Empirical and Forward Modeling of Oxygen OI 135.6nm in the Martian Thermosphere

Abstract:

The objective of this work is to develop empirical and forward models of oxygen OI 135.6nm emission in the Martian thermosphere in order to simulate accurate disk radiances as predicted to be seen from the Emirates Mars Ultraviolet Spectrometer (EMUS) - one of the three scientific instruments on board the Emirates Mars Mission (EMM) launched in July 20, 2020. The empirical model is a zonally-averaged Far Ultraviolet (FUV) model that is developed to calculate the brightness of the OI 135.6nm emission taking into consideration observation geometry parameters like emission angle, solar zenith angle and Mars-Sun distance, as informed by the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile Evolution Mission (MAVEN). To optimize the empirical model, a study had been conducted to examine the correlation between OI 135.6nm brightness and latitude, local time, and solar EUV activity. Moreover, the developed OI 135.6nm forward model inputs density profiles from the Mars Global Ionosphere Thermosphere Model (MGITM) with generated stimulation frequencies to output volume emission rates that are converted into column emission rates (R). The stimulation frequencies are calculated based on the electron impact on the species’ cross sections and have been corrected using Mars-Sun distance correction factors. The outputs of both models are examined at solar longitude (Ls) ~90 and ~270 and sampled at altitude ~130 km (homopause) where solar zenith angle less than 80 (at dayside) and emission angle less than 80. A comparative study is carried out through analyzing both models’ outputs to further understand the OI 135.6nm’s behavior sub-seasonally.