On the nature of the variability of the Martian thermospheric mass density: Results from electron reflectometry with Mars Global Surveyor

摘要

Knowledge of Mars' thermospheric mass density is important for understanding the current state and evolution of the Martian atmosphere, and for spacecraft such as the upcoming MAVEN mission that will fly through this region on every orbit. Global-scale atmospheric models have been shown thus far to do an inconsistent job of matching the mass density observations at these altitudes, especially on the nightside. Thus, there is a clear need for a data-driven estimate of the mass density in this region. Given the wide range of conditions and locations over which this must be defined, the data set of thermospheric mass densities derived from energy and angular distributions of super-thermal electrons measured by the MAG/ER experiment on Mars Global Surveyor, spanning 4 full Martian years, is an extremely valuable resource. Here we present an empirical model of the thermospheric density structure of this data set. Using this new model, we assess the global-scale response of the thermosphere to dust storms in the lower atmosphere and show that this varies with both latitude and dust opacity. Further, we examine the short-term variability of the thermospheric density and show that it exhibits a repeatable behavior with latitude and season that is indicative of atmospheric wave activity seen in the lower thermosphere. This short-term variability is consistently highest in the southern hemisphere, peaking around perihelion, which may have significant implications for studies of atmospheric escape.