Mars’ ultraviolet dayglow observations by IUVS/MAVEN: Structure and variability of Martian upper atmosphere
Résumé
Mars has been studied extensively at ultraviolet wavelengths starting from Mariner 6 and 7 (Barth et al. JGR, 1971; Stewart, JGR, 1972), Mariner 9 (Barth et al., Icaurs, 1972; Stewart et al. Icarus, 1972), and more recently by SPICAM aboard Mars Express (Leblanc et al., JGR, 2006). The results from these measurements reveal a large variability in the composition and structure of Martian upper atmosphere. However, due to the lack of simultaneous measurements of energy input in the atmosphere, such as solar electromagnetic and particle flux as well as 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 upper atmospheric structure and seasonal variability. The dayglow emission spectra show features similar to previous UV measurements at Mars. The IUVS detected a second, low-altitude peak in the emission profile of OI 297.2 nm, confirmation of 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. I will present and discuss the variability in Martian UV dayglow, its dependence on solar EUV irradiance, and the importance of IUVS observations in our current understanding of Mars’ thermosphere.