near 90° SZA, the upper peaks in the ion production rate profiles and in the ion densities are close to the photochemical equilibrium boundaries, and that there is a sharp increase in the upper ion peak height from 85° to 90°. We here model the very-near and beyond-terminator ionosphere using background neutral models appropriate to the conditions of the measurements from the MTGCM. Using the lower boundary of 80 km as the tangent point, the Martian thermosphere at 100 km is (theoretically) sunlit to 96°, and at 150 km to 101°. The limiting SZA's may actually be smaller than these values because of the attenuation of the ionizing solar fluxes by the lower thermosphere. We determine the local attenuated solar fluxes and ionization rates at post terminator SZA's and determine the actual limiting SZA's. We compare the model ion density profiles in the post-terminator region to the measured values from spacecraft, including MGS and MEX. If the local solar ionizing flux is insufficient to produce the measured ion densities, then we can conclude that ion transport from the dayside is implicated in producing ionization.


Presented at the 2006 Fall Meeting of the American Geophysical Union (AGU), San Francisco, CA.

Presentation Number SA33A-0251.

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