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 the terrestrial planets (Earth, Mars, and Venus). The seasonal
migration of the Martian He bulge has been observed and modeled (Elrod
et al., 2017, Gupta et al., 2021). The MAVEN orbit precesses around Mars
allowing for a variety of latitude and local time observations
throughout the Martian year. MY32, 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 on 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 the PEDE directly impacted upper
atmospheric circulation. Updates in modeling indicate changes in
circulation and winds can cause He to shift further north and dawn-ward
than MAVEN was able to observe. The temperature increases in the
thermosphere on the nightside during the dust storm along with changes
in gravity waves and eddy diffusion occurring during this event could
account for this circulation change.