Presentation #203.03 in the session The Martian Upper Atmosphere.
Using the Neutral Gas and Ion Spectrometer (NGIMS) on the Mars Atmosphere Volatile and Evolution spacecraft (MAVEN) we collected data from Mars Year (My) 32, 34, and 35 to examine the He bulge during the northern winter solstice (Ls ~200-280) specifically focusing on the effects from the planet encircling dust event (PEDE-2018). The MAVEN orbit precesses around Mars allowing for a variety of latitude and local time observations throughout the Martian year, for this reason, we selected He observations from My 32 and My 35, we make a comparison for a non-dust year to the PEDE observations in My 34. NGIMS observations during the PEDE during My34 are considerably lower that observations during My 32 and My 35 indicating the PEDE directly impacted the He bulge. Updates in modeling indicate changes in circulation and winds that caused He to shift further north and dawn-ward than MAVEN was able to observe.
MAVEN was in the northern hemisphere from latitude ~0 to 70 deg N lat moving from dawn to night side during the dust storm. This put MAVEN/NGIMS in an opportune position to be able to observe the previously observe norther winter He bulge during the dust storm. Not only was no bulge observed, but instead a decrease in He on the nightside was observed. This decrease is inconsistent with the MGITM modeling efforts. We have since revised the modeling efforts to include gravity waves to reflect the more turbulent atmosphere during this storm system. The new revised model indicates a redistribution of He around the northern polar region above 50 deg N latitude. While this revision agrees better with the observed NGIMS data for the dawn densities below 50 deg N latitude, there is no correlation above 50 de N latitude dawn or night side.
The included figure shows the average He density from MY 32, 34 (red) and 35. MY 32 and 35 were not substantially impacted by dust storms during northern polar winter and as a result observed a normal He bulger in the northern polar latitudes. MY 34 had the large scale dust storm from Ls 186-240 during northern polar winter and not only did not observe a He bulge but observed a decrease in He during northern polar winter.
Current revision efforts in our modeling include revising the eddy diffusion impacts, wind variability, and improving the heating mechanisms. NGIMS temperatures during the global dust storm are substantially higher than the MGITM model, particularly with the inclusion of the gravity waves. It is the temperature disagreement that can have the largest impact on the He bulge and the disagreement between model and observation.