Exploring Mars discrete aurora with synoptic images and movies from EMM EMUS

Benefiting from a large orbit and high sensitivity, the Emirates Mars mission EMUS instrument has provided the first opportunity to synoptically and regularly image Mars’ discrete FUV auroral oxygen emission at 130.4 and 135.6 nm.  Over 15-20 minutes, EMUS produces a) images by slewing its aperture slit across the disk or b) “movies” of narrow regions by staring continuously.

Discrete aurora are observed primarily where the magnetic topology is open (i.e. connected to the collisional atmosphere at one end), which occurs where Mars’ crustal magnetic fields are either very weak or primarily vertical.  Discrete aurora show a strong local time dependence, with occurrence % decreasing with increasing solar zenith angle.  The highest occurrences are generally found in the post-dusk sector, before 10 PM SLT, though a few regions (e.g. 60°-70° S, 120°-150° E) are brightest between midnight and 3 AM.   

Sinuous discrete auroras (SDA) are enigmatic, sharply-defined filamentary emissions identified in approximately 3% of observations. These emissions intersect Mars' UV terminator, aligning generally away from the Sun, tending to cluster into groups oriented to the north, south, east, and west. The occurrence of SDAs increases with higher solar wind pressure. SDAs have a tendency to form toward the direction of the solar wind convection electric field (i.e., forming in the +E hemisphere). Depending on whether they originate near dusk or dawn, there is a moderate clockwise or counterclockwise "twist" observed in the average orientation of SDAs, respectively. Based on these characteristics, we infer a connection between SDAs and Mars' magnetotail current sheet, suggesting that the emission may be a result of energized electrons within this sheet.

Lastly, near the dawn and dusk terminators, discrete aurora often display a preference for formation in regions of either positive or negative crustal magnetic field, depending on IMF direction.  This preference can be used to determine whether a dayside (magnetosheath or photoelectron) or nightside (magnetotail) source of electrons is dominant.  Overall, nightside sources dominate over dayside by 20-40%, although individual radial crustal fields can show strong preferences for day or night sources.  This tells us that local magnetic geometry plays a role in global precipitation patterns.

With more than 3000 nightside images and 400 aurora movies collected (totaling more than 12 million pixels) since April 2021, we now have a powerful tool to understand Martian aurora morphologies, variability, and dependence on internal and external drivers.