Winds in the Thermosphere of Mars: Exploring the Behavior of the Neutral Winds in the Upper Atmosphere and Examining Connections to the Lower Atmosphere

Abstract

This dissertation presents an analysis of the behavior of the neutral winds in the thermosphere of Mars, including impacts on this behavior due to coupling with the lower atmosphere. Using a 3-D ground to exobase Mars general circulation model called M-GITM (Mars Global Ionosphere-Thermosphere Model) and new in-situ thermospheric neutral wind observations at altitudes of ~140-240 km from the MAVEN (Mars Atmosphere and Volatile Evolution) mission at Mars, the behavior of the winds in the upper atmosphere of Mars and different processes which drive them were examined. These model simulations and observations were also utilized to analyze the impacts of vertical coupling between the lower and upper atmosphere of Mars, focusing on two phenomena in particular: the effects of gravity waves propagating up from the lower atmosphere as well as the impacts of the most recent global dust storm.

In the first part of this analysis, M-GITM simulations are compared to five MAVEN wind observational campaigns. Some campaigns show a clear correspondence between measured velocities and simulated velocities. However, other campaigns show notable disparities between modeled and observed speeds, directions, or both. Since M-GITM is primarily driven by forcing from EUV heating at these altitudes, this indicates that those campaigns which M-GITM was able to replicate to a better degree are also primarily being driven by solar forcing. In the areas of poorer comparison, likely other physical processes not currently represented in the model play a larger role.

The second part of this analysis examines the impacts of one of those processes that was previously not included in M-GITM. A modern whole atmosphere, non-orographic gravity wave (GW) parameterization scheme was incorporated into M-GITM so that the effects of subgrid-scale GWs could be represented in numerical simulations of the upper atmosphere. In both equinox and solstice simulations, significant GW momentum deposition was seen at altitudes from ~90-170 km, with average magnitudes of several hundred to over a thousand meters per second per sol. Additionally, mean thermospheric winds were reduced by up to a factor of two in the summer hemisphere, and mean temperatures above 120 km were cooler at most latitudes. Overall, the thermal and dynamical impact of these subgrid-scale GWs in M-GITM is substantial, indicating the importance of GWs as a coupling mechanism, and the need to include their effects in future modeling studies.

In the final section of this work, the MAVEN wind measurements are examined to determine whether the 2018 Mars global dust storm had an observable impact on the behavior of the thermospheric velocities. The campaign averaged wind speed increased slightly, up to nearly 200 m/s, near the onset of the storm before gradually decreasing during the mature phase of the storm. Substantial small-scale variability was seen during the peak of the storm and into the decay phase. However, the effects of changing local time and latitude of the MAVEN observations also need to be considered when interpreting the importance of the role of the dust storm on the behavior of the neutral thermospheric winds.

Overall, this work provides some of the first analysis of the MAVEN thermospheric wind observations, supported by numerical simulations from M-GITM. Through analysis of the impacts produced by lower atmospheric phenomena on observed and simulated thermospheric winds, this work also demonstrates the importance of vertical coupling in the Martian atmosphere.