Structure and variability of the Martian upper atmosphere: Ultraviolet dayglow observations by MAVEN/IUVS
Résumé
Mars has been studied extensively at ultraviolet wavelengths starting from Mariner 6 and 7, Mariner 9, and more recently by SPICAM aboard Mars Express. The results from these measurements reveal a large variability in the composition and structure of the Martian upper atmosphere. However, due to the lack of simultaneous measurements of energy input (such as solar electromagnetic and particle flux), and limitations in the observation geometry and data itself, this variability is still not fully understood.We report a comprehensive study of Mars dayglow observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite, focusing on vertical and global upper atmospheric structure and seasonal variability. The dayglow emission spectra show features similar to previous UV measurements at Mars. IUVS has detected a second, low-altitude peak in the emission profile of OI 297.2 nm, confirming the prediction that the absorption of solar Lyman alpha emission is an important energy source there. We find a significant drop in thermospheric scale height and temperature between Ls = 218° and Ls = 337 - 352°, attributed primarily to the decrease in solar activity and increase in heliocentric distance. The CO2+ UVD peak intensity is well correlated with simultaneous observations of solar 17 - 22 nm irradiance at Mars by Extreme Ultraviolet Monitor (EUVM) aboard MAVEN. Variations of the derived CO2 density also exhibit significant persistent global structure with longitudinal wavenumbers 1, 2 and 3 in a fixed local solar time frame, pointing to non-migrating atmospheric tides driven by diurnal solar heating. We will present and discuss the variability in Martian UV dayglow, its dependence on solar EUV flux, and the importance of IUVS observations in our current understanding of Mars’ thermosphere.