Far ultraviolet carbon and oxygen emissions in the Martian aurora as a diagnostic of electron precipitation

The EMUS far ultraviolet spectrometer on board the Emirates Mars Mission orbiter provides unique observations about the morphology and local time dependence of the atomic oxygen emission at 130.4 nm. However, this triplet transition is optically thick, so that its brightness does not directly depend on its production rate. Consequently, radiative transfer analysis is required to link its production rate to the observed nadir or limb observations.

In contrast, the intensity of the 156.1 and 165.7 nm carbon lines directly reflects the production of the corresponding excited states. We show that their dominant sources are collisions with CO₂ and CO. Based on Monte Carlo simulations, we investigate the dependence of the observed brightness with the energy of the auroral electrons. We show how the ratio of the carbon line intensity to that of the OI optically thick 130.4 nm and optically thin 135.6 nm emissions may be combined as a remote sensing diagnostic of the characteristics of the auroral electron precipitation. The sensitivity of this method to the seasonal variations of the composition is also analyzed.