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

Abstract

Response 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/CO 2 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. Plain Language Summary Characterizing the Martian upper atmosphere is extremely important as it acts as a buffer zone for escape of its gases to outer space. Toward achieving this goal, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) and ISRO's Mars Orbiter Mission (MOM) spacecraft have been providing valuable information of the Martian upper atmosphere. During 5-29 June 2018, these spacecraft have made simultaneous observations of the Martian upper atmosphere such that MAVEN was on the morningside while MOM was on the eveningside. Both the spacecraft have observed a significant increase in neutral densities in the Martian upper thermosphere (150-220 km) associated with the development of a global dust storm in the lower atmosphere. The increase, however, is larger in the evening time than in the morning. The largest changes are observed when the lower atmospheric dust activity is at its peak. These upper atmospheric changes are interpreted as due to warming and expansion of the Martian thermosphere due to radiative heating of the dust aerosols in the lower atmosphere. Furthermore, the observations suggest that radiative cooling of the Martian thermosphere, due to collisions between O and CO 2 , is more effective in the morning hours than in the evening.

Disentangling the Dominant Drivers of Gravity Wave Variability in the Martian Thermosphere

“…The NGIMS measurements constitute the longest data of the Mars thermospheric neutral densities. These data were obtained during the medium to low solar activity period and when the Mars lower atmosphere witnessed the global and regional dust storms that perturbed the thermosphere significantly (Elrod et al 2020;Jain et al 2020;Venkateswara Rao et al 2020). The main results of our study are as follows:…”

Interannual Variability of Atmospheric Gravity Waves in the Martian Thermosphere: Effects of the 2018 Planet‐Encircling Dust Event

“…Because of this diurnal asymmetry in enhancement, the diurnal pattern in GW activity ceases. It is known that during lower atmospheric dust storms, the thermosphere expands, and hence at any given altitude, the neutral densities are enhanced (e.g., Elrod et al., 2019; Jain et al., 2020; Liu et al., 2018; Venkateswara Rao et al., 2020). Thus, the thermosphere is hotter during PEDE‐2018 than normal times.…”

Latitudinal and Seasonal Asymmetries of the Helium Bulge in the Martian Upper Atmosphere

“…In addition to heating and cooling, persistent forcing by gravity waves (England et al, 2017;Fritts et al, 2006;Mahaffy et al, 2015;Medvedev & Yiğit, 2012;Terada et al, 2017;Williamson et al, 2019), thermal tides (Lee et al, 2009;Liu et al, 2017;Wilson, 2002), planetary waves (Hinson et al, 2003;Moudden & Forbes, 2010), seasonal inflation and contraction of the lower atmosphere (e.g., Bougher, Pawlowski, et al, 2015) and episodic forcings such as planet-encircling dust events (Elrod et al, 2020;Jain et al, 2020;Kuroda et al, 2020;Leelavathi et al, 2020;Liu et al, 2018;Venkateswara Rao et al, 2020) also play a key role in the energy redistribution and compositional changes in the Mars upper atmosphere. These forcings are indicative of energy transfer from the lower atmosphere to the thermosphere and hence coupling between the two regions (e.g., Bougher, Cravens et al, 2015).…”