Non-migrating atmospheric tides as a driver of variability in the nightside Martian ionosphere
S. A. Thaller, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO,
USA (scott.thaller@lasp.colorado,edu), L. Andersson, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA.

Understanding the dynamics of the nightside ionosphere of
Mars has presented more of a challenge than those on the
dayside due to the nightside being more difficult to characterize as a result of the plasma density being lower and
more variable there [e.g., Girazian et al., 2017; Nemnec et
al., 2010]. Causes of this enhanced electron density variability are not yet fully understood, but have been studied in
the context of ionizing electron precipitation [e.g., Fowler
et al., 2015; Lillis et al., 2018], combination of ionizing
electron precipitation and horizontal transport from the
dayside [e.g., Adams et al., 2018], composition and chemistry [Girazian et al., 2017], and solar flare events [Harada,
et al., 2018]. One possible source of nightside variability is
nonmigrating tides in the neutral atmosphere. It had been
shown that nonmigrating tides do play a role in the dayside
ionospheric variability [e.g., Bougher et. al 2001;
Mendillo et al., 2017; Fang et al 2021; Thaller et al., 2020;
2021]. Altitude profiles of the amplitude of the normalized
longitudinal electron density variations on the night side
have been presented by Thaller et al. [2020; 2021], but
they did not investigate the cause of those variations. The
study presented herein is motivated by the nightside observations of significant (~40% relative to the background)
longitudinal variations in electron density presented in
Thaller et al [2021], and the goal of understanding whether
and to what extent those electron density variations result
from the tidal variations observed in the neutral atmosphere, also reported in Thaller et al. [2021]. In doing so,
this investigation contributes to the broader goal of understanding the nightside dynamics of the Mars ionosphere.
In this study, we investigate whether, on the nightside of
Mars, the variation in ionospheric electron density (with
respect to the zonal mean electron density) as function of
longitude, constructed from 20-day intervals of data, is the
result of nonmigrating atmospheric tides and ionospherethermosphere (IT) coupling. Here, nightside is defined as
solar zenith angles > 110o, comprising three more specific
regions, namely, pre-midnight, deep-midnight, and postmidnight. This investigation uses a combination of in situ
neutral and electron density measurements by the Mars
Atmosphere and Volatile EvolutioN (MAVEN) spacecraft
and a simple model. The in situ neutral abundance and
electron density measurements are made by MAVEN’s
Neutral Gas and Ion Mass Spectrometer (NGIMS) and the
Langmuir Probe and Waves (LPW) instruments, respectively. From these in situ datasets, longitudinal profiles at
discrete altitudes of the density variations (percent variation with respect to the zonal mean) are produced and
studied both individually and statistically. Instances in
which the neutral and electron zonal density perturbations
either correlate or anti-correlate with one another are
found, suggesting the possibility of a causal relation. The
simple model consists of an approximate expression for
electron density as function of the neutral density, derived
from the plasma fluid equations for the conditions that ion-

neutral collisions are important, vertical transport is allowed, and the chemical and diffusion timescales are
shorter than the tidal wave period allowing for the system
to maintain approximate equilibrium. The inputs for the
model are sets of three neutral density values, one +20%
and another -20% about some baseline (zonal mean) neutral density which is the third value in the set. The range of
baseline values for the neutral densities used corresponds
to that observed by MAVEN in the region studied. The
amplitude of 20% for the neutral variations was chosen
based on the observed amplitudes of the tidal density variations in the neutral atmosphere. The model outputs are
the percent variation of the electron density from a baseline electron density. The amplitudes of the variation in the
electron density are plotted against the baseline neutral
densities, and compared to those found from the MAVEN
data. The comparison between the electron density variations predicted by the simple model and those determined
from the MAVEN data shows that, to first order, the simple model does a reasonable job predicting the amplitude
of the electron density variations as function of the neutral
density undergoing tidally driven perturbations. Another
result of the study is that the region where the IT coupling
transitions between being dominated by ionospheric chemistry, at lower altitudes (higher neutral densities), to where
diffusion becomes increasingly important, at higher altitudes (lower neutral density), is the region where tidal
variations in the neutral atmosphere result in quicky increasing amplitudes in the electron density variations with
increasing altitude (decreasing neutral density). These
results lead to the conclusion that nonmigrating tides in the
neutral atmosphere do contribute to driving variations in
the nightside ionosphere. Models with the aim of reproducing the structure and variability of the nightside ionosphere should thus include the tidal motions of the neutral
atmosphere.

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