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°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 × 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. Plain Language Summary: Helium in the upper atmospheres of Earth, Venus, and Mars is known to accumulate on the nightside which is often referred as "He bulge." The upwelling of winds on the dayside and their downwelling on the nightside, combined with large‐scale circulation, is the primary driver of the bulge formation. The densities inside the He bulge are, in general, 1–2 orders of magnitude greater than those on the dayside. In the present study, the He bulge in the Mars upper atmosphere is investigated using the neutral densities measured by the Mars Atmosphere and Volatile EvolutioN spacecraft and those of a global model. In the northern summer, the He bulge occurs in the winter polar nightside which is in accordance with the model predictions. In spring and autumn, however, the observed He bulges extend from mid‐latitudes into the southern polar region whereas the model predicts their presence at mid‐latitudes. The southern polar bulges in autumn are observed in regions where the observed winds point toward the southern pole. Inclusion of the effects of the upward propagating small‐scale waves on the Martian upper atmosphere in the model, along with a larger observational wind database, is likely to address the data‐model differences. Key Points: In equinoxes, helium bulges are observed to extend from mid‐latitudes in to the southern polar regions (>60°S)The observed helium densities in the southern polar regions in equinoxes are 10–30 × greater than the modeled onesThe observed winds seem to support the formation of helium bulge in the southern polar region during the autumnal equinox [ABSTRACT FROM AUTHOR]