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D. P. Hinson, G. L. Tyler (Stanford University), F. M. Flasar (Goddard Space Flight Center)
Mars Global Surveyor is now circling Mars at ~400 km altitude in a polar orbit with a period of ~2 hr. During the early portion of the Mission's mapping phase, radio occultation experiments were conducted 24 times per day as the spacecraft was tracked continuously from Earth. This talk will focus on measurements made at occultation entry between May 18 and May 31, 1999, which have yielded about 150 profiles of temperature and pressure versus radius and geopotential in the northern tropics (0\deg-20\degN). Consecutive measurements are separated by 28.6\deg in longitude, but the local time remained near 04:10. The season on Mars was midsummer of the northern hemisphere (141\deg < Ls < 147\deg). Our initial interpretation of these profiles is directed at two notable features. First, prominent lee waves are present in the Tharsis region. Waves westward of Ascraeus Mons have amplitudes of 4-8 K and vertical wavelengths of 6-10 km. The waves begin breaking at pressures as large as 200 Pa, which allows topography to exert a drag on the global circulation over an extended altitude interval. Second, the thermal structure varies considerably among the profiles at pressures less than ~100 Pa. This effect is most apparent near the equator, where some profiles contain a strong temperature inversion at pressures of 60-100 Pa.
For earlier results, see Hinson et al., Initial results from radio occultation measurements with Mars Global Surveyor, J.\ Geophys.\ Res., in press, 1999.