Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p41a05w&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P41A-05
Physics
0343 Planetary Atmospheres (5405, 5407, 5409, 5704, 5705, 5707), 5409 Atmospheres--Structure And Dynamics, 6225 Mars
Scientific paper
Mars Global Surveyor Accelerometer measurements of density in the martian upper atmosphere during aerobraking are now available via the PDS [Keating et al, 2001]. We are continuing our investigations of variations in density with longitude at fixed local solar time, latitude, and season, concentrating on Phase 2 of aerobraking. We find that, contrary to previous suspicions, the zonal structure is not dominated solely by wave-2 harmonics. The dominant harmonics are as follows: wave-3 in the northern extratropics, wave-2 and wave-3 in the tropics, and no clearly dominant harmonic in the southern extratropics. The relative amplitudes of the various harmonics vary with latitude. However, their phases remain very stable, despite large changes in the phasing of the corresponding harmonic of zonal topography. Relative to the mean density, the amplitudes of the various harmonics decrease as altitude increases. This is opposite to the behaviour predicted by a simple, dissipation-less model of the martian upper atmosphere, in which deviations from the background state of the atmosphere are proportional to the inverse of the square root of pressure, and indicates the presence of damping in the upper atmosphere. When the martian day was an integer multiple of the spacecraft orbital period, the accelerometer measured densities at the same latitude, local solar time, season, and longitude each martian day. This period of resonance lasted for several days as the spacecraft orbital period decreased through the critical value due to drag. This permits us to examine the true variability of the martian upper atmosphere without the complications of the zonal variability. An accurate estimate of this essentially unpredictable variability is crucial for effective and efficient aerobraking of future spacecraft missions. At an altitude of 130 km, variabilities of 15 percent were typical, though values of 30 percent were observed. This variability also decreased as altitude increased, an independent indication of damping and dissipation in the martian upper atmosphere. G.M. Keating, R.H. Tolson, J.L. Hanna, R.F. Beebe, J.R. Murphy and L.F. Huber, MGS-M-ACCEL-5-ALTITUDE-V1.0, NASA Planetary Data System, 2001.
Bougher Stephen W.
Keating Gerald M.
Withers Paul
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