Numerical prediction of changes in atmospheric chemical composition by precipitation of solar energetic particles at Mars
Y. Nakamura, F. Leblanc, LATMOS, Sorbonne Université, Paris, France (yuki.nakamura@latmos.ipsl.fr),
N. Terada, I. Murata, H. Nakagawa, S. Sakai, S. Hiruba, Department of Geophysics, Tohoku University,
Sendai, Japan, R. Kataoka, K. Murase, National Institute of Polar Research, Tokyo, Japan; The Graduate
University for Advanced Studies, SOKENDAI, Kanagawa, Japan.
Introduction:
Solar energetic particles (SEPs) are highenergetic particles that consist mainly of electrons
and protons with energies from a few tens of keV to
GeV ejected from the Sun associated with solar
flares and coronal mass ejections. SEPs that precipitate into planetary atmospheres cause ionization,
excitation and dissociation of atmospheric molecules,
leading to changes in atmospheric chemical composition via chemical network (e.g. Solomon et al., 1981;
Adams et al., 2021). The effects of SEPs on the atmospheric chemical composition on Earth has been
intensively studied. For instance, during the enormous solar flare that occurred in late October 2003,
NOx (NO + NO2) and HOx (OH + HO2) concentrations were enhanced and ozone concentration was
depleted by 40% at the polar lower mesosphere (e.g.
Jackman et al., 2005). Recently, the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft
has discovered global diffuse aurora on the nightside
of Mars during SEP events (Schneider et al., 2015),
which has been shown to be generated mainly by
SEP protons (Nakamura et al., 2022). However,
changes in the neutral chemical composition of the
Martian atmosphere during SEP events have not yet
been estimated. By coupling a Monte Carlo model,
Particle TRansport In Planetary atmospheres
(PTRIP) (Nakamura et al., 2022) and a newly developed 1-dimensional photochemical model, we investigated the effects of SEPs on the atmospheric composition at Mars.
Models:
In this study, we used PTRIP to calculate production rate of CO2+ and N2+ in the Martian atmosphere
under a SEP proton flux taken from a SEP event that
occurred on 28 October 2003 at Earth (Mewaldt et
al., 2005). Production rates of N and N(2D), which
are important drivers for NOx chemistry, were estimated assuming a ratio N2+:N:N(2D) = 1:0.84:1
(Krasnopolsky, 2009). Ionization rate by cosmic rays
as an ionization source at low altitudes were taken
from Haider et al. (2009). In the 1-dimensional photochemical model, we implemented 486 chemical
reactions consisting of HOx chemistry (Chaffin et al.,
2017), NOx chemistry (Nair et al., 1994),
ionospheric chemistry (Fox and Sung, 2001) and
water cluster ion chemistry (Molina-Cuberos et al.,
2001; Verronen et al., 2016) to fully describe chemical network during SEP events.

Results:
We first ran the photochemical model only for
neutral species to obtain the steady state neutral
composition. After that we ran the model for 10 days
including ion species with solar extreme ultraviolet
flux and cosmic ray as ionization sources to obtain
the quasi-steady state solution for ion profiles. Then
we ran the model with additional ionization source of
SEP protons for 1 day to examine the changes in ion
and neutral compositions during SEP events.
The calculated ion profiles before SEP event and
after 1 day of SEP event onset is shown in Figure 1.
Ion and electron densities were enhanced below 100
km altitude and densities of water cations were enhanced below 70 km altitude. The results indicate
that ionization by SEP protons is more effective than
ionization by cosmic rays, producing large amount of
water cations below 70 km altitude, which agree well
with the previous modeling study by Sheel et al.
(2012).

Figure 1. (upper) Calculated ion profile before SEP
event and (bottom) after 1 day of SEP event onset.

The change in neutral chemical composition was
investigated. The calculated density profiles of
ozone, HOx and NOx before SEP event, 1day after
SEP event onset, 1 day after SEP event termination
and 10days after SEP event termination are shown in
Figure 2. Enhancement of HOx and decrease of
ozone by a factor of 10 occurred in the altitude range
of 20-60 km and enhancement of NOx by a factor of
more than 100 occurred in the altitude range of 20100 km 1 day after SEP event onset. After the end of
the SEP event, ozone and HOx densities were almost
recovered 1 day after the end of SEP event, however,
NOx density was not recovered even 10 days after
the end of SEP event, due to the different loss time
scales between HOx and NOx. These results indicate
that, as on Earth, similar effects on ozone, HOx and
NOx concentrations are expected even in the case of
the CO2-dominated Mars atmosphere.

Figure 2. Calculated density profiles of ozone, HOx
(OH + HO2) and NOx (NO + NO2) before SEP
event, 1 day after SEP event onset (represented as
“during SEP”), 1 day after SEP event termination
(represented as “recovery 1 day”) and 10 days after
SEP event termination (represented as “recovery 10
day”).

References:
Adams et al. (2021), Astrobiology, 21, 8,
doi:10.1089/ast.2020.2273.
Chaffin et al. (2017), Nature Geoscience, 10, 174178, doi:10.1038/NGEO2887.
Haider et al. (2009), J. Geophys. Res., 114,
A03311, doi:10.1029/2008JA013709.
Jackman et al. (2005), J. Geophys. Res., 110,
A09S27, doi:10.1029/2004JA010888.
Krasnopolsky (2009), Icarus, 201, 1, 226-256,
doi:10.1016/j.icarus.2008.12.038.
Mewaldt et al. (2005), J. Geophys. Res., 110,
A09S18, doi:10.1029/2005JA011038.
Molina-Cuberos et al. (2001), Adv. Space Res., 27,
11, 1801-1806, doi:10.1016/S0273-1177(01)003428.

Nair et al. (1994), Icarus, 111, 1, 124-150,
doi:10.1006/icar.1994.1137.
Nakamura et al. (2022), J. Geophys. Res.:
Space
Physics,
127,
e2021JA029914,
doi:10.1029/2021JA029914.
Schneider et al. (2015), Science, 350(6261),
aad0313, doi:10.1126/science.aad0313.
Sheel et al. (2012), J. Geophys. Res., 117, A05312,
doi:10.1029/2011JA017455.
Solomon et al. (1981), Planet. Space Sci., 29, 8,
885-892, doi:10.1016/0032-0633(81)90078-7.
Verronen et al. (2015), J. Adv. Model. Earth Syst.,
8, 954-975, doi:10.1002/2015MS000592.