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S. W. Bougher (SPRL/U. of Michigan), S. Engel (LPL/U. of Arizona), D. P. HInson (Stanford University), J. R. Murphy (NMSU)
The Mars Global Surveyor (MGS) Radio Science (RS) experiment employs an ultrastable oscillator aboard the spacecraft. The signal from the oscillator to Earth is refracted by the Martian ionosphere, allowing retrieval of electron density profiles versus radius . The present analysis is carried out on four sets of occultation measurements : (1) three obtained near northern summer solstice (Ls = 74-116, near aphelion) at high northern latitudes (64.7-73.3N), and (2) one set approaching equinox conditions (Ls ~ 135-146) at high southern latitudes (64.7-69.1S). Electron density profiles (95 to 200 km) are examined over a narrow range of solar zenith angles (SZA) (76.5-86.9 degrees) for local true solar times of (1) 3-4 hours and (2) 12.1 hours.
In three of these datasets, sampling is well distributed over longitude. Specific attention is given to the height and magnitude of the primary F1-ionospheric peak observed in each of these profiles. The height of this photochemically driven peak is controlled by the neutral density structure. Variations are observed as a function of SZA (weak) and longitude (strong), with a mean height of ~134-135 km for the aphelion profiles. The magnitude of this same photochemical peak (7-9 .0 E+4 cm-3 ) reflects solar moderate conditions.
Seasonal inflation/contraction of the Mars atmosphere, dust storm expansion/abatement, and planetary wave processes are all thought to impact the integrated atmospheric column and the height of the dayside ionospheric peak. The Mars Thermospheric General Circulation Model (MTGCM) is exercised for Mars conditions appropriate to these RS observational periods in order to understand the underlying neutral atmosphere conditions giving rise to these ionospheric features (mean and variations). Solar moderate fluxes (F10.7 = 130), aphelion conditions (Ls = 90), and low dust opacities are specificed. The MTGCM simulations also incorporate wave features resulting from upward propagating migrating plus non-migrating tides as well as in-situ tidal forcing. Longitude variations in the height of the simulated ionospheric peak are contrasted with corresponding RS data. Tidal modes responsible for these longitude specific wave features are also identified. Clearly, the height of the dayside ionospheric peak is a sensitive indicator of the changing state of the Mars lower atmosphere. This research is funded by the NASA MGS Data Analysis Program.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.