FIRST GLOBAL IMAGES OF MARS FUV DISCRETE AURORA FROM THE EMIRATES MARS
MISSION EMUS INSTRUMENT

R. J. Lillis1, J. Deighan2, S. Jain2, G. Holsclaw2, M. Chaffin2, M. Fillingim1, K. Chirakkil2, 3, S. England4, D. A. Brain2, H. Al
Matroushi5, F. Lootah5, H. Al-Mazmi5, S. Xu1, X. Fang2, N. Schneider2, J. S. Halekas6, S. Ruhunusiri6, J. Espley7

UC Berkeley Space Sciences Laboratory (rlillis@berkeley.edu), 2Laboratory for Atmospheric and Space Physics,
University of Colorado Boulder, 3Khalifa University, UAE, 4Virginia Institute of Technology, 5Mohammed bin
Rashid Space Center, Dubai, UAE. 6University of Iowa Department of Physics and Astronomy, 7NASA Goddard
Space Flight Center
1

Mars Discrete Aurora: current understanding
Three major types of aurora have been discovered to
occur in the Mars upper atmosphere: diffuse aurora [1],
proton aurora [2], and discrete aurora [3]. The former
two are produced by broad energetic charged particle
precipitation during extreme solar wind disturbances
and direct precipitation of solar wind protons,
respectively. The latter is typified by discrete regions
of ultraviolet emission from atoms and molecules
electronically excited by the precipitation of
accelerated electrons into the Martian thermosphere [4,
5]. It has been observed from both and disk and limb
observations and is most common in regions of strong,
topologically open crustal magnetic fields. Aurora
observations covering significant fractions of a
hemisphere have been observed by MAVEN rarely and
only in the mid ultraviolet (> 200 nm) [6].
Data Set: Here we report the first synoptic (or “disk”)
observations of Martian discrete aurora in the far
ultraviolet (<200 nm). These are made by the Emirates
Mars Ultraviolet Spectrograph (EMUS) on board the
Emirates Mars Mission (EMM) and are well-suited to
studying aurora due to a) EMM’s high altitude vantage
point and regular observation cadence and b) the high
sensitivity of the EMUS instrument. The specific
auroral observations are from the U-OS2 observation
mode designed to study Mars’ inner corona and
includes significant coverage of the night side [7], as
shown in figure 1.

Figure 1: (left) EMM orbit and typical observation set for
U-OS2. Right: box comprised of observation swaths
across Mars' inner corona (<1.6 Mars radii).

From April 1 until December 31, 2021, 194 swaths
provide sufficient coverage of the nightside to study
discrete FUV aurora (defined as >30% of disk pixels
have solar zenith angle >110°).
Spectrum: Emission is strongest at the 130.4 nm
oxygen line (3S decay to ground state 3P), though it is
also observed in oxygen features at 98.9 nm, 102.7 nm,

and 135.6 nm, as well as possible emission in the CO
fourth positive group bands, as shown in the typical
spectrum in figure 2.

Figure 2: Typical night side Martian disk spectrum, for
both auroral and non-auroral regions.

Results: overlapping swaths can be considered
individually or stitched and averaged together to make
a single synoptic image, as shown in figure 3. Discrete
aurora show patterns broadly matching expectations
(i.e. occurring more commonly in strong, open crustal
field regions, as in figure 3, left). A notable exception
is that the brightest aurora occurred away from crustal
fields during the coronal mass ejection impact of July
21, 2021, during which discrete aurora are seen in
weak crustal field regions (figure 3, right). Detectable
aurora (>2 R in clear coherent patches) are detectable
in approximately 67% of observations, are more
common in the dusk versus dawn quadrants, and
exhibit significant variability over timescales of ~20
minutes, reflecting natural variability in electron
dynamics and magnetic field topology in the Mars
near-space environment.
Looking forward: These synoptic images, combined
with simultaneous measurements of suprathermal
electrons and magnetic geometry and topology from
MAVEN and Mars Express promise to elucidate the
complex plasma processes driving Mars’ enigmatic
aurora.
References: [1] Schneider, N. M., et al. 2015, Science,
350, [2] Deighan, J., et al. 2018, Nat Astron, 2, 802,
[3] Bertaux, J. L., et al. 2005, Nature, 435, 790, [4]
Brain, D. A., et al. 2006, Geophysical Research
Letters, 33, [5] Xu, S., et al. 2020, Geophysical
Research Letters, 47, e87414, [6] Schneider, N. M., et
al. 2018, Geophysical Research Letters, 45, 7391, [7]

Holsclaw, G. et al. 2021, Space Science Reviews, 217,
79.

Figure 3: Examples of discrete Martian FUV aurora at 130.4 nm (emission in Rayleighs, log scale). Contours show Mars'
crustal magnetic field (model from Langlais et al. [2019]) at 400 km altitude, where pink and turquoise are +10 nT and 10 nT, respectively. The left image is an example of aurora occurring in regions of strong vertical crustal magnetic field
in Terra Sirenum, while the right image (the brightest seen thus far) occurred during the passage of an interplanetary
Coronal Mass Ejection (ICME).