Latitudinal and Seasonal Asymmetries of the Helium Bulge in the Martian Upper Atmosphere

In the present study, we investigate the characteristics of helium (He) bulges in the Martian upper atmosphere using He densities and winds measured by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The observations are compared with those predicted by the Mars Global Ionosphere Thermosphere Model (M-GITM). The results of the present study show that the nightside He bulge is a persistent feature of the Martian upper atmosphere in all seasons. The He densities inside the bulges are 1-2 orders of magnitude greater than those on the dayside. In solstices, the bulges are observed in the winter polar region which is in accordance with the model predictions. In equinoxes, however, the bulges are observed to extend from mid-latitudes into the southern polar regions (>60 degrees S), which is contrary to the model predictions at mid-latitudes. These anomalous bulges are predominantly observed in the northern spring equinox and are 10-30 x greater than the modeled ones. During the autumnal equinox, the observed winds depart from the modeled winds. Furthermore, the observed winds point to the southern polar regions where the bulges are observed. Thus, the results of the present study indicate that in equinoxes the regions of local vertical advection, that are responsible for the formation of the bulges, are displaced toward the southern polar regions. The results of the present study point to the need of a larger wind database from NGIMS in southern polar region, particularly during equinoxes.