Vertical dust mixing and the interannual variations in the Mars thermosphere

Mars winter polar warming is a phenomenon of the lower thermosphere temperatures and densities that is well documented by in situ accelerometer data taken during spacecraft aerobraking maneuvers. Previous work by Bougher et al. ( 2006) simulates two specific time periods, corresponding to existing aerobraking data sets, and confirms the existence of winter polar warming structures in the Martian upper atmosphere, using the coupled Mars General Circulation Model - Mars Thermosphere General Circulation Model ( MGCM-MTGCM). The present work investigates the underlying mechanisms that drive winter polar warming in three major studies: ( 1) a systematic analysis of vertical dust mixing in the lower atmosphere and its impact upon the dynamics of the lower thermosphere ( 100 - 130 km), ( 2) an interannual investigation utilizing 3 years of lower atmosphere infrared ( IR) dust optical depth data acquired by the Thermal Emission Spectrometer ( TES) instrument on board Mars Global Surveyor ( MGS), and finally ( 3) a brief study of the MTGCM's response to variations in upward propagating waves and tides from the lower atmosphere. From the first study, we find that the vertical dust mixing greatly modifies the simulated winter polar warming features in the MTGCM's thermosphere at both solstice seasons. Furthermore, this sensitivity study confirms that an interhemispheric Hadley circulation and the concomitant adiabatic heating rates are the primary causes for these winter polar warming structures. From the second study, we find that as revealed in the lower atmosphere by Liu et al. ( 2003), the polar lower thermosphere exhibits a high degree of variability at L-S = 270, with reduced yet significant variability at L-S = 090. Finally, the third numerical experiment indicates, as was found in the lower atmosphere by Wilson ( 1997), that upward propagating waves and tides allow meridional flows to access the thermosphere's winter polar latitudes. This last investigation also indicates that the Hadley circulation responsible for thermospheric winter polar warming originates in the lower atmosphere and extends high into the upper atmosphere. In summary, these new efforts establish a baseline numerical study upon which more comprehensive model-to-data comparisons may be conducted for the Martian thermosphere.