Far Ultraviolet carbon emissions in the Mars aurora: Brightness, intensity ratios and seasonal dependence

Neutral carbon emissions have been observed in Far Ultraviolet spectra (FUV) of the Martian aurora with the Emirates Mars Ultraviolet Spectrometer (EMUS) instrument on board the Emirates Mars Mission (EMM). Oxygen emissions at 130.4 and 135.6 nm have been used to map the auroral morphology and its dependence on solar wind parameters. The CI emissions at 156.1 and 165.7 nm are excited by collisions between energetic electrons and CO2, CO and C. We present Monte Carlo simulations of the altitude distributions of both features showing that electron collisions with ambient CO2 molecules are the dominant source, followed by collisions on CO, while direct impact on C atoms is several orders of magnitude smaller. Since the C atom fragments resulting from dissociation are produced with a speed exceeding by far the thermal velocity, the optical depth is negligible. Consequently, the 156.1 and 165.7 nm photons are not self-absorbed, and the calculation of the emission rates does not require consideration of radiative transfer, unlike the OI 130.4 nm triplet. We present calculations of the efficiency of the carbon emissions for a range of incident auroral electrons energies. We illustrate the energy dependence of the intensity ratios between the oxygen 130.4 and 135.6 nm and the carbon 165.7 nm emissions. These ratios may be used to remotely characterize the spatial distribution of the incident electron energies in spectral FUV maps of the aurora.