Helium Bulges in the Upper Atmosphere of Mars: Seasonal and Latitudinal Variations in Helium Densities from NGIMS Observations and Mars-PCM Simulations

Light atmospheric species such as helium (He) serve as tracers of global circulation in Mars' upper atmosphere (>100 km). Due to its low mass and large scale-height, He exhibits unique behaviour, including the formation of He bulges, their spatiotemporal variations, and their response to Global Dust Storm (GDS). The significant variability observed during nominal dust conditions (i.e., in the absence of a GDS) highlights helium's sensitivity to global circulation across different locations and seasons on Mars. In recent years, seminal studies have explored the small- and large-scale variabilities in He bulges during nominal dust conditions using data from the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission. These observations were supplemented by simulations from the Mars Global Ionosphere-Thermosphere Model (M-GITM). However, no other Global Climate Models (GCMs) simulations have been compared with NGIMS He observations, despite notable discrepancies between NGIMS data and M-GITM outputs. Consequently, He climatology under nominal dust conditions using a GW-parameterized GCMs remains unexplored.

MAVEN dataset, spanning Mars Years (MY) 32–37, excluding the period of MY34 GDS, Solar Longitude (L_s) ~ 180-290°, obtained through the NGIMS instrument onboard MAVEN, provides sufficient global coverage and a rare opportunity to study the long-term climatology of He in the Martian upper atmosphere. We use this dataset at an altitude of ~200 km to understand the latitudinal, seasonal, and local-time variability of He bulges in the upper atmosphere during nominal dust conditions on Mars. Additionally, we compare these observations with simulations from a GW-parameterized version of Mars-PCM, which is prescribed with a ‘climatology’ dust scenario. This scenario uses column dust opacity derived by averaging dust opacities observed during MY 24 to 35, excluding MY 25, 28, and 34, to enable an unbiased investigation of He bulges independent of the effects of GDS. In addition, a comparative analysis of NGIMS observations and Mars-PCM simulations, with gravity waves turned on will allow us to study discrepancies between observations and simulations reported in previous studies. The result of this study shows a stronger agreement between NGIMS observed He bulges with those simulated by Mars-PCM as compared to the models used previously. Particularly, the latitudinally extended He bulges shown in this study discard the anonymity of their presence in the high latitude regions (>50°) of Mars, as suggested in previous studies. Furthermore, the sol-to-sol simulations from Mars-PCM for a typical Martian year provides an insight on the seasonal migration of He bulges throughout the year. The He bulges shift toward the southern hemisphere around L_s of ~50° as Mars transitions from the northern spring equinox to northern summer. Conversely, they migrate to the northern hemisphere around L_s ~183° as Mars moves from the northern autumn equinox to northern winter. Thus, the results of this study further our understanding of spatiotemporal variability and migration of He bulges, highlighting the significance of gravity waves induced changes, particularly at the high latitude regions in the upper atmosphere of Mars.