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

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 similar to 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 similar to 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., Yigit, 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.

Automated summary

Using the NGIMS instrument on board MAVEN, data from MY 32, 34, and 35 were analyzed to study the He bulge during the northern winter solstice and its interaction with the planet encircling dust event (PEDE-2018). The observations confirmed the seasonal migration of the He bulge and its absence during the PEDE, indicating a direct impact on upper atmospheric circulation.