Seasonal and Local Time Dependence of Martian FUV Discrete Aurora Observed by EMM EMUS
Authors: Chirakkil Krishnaprasad , Lillis Robert , Deighan Justin , Chaffin Michael , Jain Sonal , Brain David , Fillingim Matthew , Raghuram Susarla , Evans Scott , Holsclaw Gregory , Al Matroushi Hessa , England Scott , Al Mazmi Hoor , Ramstad Robin , Halekas Jasper , Espley Jared , Xu Shaosui , Fang Xiaohua , Schneider Nick , Curry Shannon 
Published: February 2023

Discrete aurorae are produced by charged particle precipitation (mostly electrons) into the upper atmosphere. Electron impact causes electronic excitations of atoms and molecules in the atmosphere, whose deexcitation releases ultraviolet photons. Discrete aurora was first discovered as an ultraviolet glow coming from &#8220;magnetic umbrellas&#8221; in the southern hemisphere. These are strong crustal magnetic field regions on Mars, which are remnants of a global field that decayed billions of years ago. Both Mars Express (Bertaux et al., 2005) and MAVEN (Schneider et al., 2021) have observed cases of discrete aurora events using their limb viewing observations. Emirates Mars Mission (EMM) provides the first synoptic (or disk) images of discrete aurora at Mars (Lillis et al., 2022), thanks to its large orbit and high sensitivity UV spectrograph.Using observations from Emirates Mars Ultraviolet Spectrometer (EMUS) onboard EMM, the geographic, local time and seasonal distributions of FUV discrete aurora in oxygen auroral emissions (130.4 nm and 135.6 nm) are investigated. Interesting local time asymmetry is observed in the aurora occurrence rates, brightnesses and emission line ratios. More aurora occurrence is observed during pre-midnight (dusk) as compared to post-midnight (dawn). Strong radial crustal field regions (SCFR) have aurora mostly during dusk, and not during dawn. Aurora also tend to occur more in open magnetic field regions away from SCFR. Brighter aurora is observed in the southern hemisphere during dusk, while in the northern hemisphere during dawn. Low brightness ratio [O I 130.4 nm/O I 135.6 nm] is observed in SCFR, but higher ratio in regions away from SCFR in the southern hemisphere. Also, the occurrence rate is found to be enhanced during the perihelion season as compared to the aphelion season. Statistical analysis of the dependence of discrete aurora on observation geometry, upstream solar wind and interplanetary magnetic field conditions will also be presented.References:[1] Bertaux, JL., Leblanc, F., Witasse, O. et al. (2005). Discovery of an aurora on Mars. Nature, 435, 790&#8211;794, https://doi.org/10.1038/nature03603.[2] Schneider, N. M., Milby, Z., Jain, S. K., G&#233;rard, J.-C., Soret, L., Brain, D. A., et al. (2021). Discrete aurora on Mars: Insights into their distribution and activity from MAVEN/IUVS observations. Journal of Geophysical Research: Space Physics, 126, https://doi.org/10.1029/2021JA029428.[3] Lillis, R. J., Deighan, J., Brain, D., Fillingim, M., Jain, S., Chaffin, M., et al. (2022). First synoptic images of FUV discrete aurora and discovery of sinuous aurora at Mars by EMM EMUS. Geophysical Research Letters, 49, https://doi.org/10.1029/2022GL099820.