Characteristics of Mars UV dayglow emissions from atomic oxygen at 130.4 and 135.6 nm: MAVEN/IUVS limb observations and modeling.

We present an overview of two Martian years oxygen dayglow limb observations of the
ultraviolet (UV) emissions at 130.4 nm and 135.6 nm. The data have been collected with the
IUVS instrument on board the 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, SZA 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 datasets
collected at LS=350° in Martian years 32 and 33.