The impacts of solar wind on the Martian upper atmosphere

Abstract

Since the first in-situ measurements of the altitude profile of upper atmospheric density and composition were carried out by the Viking lander missions in 1976, similar data are continuously gathered by MAVEN and MOM spacecraft orbiting Mars since their launch in September 2014 with mass spectrometers and other related payloads. Using near-simultaneous observations by the two orbiters, it is seen that both data sets indicate significant day-to-day variations of Argon (Ar) density profiles in the thermosphere-exosphere (∼150-300 km) region during the period 1-15, June 2018, when the solar EUV radiation did not show any appreciable change but the solar wind energetic particle fluxes did so. Extending this study to include the other parent atmospheric constituents (CO 2 , He, N 2 ) and their photochemical products (O, CO) during the same period it is found that the density profiles of CO 2 and O also show similar variations with CO 2 densities showing an increasing trend similar to Ar, but a reversal of this trend for O densities. Using in-situ and near-simultaneous measurements of solar EUV fluxes and the solar wind plasma (e, H + ) velocities and densities near MAVEN periapsis it is noted that unlike the solar EUV radiation (which decreased only by ∼10% between the first and second week of June 2018), solar wind parameters showed a decrease by a factor of 2-3. Hence, it is inferred that the energetic and penetrating solar wind charged particle impact-driven dissociation, ionisation and ion-chemical processes could decrease the CO 2 densities leading to the increase in O densities. This result is also discussed from the considerations of the proton gyro radius effect, pickup ions, sputtering, energetic neutral atoms (ENAs) driven ionisation and ion losses. Further data and modelling efforts would be necessary to confirm this finding.

Martian Thermosphere Response to Solar Flares: MAVEN NGIMS Observations

“…In addition to the in situ and remote measurements, a new Mars Global Ionosphere-Thermosphere Model (MGITM) is developed to accurately describe the features of the compositional and thermal structure of the Martian thermosphere (Bougher et al 2015). Observations and simulations have revealed that the structure and circulation of this region were driven both by energy and momentum fluxes from the lower atmosphere and by solar X-ray/Extreme Ultraviolet (EUV) radiation, solar energetic particle as well as solar wind influx from above (e.g., Bougher 1995;Krasnopolsky 2002;Bougher et al 2015;Nagaraja & Chakravarty 2022;Wu et al 2022).…”