The OI 130.4 / 135.6 nm emission ratio in the Mars aurora: a radiative transfer model to simulate the EMM/EMUS observations

Since April 2021, the Emirates Mars Ultraviolet Spectrometer (EMUS) instrument (Holsclaw et al., 2021) on board the EMM orbiter has been collecting spectral images in the far ultraviolet (FUV) domain. Images of the oxygen auroral emission have been acquired both at 130.4-nm and 135.6-nm (Lillis et al., 2022). The 130.4-nm oxygen emission detected for the first time in the Martian aurora with SPICAM/Mars Express (Soret et al., 2016). It corresponds to the O 3P-3S transition and is mostly excited by direct impact of energetic electrons. However, the 130.4-nm triplet is optically thick and affected by both multiple scattering and absorption by CO2. The OI 135.6-nm emission corresponding to the 3P-5S forbidden transition has also been observed by EMUS (Jain et al., 2022).
To simulate the EMUS observations, a Monte-Carlo electron code associated with a photochemical model as well as a radiative transfer code for the 130.4-nm triplet are needed. Simulations of both emissions have been performed for mono-energetic electrons up to 1000 eV. We show that the atomic oxygen emissions are mainly produced by electron impact on O. While intensities vary with the input flux, their ratio only depends on the initial energy of the electrons and the oxygen density. The ratio appears then as a good proxy of the auroral electron energy.
We show that the I(130.4 nm/I(135.6 nm) nadir intensity ratio is expected to vary between ~2 and 9 with the initial electron energy and with a maximum at 70 eV. These values do not depend much on the oxygen density and the season.
According to EMUS observations, the ratio is higher in the strong crustal field region, suggesting that the auroral electrons in this area have energies lower than 200 eV.

References
Jain, S. et al. (2022), poster presented at AGU fall meeting.
Holsclaw, G. et al. (2021), Space Science Reviews, 217, 1-49.
Lillis, R. J. et al. (2022), Geophysical Research Letters, 49, e2022GL099820.
Soret, L. et al. (2016), Icarus, 264, 398-406.