The Impact of Solar Radiation on the Martian Upper Atmosphere
The first in-situ measurements of the altitude profile of Martian upper atmospheric density and composition were carried out by the Viking lander missions in 1976. The MAVEN and MOM spacecraft launched in September 2014 with mass spectrometers and solar radiation measuring payloads have vastly expanded this initial database. Using a rare set of near-simultaneous data from these two orbiters, we find that there is either an increasing (e.g., for $CO_2$ and $Ar$) or a decreasing (e.g., for $O$) trend of the density profiles by a factor of 2 between June 1 to June 15, 2018, in the height region of $\sim$150-300 km. A time series analysis of the concurrent in-situ solar EUV spectral flux and the $H^+$ ion velocities of the incident solar wind measured near MAVEN periapsis showed the former going through a decrease of only $\sim$10\% compared to the latter's decrease by a factor of 4 within the same non-solar-flare period of observation. The estimates of standard errors and the use of linear regression analysis for the correlation coefficients between densities and solar radiation components have been carried out. Invoking simple photochemical equilibrium conditions with the dissociation of $CO_2$ (producing $CO$ and $O$) through solar EUV radiation and the solar wind $H^+$ ion impact process, the day-to-day variations of these constituents are estimated. The high and significant anti-correlation between the density variations of $CO_2$ and $O$ due to the dissociation of $CO_2$ by the solar wind particle radiation is clearly demonstrated. The cause for the increasing densities of $Ar$ like that of $CO_2$ during this period is more complex and would likely be governed by the temperature variations due to absorption of solar EUV/charged particle radiation and other interacting dynamical effects.