Winds in the martian upper atmosphere from MGS aerobraking density profiles

We have used a novel technique to measure the zonal wind in the martian upper atmosphere using MGS Accelerometer aerobraking density profiles. Typical results for the northern hemisphere (NH) at about Ls=40, 115 km altitude and midafternoon local solar times (LSTs) show a westward speed of 50 to 100 m/s; those for the southern hemisphere (SH) at about Ls=80, 110 km altitude show an eastward speed of 0 to 50 m/s. Solar activity is moderate for both periods with an F10.7 index of about 140 units. In the NH, wind speed shows no dependence on longitude, decreases as latitude increases poleward, and increases as altitude increases. In the SH, repeated measurements of wind speed at fixed latitude, altitude, LST, and longitude during the 8:1 resonance between MGS's orbit and Mars' rotation show a significant dependence of wind speed on longitude. At 20E longitude the typical wind speed is 50 m/s westward, whereas at 335E it is 120 m/s eastward. The dependence of wind speed on latitude and altitude is difficult to examine, because periapsis altitude steadily decreased as periapsis precessed poleward. The two variables are strongly correlated. In some longitude regions, eastward wind speeds increase as periapsis moves poleward and downward, but in others the eastward wind speeds stay constant. At 60S latitude and nighttime LSTs, wind speeds differ from their daytime values. Nighttime wind speeds at a given longitude show much less variability than their daytime counterparts. These results will be compared to MTGCM simulations. Other applications of this technique will be suggested.