Discrete aurora are sporadic ultraviolet emissions on Mars. These emissions, which
occur globally and in the upper atmosphere of Mars, are strongly correlated with martian crustal
magnetic field. Crustal fields on Mars form from remanent magnetism locked in the crust, and
vary in strength across the disk, with the strongest fields located in the southern hemisphere
Discrete aurora on Mars was first identified by the ultraviolet spectrograph (SPICAM) on
Mars Express (Bertaux et. al. 2005) which appeared as clustered, sporadic, and localized
emissions in the mid-ultraviolet. Further work conducted by the SPICAM team (Leblanc et al.,
2006, Leblanc et al., 2008, Gerard et al., 2015) expanded the list of detections to 19, 16 of which
were identified in nadir-viewing geometry. This work showed that (1) the lifetime of discrete
aurora is on the second timescale and (2) correlations between detection time and electron energy
measurements (taken by the ASPERA-3 instrument aboard the Mars Express) showed that
detection locations were highly correlated with locations of increased precipitating electron
fluxes (Gerard et al., 2015.) These SPICAM detections were plotted on top of a global crustal
field map (Brain et al., 2007) and found to be correlated with local field strengths. Analysis of
precipitating electron populations with contemporaneous SPICAM aurora detection times (Brain
et al., 2006, Gérard et al., 2015, Leblanc et al., 2006, Leblanc et al., 2008) with models of
electron energy deposition and resultant emission altitudes (Leblanc et al., 2006, Soret et al.,
2016) revealed the emission populations causing discrete aurora and altitude. These electron
populations are likely between 40-200 eV and peak near 130 km altitude (Brain et al., 2006,
Soret et. al. 2021.)
Later work, conducted 10 years later using MAVEN-IUVS observations, revealed the
first nadir-viewing full-disk image of discrete aurora on Mars. In this work, the IUVS team
identified a single orbit during the September 2017 space weather event with multiple discrete
aurora emissions (Schneider et al. 2018.) Due to the lack of a large nadir-viewing dataset of
detections, it remained unanswered at the time if discrete aurora were visible in other nadirviewing observations.
Analysis of MAVEN-IUVS limb observations confirms previously reported knowledge
of crustal field strength correlations with both detection location and brightness and auroral
emission altitude (Schneider et. al., 2018, 2021.) This analysis also showed events mainly
occurring before midnight with a preference for negative interplanetary magnetic field (IMF)
clock angles in the strong crustal field region (Schneider et al., 2021.) Due to observational bias

and spacecraft viewing limitations, this dataset did not have good morning coverage. It remained
unanswered how local time controlled where discrete aurora occurred.
The work presented here builds off previous understanding and examines an additional
seven thousand nadir-viewing observations of Mars using MAVEN-IUVS data. Previous studies
using data taken by the MAVEN spacecraft has revealed hundreds of detections in limb viewing
and a single detection in the nadir observations. Further analysis of all nightside MUV nadirviewing observations taken by the MAVEN Imaging Ultraviolet Spectrograph (IUVS)
instrument has revealed approximately 200 additional discrete aurora detections. While aurora
occur globally and sporadically, events in the strongest crustal magnetic field regions of the
Martian southern hemisphere show high repeatability. Previous work reported using IUVS limb
observations (Schneider et. al., 2021) revealed detections occurring mainly before midnight; the
emissions identified in the nadir dataset show detections both occurring before midnight and
after midnight in the strong field region. This independent and in-depth analysis has resulted in
an additional 183 detections, confirms previous understanding of discrete aurora, allows for
tighter confinement on detection locations and dimensions, and reveals a previously unreported
regional local time correlation of discrete aurora on Mars. The regional local time control
identified in this new dataset reveals two distinct auroral event groups manifesting pre- and postmidnight in separate but adjacent locations in the strong field region (figure 1), which may be
explained by magnetic field reconnection between local crustal martian fields and the draped
interplanetary magnetic field.

Figure 1. Locations of discrete aurora detections in the strong crustal field region
organized by local time, overplotted ono a magnetic field map of Mars. The blue-white colorbar
corresponds to the probability of open magnetic field lines, with dark blue being ~100 probably
for open field lines and white corresponding to closed loops. Red detections correspond to premidnight detection locations and black to post-midnight detection locations. There is a
noticeably clear distinction between pre-and-post midnight at -52° latitude and between 150° –
210° longitude. Analysis of detection frequency vs local time histograms shows that most premidnight detections from -35° to -52° latitude occur shortly after dusk. Post-midnight detections
from -52° to -67° latitude mainly occur before dawn.