Dawn-Dusk Asymmetries in the Martian Upper Atmosphere

In this paper, we report the first comprehensive observations of local time asymmetries in densities and scale heights (temperatures) of the Martian upper atmosphere (between 150 and 300 km) measured by the Mars Atmosphere and Volatile Evolution mission/Neutral Gas and Ion Mass Spectrometer. For this purpose, we use the densities and temperatures of Ar. In general, the daytime densities and temperatures are greater than the nighttime values. The maximum and minimum values, however, are observed at the dusk and dawn terminators, respectively. An enhancement at the dusk terminator is persistently observed at all altitudes; however, the time of the peak enhancement shifts toward sunlit hours with increasing altitude. At the dawn terminator, a minimum in density is observed at altitudes of 150-170 km. At higher altitudes, the minimum is observed close to midnight. Accordingly, the dawn-dusk asymmetry is more prominent at 150-170 km and decreases with increasing altitude. A maximum ratio of six is observed between the dusk and dawn densities at 160 km. In addition, the local time for the maximum ratio at each altitude moves toward sunlit hours with increasing altitude. The observed asymmetry is explained in terms of dynamical heating and cooling due to convergent and divergent winds at the dusk and dawn terminators, respectively. In addition, upward propagating gravity waves generated by the solar terminator wave and O/CO2 radiative cooling are also proposed as important mechanisms contributing to the observed asymmetry.