GLOBAL STRUCTURE AND VARIABILITY OF THE INNER HOT
OXYGEN CORONA AS IMAGED BY EMM/EMUS
J. Deighan1 (justin.deighan@lasp.colorado.edu), Michael Chaffin1, Krishnaprasad Chirakkil1,2, Hessa Rashid Almatroushi3, Robert J. Lillis4, Matthew O. Fillingim5, Scott England6, Sonal Jain1, Greg Holsclaw1,
Fatma Hussain Lootah7, Hoor Abdelrahman AlMazmi8, F. G. Eparvier1, E. M. B. Thiemann1, P. C.
Chamberlin1
(1)Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, United
States,(2)Space and Planetary Science Center, Khalifa University of Science Technology and Research, Abu
Dhabi, United Arab Emirates, (3)Mohammed Bin Rashid Space Center, Al Khawaneej, United Arab Emirates,
(4)Space Sciences Laboratory, University of California Berkeley, Berkeley, CA, United States, (5)University of
California, Berkeley, CA, United States, (6)Virginia Polytechnic Institute and State University, Aerospace and
Ocean Engineering, Blacksburg, VA, United States, (7)Mohammed Bin Rashid Space Centre, Dubai, United
Arab Emirates, (8)UAE Space Agency, Abu Dhabi, United Arab Emirates

Introduction:
The hot atomic oxygen corona of Mars is composed of a hyper-thermal population (energies of
order ~ 1 eV) resulting from solar EUV driven photochemistry in the planet’s upper atmosphere. A fraction of this population has sufficient velocity to escape from the planet, representing a mechanism for
loss of atmosphere that has driven planetary evolution over the history of the solar system. The gravitationally bound portion of the corona extends to many
planetary radii, but is mostly confined within the first
couple radii of the planet. At distances within ~1/2
Martian radius above the surface, the structure of
corona is expected to retain the strongest imprint of
any spatial variations in the photochemical source
around the globe. At more distant altitudes of multiple Martian radii the corona is expected to become
more uniform as spatial variations become more
globally averaged.
Previous Work:
The hot oxygen corona is directly observable via
remote sensing in the far ultraviolet by measuring
solar resonant fluorescence of the O I 130.4 nm
emission line. However, the signal is quite dim (1-10
Rayleighs within the first Martian radius), and very
sensitive instrumentation is required for a detection,
let alone sampling profiles or capturing images. Detection at a few select altitudes at the base of the corona was first accomplished by the ALICE instrument aboard the Rosetta mission during a Mars flyby (Feldman et al. 2011). The MAVEN IUVS instrument has been orbiting Mars since 2014, and has
been collecting vertical 1D profiles of hot oxygen
within the first Martian radius at a roughly daily cadence (Deighan et al. 2015). Interpretation of the
IUVS data has three key complicating factors (1) the
1D profiles are obtained while MAVEN is deeply
embedded within the hot oxygen corona structure,
(2) the roughly daily sampling cadence makes longi-

tudinal information sparse, and (3) the local time
sampling varies with the MAVEN orbit precession
over the course of months, during which time there
can be significant variations in season and solar activity. These factors present challenges in
deconvolving the drivers of the observed O I 130.4
nm brightness in order to reconstruct a global 3D
description of the hot oxygen.
Current Work:
We report here on an extensive new set of data
for studying the Martian hot oxygen corona that is
now available from the Emirates Mars Ultraviolet
Spectrometer (EMUS) aboard the Emirates Mars
Mission (EMM). The EMUS instrument was designed with a driving requirement for high sensitivity
and low background in order to obtain comprehensive 2D images of the inner Martian corona
(Holsclaw et al. 2021). In addition, the EMM spacecraft orbit (19,970 km periapse altitude, 42,650 km
apoapse altitude, 25° inclination, 54.5 hour period) is
configured to allow imaging of the planet from outside the majority of the corona across a wide range of
positions around the planet with a full image set cadence of at least once per week (Amiri et al. 2022,
Almatroushi et al. 2021). These key features enable
major advancements in our understanding of the
structure and variability of the hot oxygen corona,
giving insight to its drivers and its connection to atmospheric loss.
Data:
Here we present results from the EMUS UOS-2
observations which image the inner corona out to at
least 1800 km from at least six points of view around
a single EMM orbit in order to provide 3D information about the structure of the corona. These image sets are obtained at least once per week (approximately 3 orbits), and occasionally on every orbit
during a “high cadence” week every 15 of LS in
order to look for short term variability. With the

nominal science mission beginning 23 May 2021,
there are now multiple Mars seasons of data available and a modest range of solar activities to examine
as Solar Cycle 25 begins ramping up.
Method:
Due to the highly sensitivity nature of the measurement, faint background stars in the sky represent a
major nuisance. Affected data are identified and removed from the analysis by interrogating the spectrum for continuum signal at 133–161 nm.
To examine spatial structure we compare the
EMUS images to a simple 3D model of the hot oxygen corona density that includes an inverse radial
power law dependence and a cosine power law dependence in solar zenith angle. This two parameter
model captures the most basic structure of the
photochemically generated corona and allows for a
reference point by which asymmetries and trends can
be identified. The density model is integrated
through for every line of sight in the observations in
order to provide a direct comparison to the measured
brightnesses.
To examine temporal variability we average the
dayside corona (SZA > 70) across altitudes of 7001500 km for each observation to provide a consistent
geometry for sampling across time. This criterion
ensures good signal and also avoids the denser thermal oxygen corona that dominates at lower altitudes.
Results:
We find while that the simple reference model of
the inner hot oxygen coronal density can reproduce
the gross structure of the observations, it tends to
systematically under predict the brightness in the
dimmer areas of the EMUS images, both at higher
altitudes and on the nightside of the planet. Interrogation of the spectra confirm the presence of O I
130.4 nm signal and not a spurious background. We
interpret this behavior as a foreground contribution
of the hot oxygen population residing in the extended
middle corona (1.6-6 Martian radii), a region through
which the EMUS line of sight must pass when observing the inner corona (< 1.6 Martian radii) from
the elevated orbit of EMM.

creased over the EMM mission to date, due to a
combination of the Mars-Sun distance and increase
solar activity. This is consistent with other indicators
for airglow variability measured on the disk of the
planet, namely O I 130.4 nm reflecting from the
thermospheric cold atomic oxygen population and C
I 128.0 nm emission from CO2 dissociation. It is also
well correlated with direct measurements of solar
activity as recorded by the MAVEN EUVM instrument (Eparvier et al. 2015).

Figure 2. Timeline of average dayside inner hot
oxygen brightness measured by EMM/EMUS.
References:
Feldman, P. D., et al. (2011), Rosetta-Alice observations of exospheric hydrogen and oxygen on
Mars, Icarus, 214, 394–399,
doi:10.1016/j.icarus.2011.06.013
Deighan, J., Chaffin, M. S., Chaufray, J.-Y., Stewart,
A. I. F., Schneider, N. M., Jain, S. K., Stiepen,
A., Crismani, M., McClintock, W. E., Clarke, J. T.,
et al. (2015), MAVEN IUVS observation of the hot
oxygen
corona
at
Mars, Geophys.
Res.
Lett., 42, 9009– 9014, doi:10.1002/2015GL065487
Holsclaw, G.M., Deighan, J., Almatroushi, H. et
al. The Emirates Mars Ultraviolet Spectrometer
(EMUS) for the EMM Mission. Space Sci
Rev 217, 79 (2021). https://doi.org/10.1007/s11214021-00854-3
Amiri, H.E.S., Brain, D., Sharaf, O. et al. The Emirates Mars Mission. Space Sci Rev 218, 4 (2022).
https://doi.org/10.1007/s11214-021-00868-x

Figure 1. Data/Model/Ratio comparison for the
three swaths of a UOS2 observation.

Almatroushi, H., AlMazmi, H., AlMheiri, N. et
al. Emirates Mars Mission Characterization of Mars
Atmosphere Dynamics and Processes. Space Sci
Rev 217, 89 (2021). https://doi.org/10.1007/s11214021-00851-6

In terms of temporal variability, we find that that
the hot oxygen coronal brightness has generally in-

Eparvier, F. G., P. C. Chamberlin, T. N. Woods,
and E. M. B. Thiemann (2015), The solar extreme

ultraviolet monitor for MAVEN, Space Sci. Rev.,
doi:10.1007/s11214-015-0195-2