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S. W. Bougher, S. Engel (LPL, Univ. of Arizona), D. P. Hinson (Stanford Univ.)
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 and geopotential. The present analysis is carried out on a set of occultation measurements obtained near northern summer solstice (Ls = 74-77) at aphelion. Electron density profiles (95 to 200 km) obtained over Dec. 24-31, 1998 are presented, spanning 64.7-67.3 N latitude, local true solar time (LTST) of 3-4 hours, and 78-81 degrees solar zenith angle (SZA). The sampling during this time period is well distributed over longitude. Specific attention is given to the height and magnitude of the 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 134.4 km over the 32-profiles contained in this dataset. The magnitude of this same photochemical peak is controlled by the changing solar flux (incident at the top of the atmosphere). A mean ionospheric peak density of 8.1 x 104 cm-3 was obtained, with variations of as much as ±15%.
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 University of Arizona Mars Thermospheric General Circulation Model (MTGCM) is exercised for Mars conditions appropriate to this RS observational period in order to understand the underlying neutral atmosphere conditions giving rise to these ionospheric features. Solar moderate fluxes (F10.7 = 130), aphelion conditions (Ls = 90), and low dust opacities (tau = 0.3) are specified. Only mean conditions are reflected by initial MTGCM simulations (no longitude specific wave features are incorporated). MTGCM electron density profiles are extracted at 62.5 and 67.5 N latitude for LTST = 4.0, yielding a mean ionospheric peak height of ~135 km and peak electron density of ~1.0 x 105 cm-3. Longitude variations in the height of the F1-peak will be contrasted to MGS Accelerometer longitude variations in measured densities. Clearly, the height of the dayside F1-ionospheric peak is a sensitive indicator of the state of the Mars lower atmosphere. This research is funded by the NASA Mars Data Analysis Program.
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