Seasonal variations of Mg and Ca in the exosphere of Mercury
Résumé
We simulate the seasonal variations of the Mg 285.3 nm and Ca 422.7 nm brightness and compared our results to the MESSENGER/MASCS observations at dawn. Our results are consistent with the previous studies of Ca while for Mg we used another seasonal variation for the g -value (excitation frequency) at 285.3 nm. We find that both emissions are well reproduced from micrometeoroid impacts when the true anomaly angle (TAA) of Mercury is larger than 80°. For true anomaly angle lower than 80°, an additional source is needed to reproduce the Ca observations in agreement with previous studies, and possibly the Mg observations. We compare several solar spectra (observed or modeled) to study the Mg g-value and found that the seasonal variation of the g-value peaking near TAA = 60° used by previous studies to analyse the MESSENGER observations of Mg may be due to an artefact not present in the solar spectrum. The observed seasonal variations of the Mg brightness are better reproduced without this artefact. However, observations of the solar spectrum near 285.3 nm at a spectral resolution of ~20 mA would be needed to better estimate the seasonal variations of the Mg excitation frequencies and then to better understand the possible differences in the source of these two species in the exosphere of Mercury.
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