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       HR: 0830h
       AN: P41A-05
       TI: Harmonic Analysis of Zonal Density Structures in Martian Upper Atmosphere
       AU: * Withers, P 
       EM: withers@lpl.arizona.edu
       AF: University of Arizona, Lunar and Planetary Laboratory 
       University of Arizona, Tucson, AZ 85721 United States 
       AU: Bougher, S W
       EM: sbougher@lpl.arizona.edu
       AF: University of Arizona, Lunar and Planetary Laboratory 
       University of Arizona, Tucson, AZ 85721 United States 
       AU: Keating, G M
       EM: G.M.KEATING@larc.nasa.gov
       AF: George Washington University, The George Washington University 
       at NASA Langley, MS 269, Hampton, VA 23681 United States 
       AB: 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. 
       DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
       DE: 5409 Atmospheres--structure and dynamics
       DE: 6225 Mars
       SC: P
       MN: 2001 Spring Meeting