Enhanced Densities in the Martian Thermosphere Associated With the 2018 Planet‐Encircling Dust Event: Results From MENCA/MOM and NGIMS/MAVEN

Abstract

AbstractResponse of the Martian upper thermosphere to the lower atmospheric dust activity is studied using unique observations made together by the Mars Orbiter Mission (MOM) and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. The Mars Exospheric Neutral Composition Analyzer (MENCA)/MOM and the Neutral Gas and Ion Mass Spectrometer (NGIMS)/MAVEN have simultaneously (on the same day) measured the neutral densities in the Martian thermosphere on 5, 8, 10, 13, 16, and 29 June 2018. The measurement period falls in the onset and progression time of the planet‐encircling dust event (PEDE) in the Mars lower atmosphere. During this time, the inbound trajectories of MAVEN and MOM spacecraft occurred on the dawnside and duskside, respectively. Using these observations, we found that thermospheric densities both on the dawnside and duskside are enhanced associated with the onset and growth of PEDE‐2018. The enhancement, however, is more on the duskside than on the dawn, showing the dawn‐dusk asymmetry. The densities on the duskside reach their maximum on 29 June 2018, close to the time of peak dust activity. These results are explained by considering the local time asymmetries in radiative heating of the lower atmosphere and subsequent expansion of the thermosphere due to PEDE‐2018. Furthermore, O/CO2 ratios below 220‐km altitude become one on the dawnside, whereas they are always <0.2 at the dusk. This indicates that radiative cooling is more effective on the dawnside than the duskside.