Surprising Decrease in the Martian He Bulge During PEDE-2018 and Changes in Upper Atmospheric Circulation

Abstract

Using the Neutral Gas and Ion Mass Spectrometer (NGIMS) on the Mars Atmosphere Volatile and Evolution spacecraft (MAVEN), we analyzed data from Mars Year (MY) 32, 34, and 35 to examine the He bulge during the northern winter solstice (Ls ∼ 180–240), specifically focusing on the effects from the planet encircling dust event (PEDE-2018). He collects on the dawn/nightside winter polar hemisphere of Mars. The seasonal migration of the He bulge has been observed and modeled (M. Elrod et al., 2017, https://doi.org/10.1002/2016JA023482; Gupta et al., 2021, https://doi.org/10.1029/2021JE006976). The MAVEN orbit precesses around Mars allowing for a variety of latitude and local time observations throughout the Martian year. MY 32, 34, and 35 had the best possible opportunities to observe the He bulge during northern winter (Ls ∼ 180–240). NGIMS observations during MY 32 and MY 35 revealed a He bulge from the nightside to dawn in alignment with modeling and previous publications. However, in MY 34, during the PEDE, the He bulge was not present, indicating that the PEDE directly impacted upper atmospheric circulation. Updates in modeling indicate changes in circulation and winds can cause He to shift further north than MAVEN was able to observe. While adding a simple static version of gravity waves to the Mars Global Ionosphere Thermosphere Model model may account for some of the variations in the global circulation during the dust event, other studies (e.g., Yiğit, 2023, https://doi.org/10.1038/s41561-022-01118-7) have posited that the gravity waves during the dust storm were more variable than the initial parameters we have included.Plain Language SummaryMars is regularly subjected to large dust storms that typically start during the northern winter season. Approximately every 7–10 years these large storms can merge and grow and become planet-sized dust storms that cover 80%–95% of the surface. These rare and massive planet sized storms last for about a month before slowly dissipating, changing not only the surface of the planet but also the structure and composition of the atmosphere. Mars has regular helium bulges that collect in the cold part of the upper atmosphere (e.g., polar winter regions on the night, dawn side). Global circulation models of the atmosphere have shown this helium bulge as predictable throughout the Martian year. The Mars Atmospheres and Volatiles EvolutioN spacecraft made observations of Helium over the course of 4 Martian years, but during the last major global dust storm, where a Helium bulge in the upper atmosphere should have been observed, no increase in Helium associated with a bulge was observed. This could have implications for the broader impact of regular global dust storms, winds, and upper atmosphere circulation on the Martian atmosphere.Key PointsHe collects in the upper atmosphere above the dawn winter polar atmosphere regionHe bulge density is unexpectedly lower in Mars Year (MY) 34 northern winter during the (planet encircling dust event) than other observed northern winters during MY 32 and 35He is a tracer for upper atmosphere circulation and changes in the bulge indicate the circulation changed as shown through modeling