Solar Rotation Effects in Martian Thermospheric Density as Revealed by Five Years of MAVEN Observations

The quasi-periodic 27-day solar rotation variation in extreme ultraviolet radiation represents the most prominent source of short-term (< $< $40-day) solar-driven variability in Mars' thermosphere (ca. 125-250 km). Despite recent advances in our understanding of solar-driven variations in Mars' thermosphere density and composition associated with solar flares, orbital eccentricity, and solar cycle afforded by new Mars Atmosphere and Volatile EvolutioN (MAVEN) observations, relatively little is known about this response to short-term periodic solar flux variability. This work presents a detailed investigation of the solar rotation effects on the whole Martian thermosphere by applying correlation analysis techniques to over five years of coincident MAVEN Neutral Gas and Ion Spectrometer and Extreme Ultraviolet Monitor observations. Least squares methods are used to estimate the response of CO2 ${\mathrm{C}\mathrm{O}}_{2}$, Ar, and N2 ${\mathrm{N}}_{2}$ densities to the quasi-27-day solar rotation variability over the 0-7 nm, 17-22 nm, 0-45 nm, and 117-125 nm spectral bands. Results clearly reveal the presence of prominent solar rotation effects in the Martian thermosphere density for all species, irradiance bands, and altitudes. These effects are strongest at higher altitudes (200-250 km), while the dependence of the slope on solar zenith angle depends on the species, with CO2 ${\mathrm{C}\mathrm{O}}_{2}$ having a higher slope on the nightside, Ar having a higher slope on the dayside, and N2 ${\mathrm{N}}_{2}$ having a different behavior depending on altitude.

Automated summary

This study investigates the effects of solar rotation on the Martian thermosphere by analyzing MAVEN observations. The results show that the density of the Martian thermosphere exhibits significant response to the quasi-27-day solar rotation variability.