Production and radiative transfert of the OI 130.4 and 135.6 nm emissions in the Mars aurora

The presence of a weak oxygen emission at 130.4-nm resulting from the O ³P-³S transition was first detected in limb observations of the Martian aurora with SPICAM/Mars Express (Soret et al., 2016). It is mostly excited by direct impact of energetic electrons on ground-based O(³P) atoms, but the 130.4-nm radiation is affected by multiple scattering and absorption by CO₂. In April 2021, the Emirates Mars Ultraviolet Spectrometer (EMUS) instrument (Holsclaw et al., 2021) on board the HOPE Emirates orbiter started collecting spectral images in the 110-180 nm range with a much increased sensitivity. The OI 135.6-nm emission corresponding to the ³P-⁵S forbidden transition has also been observed (Jain et al., 2022). In addition, EMUS is taking images of the discrete and sinuous aurora at 130.4 nm. We present Monte Carlo model simulations of the production of the O ³S and ⁵S excited states for different initial electron energies and discuss possible seasonal variations. We solve the radiative transfer equation for the 130.4-nm triplet and show that the I(130.4 nm/I(135.6 nm) nadir intensity ratio is expected to widely vary with the initial electron energy. These variations result from two effects:

• The different shapes of the two emission cross sections since the optically thick ³P-³S resonance transition is permitted while ³P-⁵S is forbidden
• the radiation entrapment of the 130.4 nm triplet by atmospheric atomic oxygen coupled with absorption by CO₂.

We also discuss the sensitivity of the 130.4-nm nadir brightness to the energy distribution of the incoming auroral electrons.

 

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.