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Abstract :
[en] MAVEN's Imaging Ultraviolet Spectrograph (IUVS) has made extensive use of mid-UV observations to advance our understanding of auroral processes at Mars. Recent studies have focussed on the concentration of discrete aurora around crustal fields, and their dependence on local time and magnetospheric conditions. The MUV emissions all arise from electron impact on CO2, which ionizes and dissociates the molecules, and causes emissions in the CO Cameron bands, the CO2+ ultraviolet doublet and the oxygen 297 nm emission.
This new study highlights discrete aurora emissions in the far-UV, specifically emissions from atomic oxygen at 130 and 135 nm. We find the FUV emissions are not simply scaled-down versions of their longer-wavelength counterparts. In fact, we find no simple patterns or correlations between the FUV and MUV, nor even between the 130 nm and 135 nm emissions. We even find faint FUV emissions present without any MUV emissions. Furthermore, the emissions at different wavelengths do not always have the shape or originate from the same locations. The figure below shows scan images of all confirmed 130 and 135 nm emissions, with confirmed simultaneous emissions at other wavelengths (if any) highlighted with a border.
We investigate multiple causes for these unanticipated differences: (1) electron impact on atomic oxygen instead of CO2, as oxygen becomes a significant fraction of the atmosphere at auroral altitudes; (2) absorption of FUV emissions by the background CO2 atmosphere, which does not affect MUV emissions; (3) optical thickness effects in resonant scattering of O 130 nm emission by atomic oxygen, which does not affect 135 nm emission, and (4) potential influences of observational viewing geometry.
We will report the results of these investigations, both in the context of interpreting the full MUV/FUV IUVS dataset and in understanding the high-sensitivity FUV/EUV discrete aurora observations made by the EMUS instrument on the Emirates Mars Mission.
