Observations of seasonal and diurnal patterns on the He bulge during the PEDE-2018 as observed by MAVEN NGIMS

Using the Neutral Gas and Ion Spectrometer (NGIMS) on the Mars Atmosphere Volatile and Evolution spacecraft (MAVEN) we collected data from 2015 through 2020 to examine the He bulge specifically focusing on the effects from the planet encircling dust event that occurred in 2018 (PEDE-2018). This study expands on Elrod et al. (2016) that first examined the changes of the He bulge but was limited to the first year of MAVEN data. An analysis of the first year revealed higher densities of He on the nightside and in the polar regions. Using Mars Global Ionospheric Thermosphere Model (M-GITM) we modeled the seasonal and diurnal variations of the He bulge. These predictions show the He bulge forming in the dawn hours (LST 2-5hrs) and shifting between the northern and southern poles (>45° & <-45°) with the seasons (Elrod et. al, 2016). In 2018, a planet encircling dust event occurred (PEDE-2018) at Ls 180-280 Mars Year 34 (MY34). During this event, MAVEN revealed changes in the circulation and temperature, increasing the CO₂ and Ar densities, simultaneously decreasing the O and He densities (Elrod et al. 2018). While M-GITM models have explained some of the decrease in O, it has not explained the decrease in He. During the PEDE-2018 He was expected to show a peak on the nightside in the northern hemisphere, however, He was lower when compared with other nightside densities at various Ls. In MY 33 at Ls 180-280 (same time frame as PEDE-2018 MY 34) there was a decrease in He in the southern to mid-latitude range (-20° through -75°) which was expected for this time of year. In contrast, during PEDE-2018, the spacecraft was in the northern hemisphere where a He bulge was expected, but NGIMS observed He density to be more diffused to all latitudes and was overall lower when compared to MY 33 for the same Ls. We examined the NGIMS data at different latitudes sorted by Ls to reveal seasonal effects and at different LST to compare the nightside of the PEDE-2018 to other nightside’s throughout the trend.