Characteristics of Mars UV Dayglow Emissions From Atomic Oxygen at 130.4 and 135.6 nm: MAVEN/IUVS Limb Observations and Modeling

Abstract

AbstractWe present an overview of two Martian Years (MYs) oxygen dayglow limb observations of the ultraviolet (UV) emissions at 130.4 and 135.6 nm. The data have been collected with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We use solar flux measurements of EUVM on board MAVEN to remove the solar‐induced variation and show the variations of the maximum limb brightness and altitude with season, solar zenith angle, and latitude, which reflects the strong variability of the Martian atmosphere. The 130.4‐ and 135.6‐nm peak brightness and altitudes are strongly correlated and behave similarly. Both emissions are modeled for selected data using Monte Carlo codes to calculate emissions arising from electron impact on O and CO2. Additional radiative transfer calculations are made to analyze the optically thick 130.4‐nm emission. Model atmospheres from the Mars Climate Database serve as input. Both simulated limb profiles are in good agreement with the observations despite some deviations. We furthermore show that the observed 130.4‐nm brightness is dominated by resonance scattering of the solar multiplet with a contribution (15–20%) by electron impact on O. Over 95% of the excitation at 135.6 nm arises from electron impact on O. Simulations indicate that the limb brightness is dependent on the oxygen and CO2 content, while the peak emission altitude is mainly driven by the CO2 content because of absorption processes. We deduce [O]/[CO2] mixing ratios of 3.1% and 3.0% at 130 km for data sets collected at LS = 350° in Martian years 32 and 33.