Temporal and Spatial Variability of the Martian Hot Oxygen Corona During the MAVEN Mission

The dominant loss mechanism of oxygen from Mars to space in the current epoch is thought to be photochemical escape of non-thermal "hot" oxygen produced by dissociative recombination of O₂⁺. This ion is ultimately sourced from CO₂⁺, which is the primary product of photoionization in the upper atmosphere. The escaping hot oxygen population is accompanied by a gravitationally bound hot oxygen corona produced by the same mechanism. The MAVEN spacecraft has been at Mars since November 2014, with nearly continuous observations using the IUVS instrument to measure the hot oxygen corona via fluorescence of the O I 130.4 nm triplet. This provides the opportunity to examine temporal variations associated with changes in the photoionizing solar EUV radiation which produces CO₂⁺ and O₂⁺ ions. We present results based on the MAVEN mission to date, extending from L_S = 270 in Mars Year 32 near the maximum of Solar Cycle 24 until L_S = 60 in Mars Year 35 at the end of the declining phase of the same Solar Cycle. The data exhibits variations due to the 27-day solar rotation period, seasonal effects, and the overall decline in solar activity. The peak brightness of ionospheric CO₂⁺ UVD emission from IUVS limb scans is also used as a direct indicator of the photoionization frequency. As expected, there is a strong correlation between solar EUV input, observed ionization frequency, and the density of the hot oxygen corona. In addition, the 1D coronal scans can provide 3D shape information about the hot oxygen corona by leveraging the precession of the MAVEN orbit and using complimentary observations on opposite sides of the spacecraft orbit. Analysis reveals significant hemispherical asymmetries, giving insight into how the structure of the Martian upper atmosphere modulates the hot oxygen corona and its escape to space.