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# The effects of atmospheric dust and solar radiation on the dayside ionosphere of Mars derived from 17 years of Mars Express radio science observations

Kerstin Peter, M. Pätzold, L. Montabone, E. Thiemann, F. González-Galindo,

O. Witasse, S. Tellmann, M. K. Bird

► To cite this version:

Kerstin Peter, M. Pätzold, L. Montabone, E. Thiemann, F. González-Galindo, et al.. The effects of atmospheric dust and solar radiation on the dayside ionosphere of Mars derived from 17 years of Mars Express radio science observations. Icarus, 2023, 400, 10.1016/j.icarus.2023.115565. insu-04195484

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https://insu.hal.science/insu-04195484v1

Submitted on 16 Sep 2025

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# The effects of atmospheric dust and solar radiation on the dayside ionosphere of Mars derived from 17 years of Mars Express radio science observations

5 Kerstin Peter (1), M. Pätzold (1), L. Montabone (2,3), E. Thiemann (4), F. González-Galindo (5), O.

6 Witasse (6), S. Tellmann (1), and M. K. Bird (1,7)

7 (1) Rhenish Institute for Environmental Research, Cologne, Germany, (Kerstin.peter@uni-koeln.de)

8 (2) Laboratoire de Météorologie Dynamique, IPSL/CNRS/Sorbonne Université, Paris, France

9 (3) Space Science Institute, Boulder, CO, USA (Imontabone@spacescience.org)

10 (4) University of Colorado at Boulder, Laboratory for Atmospheric and Space Physics, Colorado, USA

11 (Ed.Thiemann@lasp.colorado.edu)

12 (5) Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain (ggalindo@iaa.es)

13 (6) Research and Scientific Support Department of ESA, ESTEC, Noordwijk, The Netherlands

14 (owitasse@cosmos.esa.int)

15 (7) Argelander Institut für Astronomie, Bonn, Germany (mbird@astro.uni-bonn.de)

## 16 Abstract

17 This work combines 17 years of Mars Express radio science (MaRS) observations with proxies for
18 insolation and local/global atmospheric dust to investigate the combined and individual effects on
19 the dayside ionosphere of Mars from the top down to the ionospheric base.

20 The increase in insolation from orbital apocenter to pericenter in combination with Mars' dust cycle
21 causes an average rise of the whole photochemically dominated region of the dayside ionosphere,
22 ranging from 13 km at the ionospheric base up to 22 km above the main peak during conditions
23 without a global dust storm. The declining phase of the 2018 global dust storm was observed by
24 MaRS on the southern hemisphere and close to pericenter. The observed lifting effect on the whole
25 photochemically dominated region of the ionosphere from the increased insolation and the high
26 local and global atmospheric dust levels exceeds that seen by MaRS from similar seasons during
27 years without a global dust storm.

28 The average ionospheric peak altitude at the subsolar point rises for increasing levels of local
29 atmospheric dust until a maximum elevation is reached. This maximum depends on the available
30 insolation at the top of the planetary atmosphere. Further increases of the local atmospheric dust
31 levels do not lead to a further rise of the average ionospheric peak altitude in the investigated data
32 set. This indicates a limit for the warming/expansion of the lower neutral atmosphere and the
33 consecutive lifting of the ionosphere based on the available insolation and explains why regional dust
34 storms can cause a similar lifting of the ionospheric main peak region as global dust storms.
# 1. Introduction

The ionosphere of Mars originates from the primary and secondary ionization of the planet's neutral atmosphere by solar radiation. On the dayside ionosphere, photochemistry is the dominant process below ~180 km altitude, while plasma transport processes dominate above that altitude. The two main features of the undisturbed Martian dayside ionosphere are the main peak region M2¹ (Figure 1), which is primarily caused by solar radiation in the Extreme Ultraviolet and the secondary layer M1, which is mainly produced by the primary and secondary impact ionization of soft solar X-rays < 10 nm (Fox et al. 1996). Below M1, Pätzold et al. (2005) discovered small-scale sporadic electron density structures in Mars Express (MEX) Radio Science (MaRS) observations which were attributed to the ablation and ionization of metallic atoms from infalling meteoroids (e.g. Pesnell and Grebowsky 2000, Molina-Cuberos et al. 2003, Whalley and Plane 2010). Detailed investigations by Peter et al. (2021) revealed that a subset of identified disturbances could be attributed to the ionization of the local neutral atmosphere by solar X-ray radiation < 2nm. An exclusive meteoric origin of the identified features is unlikely (Crismani et al. 2017). One recurrent feature in the highly variable ionospheric topside is shown in Figure 1a. The local enhancement in electron densities above the main peak, effectively a topside "bulge", has been addressed early by the MaR team (e.g. Peter et al. 2014), and is also referred to in the literature as the second layer (Gurnett et al. 2008), transient topside layer (Kopf et al. 2008, 2017) and M3 (Mayyasi et al. 2018). Current theories include energy deposition by solar wind electron precipitation or Kelvin-Helmholtz plasma instabilities. The true origin of this feature is, however, still unclear.

The Chapman theory (Chapman 1931a, 1931b) describes the basic features of the ionosphere formation for a horizontally stratified, isothermal, single-species neutral atmosphere with constant scale height in photochemical equilibrium which is ionized by a single solar wavelength. The electron density of the ideal ionosphere n in dependence of altitude h and solar zenith angle χ is described by

$$n(h, \chi) = n_0 \exp \left( \frac{1}{2} \left[ 1 - \frac{h-h_0}{H} \right] - \frac{1}{\cos \chi} e^{-(h-h_0)/H} \right) \quad (1)$$

where H is the neutral scale height and $n_0$ and $h_0$ are the peak electron density and the peak altitude at $\chi=0°$, respectively. Chapman theory indicates, that the idealized ionospheric peak forms at that altitude where the optical thickness for the ionizing wavelength is $\tau=1$ and that the ionospheric peak at a fixed $\chi$ forms at a constant pressure level. The increase of atmospheric density at a certain neutral atmospheric level below the ionospheric peak therefore leads to an increase in peak altitude (see associated equations in Girazian et al. (2020), illustrated by general circulation model calculations in Fig. 5 of González-Galindo et al. (2013)). It is expected from Chapman theory that the

<footnote n="1">¹ Nomenclature after Rishbeth and Mendillo (2004).</footnote>
<figure><img src="image_3.png" /></figure>

Figure 1: The dayside ionosphere of Mars observed by MaRS on (a) Day of Year (DoY) 065 (2018) before and (b) DoY 225 (2018) during the declining phase of the 2018 global dust storm. The gray dashed line indicates the noise level of the observation (3σn); the horizontal black dashed line marks the lowest valid altitude (h<sub>lva</sub>) of the individual observation (details about the individual parameter derivation are found in Peter et al. (2021)). CDOD<sub>G</sub> in Panel (a) is shown for completeness only. It is not used in further analysis because the observational coverage in the associated dust map is only 63% of the planetary surface for the time of the observation (details in Section 2).

ideal peak electron density $n_m$ decreases for increasing solar zenith angle with $n_m = n_0\sqrt{\cos\chi}$, that the peak altitude increases with $h_m = h_0 - H \ln(\cos\chi)$ and that the associated integrated Total Electron Content (TEC) of the ionosphere approximately decreases with $4.13n_0H\sqrt{\cos\chi}$ for increasing solar zenith angle (see e.g. Withers (2009)).

Equation 1 is valid for a horizontally stratified neutral atmosphere. For larger solar zenith angles the curvature of the planetary atmosphere becomes increasingly important in the ionospheric formation process. It is therefore feasible to replace the expression $1/\cos\chi$ in (1) by the "Chapman function for grazing incidence angle" $ch(X_p, \chi)$, where $X_p=r_p/H$ and $r_p$ is the distance between the point of interest P and the planet center. This work applies the numerical approximation $ch^*(X_p, \chi)$ of Smith and Smith (1972) for $ch(X_p, \chi)$, which is sufficiently accurate for higher solar zenith angles.

In recent years the described connection between the M2 peak altitude and the state of the lower neutral atmosphere was used to investigate the impact of e.g. Mars seasons, regional/global dust storms and atmospheric tides. The observed seasonal variations of the Mars ionosphere can be divided into obliquity and orbital investigations. The large Mars orbit eccentricity of 0.09 leads to a roughly 45% increase in solar insolation at the subsolar point from aphelion ($L_s^{2}=71^\circ$) to perihelion ($L_s=251^\circ$). Mars' perihelion is close to the south hemisphere summer solstice, therefore the orbital seasons (perihelion/aphelion) and obliquity seasons (summer/winter) align at the southern hemisphere and disalign on the northern hemisphere. This results in higher insolation on the southern hemisphere during southern summer than on the northern hemisphere during northern

<footnote n="2">² $L_s$ represents the orbital angle between Mars and the northern hemisphere vernal equinox. $L_s=90^\circ$ is therefore equivalent to summer solstice, $L_s=180^\circ$ to the autumn equinox and $L_s=270^\circ$ to the winter solstice on the northern hemisphere.</footnote>
summer. Obliquity effects are seen in a subset of Mars Global Surveyor (MGS) radio science ionospheric observations close to $L_s=140°$. Derived subsolar ionospheric peak altitudes were found on average 7.7 km higher on the northern (late summer) than on the southern (late winter) hemisphere (Zou et al. 2005). Orbital season effects on the ionospheric peak height at the subsolar point were identified in observations of the Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) experiment. Morgan et al. (2008) found that subsolar ionospheric peak altitudes derived from southern summer observations were elevated above those derived from northern summer observations. Němec et al. (2011) identified an increase in ionospheric peak altitudes at the subsolar point for decreasing Mars-Sun distances. González-Galindo et al. (2021) combined MARSIS and MaRS radio science observations to show that obliquity as well as orbital season effects affect the Mars altitude of the ionospheric main peak.

Another factor affecting the ionospheric peak height is the presence of dust aerosols in the Mars atmosphere. The Mars dust cycle can be roughly divided into two seasons. The Mars non-dusty season, where the atmospheric dust load is generally low, occurs approximately between $L_s=0°$ and $135°$. The atmospheric dust aerosol levels increase during the dusty season between approximately $L_s=135°$ and $360°$ (see e.g. review by Kahre et al. (2017)). Observed dust-associated effects range from local to regional and upward to planet encircling Global Dust Storms (GDS) with the largest dust events observed during the dusty season (Wang and Richardson 2015). Dust particles are effectively lifted into the atmosphere by near-surface wind stress and convective vortices ("dust devils") (e.g. Greenley and Iversen (1985), Greenley et al. (2006)) and can reach altitudes of ~80 km during global dust storms (Clancy et al. 2010, Kass et al. 2020). Floating dust efficiently absorbs and scatters solar radiation and emits in the infrared, which results in local heating and cooling effects on the surrounding atmosphere (see e.g. reviews of Kahre et al. (2017), Wolff et al. (2017) and Wolkenberg et al. (2018)). During the global dust storm in Mars Year<fnref n="3" /> (MY) 25, a large temperature increase was identified on the southern summer hemisphere between 15 and 25 km altitude, with a recognizable temperature effect up to 60 km altitude. Near-surface temperature dropped by 10 K due to the block of solar radiation by dust (see review of Medvedev et al. (2011) and references therein). The described temperature changes produce a dynamic feedback on the local and global Mars atmosphere and can lead to an expansion of the atmosphere, to a change of global circulation patterns (e.g. Bougher et al. (1997), Heavens et al. (2011)) and to a variation in intensity of the atmospheric solar tidal forcing (Wu et al. 2021). Mariner 9 radio science observations conducted during the declining phase of the great 1971/1972 global dust storm indicate a rise of the main ionospheric peak by 20-30 km compared to earlier radio science observations. This rise was

<footnote n="3">³ The numbering of Mars Years used in this work follows the calendar introduced by Clancy et al. (2000), where Mars Year 1 began on April 11, 1955 with $L_s=0°$.</footnote>
attributed to the expansion of the lower neutral atmosphere by local heating from increased
absorption of solar radiation by the suspended dust (e.g. Kliore et al. (1973), Hantsch and Bauer
(1990), Wang and Nielsen (2003)). During aerobraking maneuvers in 1997/1998 the Mars Global
Surveyor spacecraft observed an altitude increase of a thermospheric reference pressure level at
mid northern latitudes, indicating a rise of the ionospheric peak altitude by ~8 km which coincided
with the occurrence of a regional dust storm in the southern hemisphere (e.g. Keating et al. (1998),
Bougher et al. (1999a)). In MY27 the Mars Express Spectroscopy for the Investigation of the
Characteristics of the Atmosphere of *Mars* (SPICAM) UV instrument observed an expansion of the
neutral atmosphere at ionospheric heights during the occurrence of a weak dust event in equatorial
regions. Statistics also indicate an associated small increase in ionospheric peak altitudes between
62° and 81° northern latitudes seen by Mars Global surveyor radio science (Withers and Pratt 2013).
A correlation was found between an abrupt rise of the ionospheric peak altitude by ~9 km at lat=+52°
and a small regional dust event on southern hemisphere in the Mars Atmosphere and Volatile
Evolution N (MAVEN) radio science observations in 2017 (Withers et al. 2018). Felici et al. (2020)
extended the work of Withers et al. by identifying a correlation between the occurrence of the 2018
global dust storm and a rise of the ionospheric peak altitude on the ionospheric dayside by 10-15 km
in the MAVEN ROSE radio science observations. They also discovered a rise of the ionospheric peak
altitude by approximately 20 km during a regional dust event in 2016. The observed ionospheric peak
altitudes are roughly found at the same altitude as those seen by Mariner 9. This indicates that
regional dust storms can trigger strong atmospheric responses comparable to those of a global dust
storm. Those results are supported by Girazian et al. (2020) who find an increase in M2 peak altitude
in MARSIS observations by ~10-15 km during six different dust events ranging from local to global
dust events accompanied by an increase in overall ionospheric variability.

Since December 2003, when the Mars Express spacecraft went into orbit around Mars, the Mars
Express Radio Science experiment (MaRS) has observed more than 1,000 high-resolution height
profiles of the Mars ionospheric electron density from the topside down to the ionospheric base (e.g.
Pätzold et al. (2004), Pätzold et al. (2009), Pätzold et al. (2016), Peter et al. (2014), Peter et al.
(2021)). Observations occur in occultation seasons due to geometrical constraints of the observation
method, but are well distributed in solar longitude, planetary latitude and available for solar zenith
angles between approximately 50° and 130°. While radio science observations could not be
conducted during the global dust storm in MY 28, MaRS had the unique opportunity to observe the
ionospheric response to the declining phase of the MY 34 global dust storm. The first elevated
atmospheric dust levels of this global dust event were detected by the Mars Climate Sounder
radiometer on the Mars Reconnaissance Orbiter on 2 June 2018. The mature phase of the storm was
reached by 22 June with heavy impacts on the amount suspended dust, atmospheric temperatures
and water ice profiles. The decay phase with decreasing temperatures and dust column opacities started on 18 July, reaching ambient conditions in October 2018 (Kass et al. 2020). MaRS was able to observe the effects of the declining phase of the global storm between August and October 2018. While the ionospheric profile in Figure 1a was observed before the onset of the global dust storm at a Mars-Sun distance of 1.56 AU, the MaRS observation presented in Figure 1b was conducted during the storm's declining phase at an almost similar solar zenith angle at 1.39 AU. The altitude increase of the entire ionospheric photochemical region by ~31 km from DoY 065 to DoY 225 correlates with a large increase in locally suspended atmospheric dust and also with a decrease of Mars-Sun distance.

While the individual contributions of Mars' seasons and atmospheric dust to the variability of the ionospheric main peak region have been investigated by several researchers, it is, however, expected that the impact of suspended dust on the lower neutral atmosphere and the associated effects on the ionosphere are also closely connected to the available amount of insolation at Mars. This work combines 17 years of MaRS observations with proxies for solar insolation and the local/global atmospheric dust level to determine their combined and individual contributions to the observed variability of the dayside ionosphere of Mars. Section 2 of this paper gives a short introduction to the radio science method and the derivation of ionospheric and environmental parameters used in this work. Section 3 provides a summary of the derived results, while Section 4 provides the discussion.

## 2. MaRS ionospheric parameters and environmental proxy derivation

With the Mars Express mission in orbit around Mars for more than 19 years, the MaRS radio science experiment provides a unique dataset for the statistical analysis of the Mars ionospheric electron density from Mars Year (MY) 27 to 37. Key parameters that characterize the state of the Mars ionosphere are extracted from MaRS observations for comparison with proxies for insolation and dust suspended in the planetary atmosphere to provide valuable insights into potential drivers of the ionospheric dayside variability.

**MaRS radio science:** The MaRS experiment onboard the Mars Express spacecraft (Pätzold et al. 2004, 2009, 2016) observes the Mars atmosphere during two-way Earth occultations in either single frequency X-Band (~8.4 GHz), S-Band (~2.1 GHz) or in coherent dual-frequency X- and S-Band mode. When the spacecraft is moving behind the planetary disk as seen from the Earth, the radio signals propagate through the atmosphere of Mars from top above the ionopause to the surface during occultation ingress, thereby providing information about the planetary ionosphere and lower neutral atmosphere (see e.g. Fjeldbo and Eshleman (1965), Fjeldbo et al. (1971), Pätzold et al. (2016)). Mars Express radio science observations are not continuous in time and are constrained to solar zenith angles between approximately 50° and 130°. The necessary geometric orientation of spacecraft,
Mars and Earth are typically satisfied for periods of several months, separated by gaps of a similar extent. Radio science observations provide almost vertical profiles of the ionospheric electron density from the top down to the ionospheric base at an altitude resolution of about 500m (Figure 1). The MaRS data set used in this work contains occultation ingress and egress<fnref n="4" /> observations from 2004 to 2020 which meet the following criteria: (i) ionospheric dayside observation, i.e. a solar zenith angle $\chi < 90°$, (ii) a complete ionospheric profile ranging from the upper observational noise level down to the lower base, and (iii) a large scale noise level $\sigma_{n,L} < 1.0 \cdot 10^9$ m$^{-3}$. $\sigma_{n,L}$ indicates the amount of large external disturbance in an ionospheric profile and is calculated with the procedure described in Peter et al. (2014). The above criteria result in 361 MaRS observations in the MaRS<sub>Data</sub> data set (coverage illustrated in Figure 2). If single and dual frequency modes are available for a certain observation, the ionospheric profile with the lower $\sigma_{n,L}$ value is selected. Observations of the MY34 global dust storm were conducted between 10 August 2018 and 30 October 2018.

The upper baseline noise level $\sigma_n$ of an electron density profile is the standard deviation of the electron density samples above the local areoid altitude +800 km. The lowest valid altitude of the ionospheric electron density $h_{ivo}$ is defined as that altitude where the neutral atmosphere is strong enough to potentially bias the observation of the ionospheric electron density (details in Peter et al. (2021)). The solar longitude $L_s$, Mars-Sun distance $D$ (with low ($D_L$), moderate ($D_M$) and high ($D_H$) Mars-Sun distance intervals, Table 1), solar zenith angle $\chi$ and planetary longitude $lon$ and latitude $lat$ of an individual MaRS observation are determined from the time when the spacecraft is sounding 130 km altitude above the areoid. The ionospheric main peak electron density $n_{M2}$ and associated altitude above the areoid $h_{M2}$ are determined from a maximum search between 3,479.5 and 3,789.5 km distance to the planet center (Figure 3). The parameters of the M1 region $n_{M1}$ and $h_{M1}$ are determined from the residuals of an automated straight line fit on a sub-region between M2 and M1 (fit boundaries $h_{M1,1}$ and $h_{M1,2}$ in Figure 3a) where the ionosphere is not yet strongly affected by the presence of M1 (procedure described in Appendix B of Peter et al. (2021)). For this work the same procedure with similar boundary conditions is applied to the region above M2 to determine the region where the shape of the ionosphere is not yet affected by the presence of a potential bulge (fit boundaries $h_{BL,1}$ and $h_{BL,2}$ in Figure 3a). The total electron content (TEC) of the whole ionospheric profile (TEC<sub>All</sub>) is determined by the integration of the electron density between the upper and lower ionospheric boundaries where the profile reaches $3\sigma_n$. The TEC of the M1 region (TEC<sub>M1</sub>) is determined from the derived M1 residuals illustrated in Figure 3.

<footnote n="4">⁴ The first successful egress occultation with Mars Express was conducted in 2020.</footnote>
<figure><img src="image_4.png" /></figure>

Figure 2: Overview of derived environmental parameters. Continuous lines show the evolution during the MEX mission, while blue dots highlight the times covered by MaRS<sub>data</sub>. The vertical dashed lines indicate Mars pericenter. Panel (a) contains the Mars-Sun distance D, while Panels (b), (c) and (d) show the integrated solar radiation Φ<sub>190</sub> < 190 nm, 10 nm ≤ Φ<sub>EUV</sub> ≤ 95 nm and Φ<sub>X</sub> < 10 nm, respectively. Panel (e) and (f) contain the local (CDOD<sub>L</sub>) and global (CDOD<sub>G</sub>) Column Dust Optical Depth of the Mars atmosphere. CDOD<sub>L</sub> and CDOD<sub>G</sub> are provided for the subset of MaRS observations where valid proxies are available from the Mars dust maps. Individual definitions for the parameters are provided in Section 2.

The electron density vs. altitude distribution of each individual ionospheric profile in MaRS<sub>data</sub> is parametrized to determine the effects of Mars' seasons and atmospheric dust on the shape of the ionospheric profiles. Altitude for six predefined electron density values are extracted (Figure 3b). h<sub>1</sub> at 0.2·n<sub>M2</sub> below the main peak describes the behavior of the ionospheric base. h<sub>1</sub> is usually found below h<sub>M1</sub> (see Figure 5) and above 90 km altitude, below which transient accumulations of electron density are regularly found (Peter et al. 2021) and might add increased variability to the derived h<sub>1</sub>
<figure><img src="image_5.png" /><figcaption>Figure 3: MaRS observation of DoY 035 (2006) with (a) M1 and bulge line fits for the determination of the fit range for the Chapman function and (b) parametrization of the ionospheric shape at six selected electron density values.</figcaption></figure>

224 altitudes. $h_2$ at $0.5 \cdot n_{M2}$ above $h_{M2}$ was chosen to investigate the ionospheric topside behavior close to
225 the main peak. $h_3$ at $0.2 \cdot n_{M2}$ above $h_{M2}$ was chosen to mirror $h_1$ in the ionospheric topside. $h_4$ at $5 \cdot 10^9$
226 $m^{-3}$ (for which the MaRS electron density is smoothed by a small window moving average to reduce
227 $\sigma_n$) was chosen to investigate the behavior of the ionospheric transport region close to the noise
228 level of the observation. The parameters set is completed by $h_{M2}$ and $h_{M1}$, the altitudes of the most
229 prominent features of the dayside ionosphere.

230 **M2 peak altitude at $\chi=0^\circ$:** The Chapman function in Equation 1 describes the electron density
231 distribution of an idealized ionosphere. Fitting it to the main peak region of an observed ionospheric
232 electron density profile provides a rough approximation of the underlying neutral scale height $H$,
233 ionospheric M2 peak density $n_{M2}^0$ and peak altitude $h_{M2}^0$ at $\chi=0^\circ$.

234 Equation 1 is applied to three different altitude ranges of each profile in MaRS<sub>data</sub> to reduce the
235 uncertainties introduced into the derived parameters by a fixed fit range around $h_{M2}$ (Figure 3a): i.)
236 [$h_{M1,1}$ : $h_{BL,1}$], ii.) [$h_{M1,2}$ : $h_{BL,2}$] and iii.) the altitude range around $h_{M2}$ where the electron density $n_e$
237 exceeds $0.7 \cdot n_{M2}$. The derived parameters $n_{M2}^0$ and $h_{M2}^0$ of a Chapman function fit are accepted as
238 valid if i.) $3\sigma_n > 1/N \cdot \sum_{i=1:N} |n_{MaRS}(i) - n_{Chap}(i)|$, where N is the number of data points in the
239 selected fit range and $n_{MaRS}$ and $n_{Chap}$ are the MaRS and Chapman fit electron densities at a certain
240 altitude, respectively. ii.) ($n_{MaRS}(h) + 3\sigma_n$) > $n_{Chap}(h)$ at those altitudes where the MaRS observation
241 reaches $0.5 \cdot n_{M2}$ and $0.25 \cdot n_{M2}$ above and below $h_{M2}$. Essentially the criteria indicate that the average
242 deviation between fit and observation is not allowed to be larger than $3 \cdot \sigma_n$ and the Chapman fit
243 needs to be enveloped by the observed ionospheric shape at the altitude of $0.5 \cdot n_{M2}$ and $0.25 \cdot n_{M2}$. If
244 one valid Chapman fit is available for an observation, the derived M2 peak altitude at $\chi=0^\circ$ is set as
245 $h_{M2}^0$ without any error bars. If two valid Chapman fits are available, $h_{M2}^0$ is set as their mean value,
<table><caption>Table 1: Derived boundaries for the environmental parameters</caption><thead><tr><th>Proxies</th><th>Integr. Range</th><th>Units</th><th>low</th><th>moderate</th><th>high</th></tr></thead><tbody><tr><td>Φ<sub>X</sub></td><td>&lt; 10 nm</td><td>[10<sup>12</sup> s<sup>-1</sup>m<sup>-2</sup>]</td><td>Φ &lt; 2.31</td><td>2.31 ≤ Φ ≤ 5.11</td><td>Φ &gt; 5.11</td></tr><tr><td>Φ<sub>EUV</sub></td><td>10 - 95 nm</td><td>[10<sup>14</sup> s<sup>-1</sup>m<sup>-2</sup>]</td><td>Φ &lt; 1.87</td><td>1.87 ≤ Φ ≤ 2.59</td><td>Φ &gt; 2.59</td></tr><tr><td>Φ<sub>190</sub></td><td>&lt; 190 nm</td><td>[10<sup>16</sup> s<sup>-1</sup>m<sup>-2</sup>]</td><td>Φ &lt; 1.68</td><td>1.68 ≤ Φ ≤ 2.14</td><td>Φ &gt; 2.14</td></tr><tr><td>D</td><td></td><td>[AU]</td><td>D &lt; 1.44</td><td>1.44 ≤ D ≤ 1.62</td><td>D &gt; 1.62</td></tr><tr><td>CDOD<sub>L</sub></td><td></td><td></td><td>CDOD &lt; 0.07</td><td>0.07 ≤ CDOD ≤ 0.15</td><td>CDOD &gt; 0.15</td></tr><tr><td>CDOD<sub>G</sub></td><td></td><td></td><td>CDOD &lt; 0.10</td><td>0.10 ≤ CDOD ≤ 0.16</td><td>CDOD &gt; 0.16</td></tr></tbody></table>

the two derived altitude values are set as associated error bars. If all three Chapman fits are valid, the middle altitude value is assigned as $h_{M2}^0$, while the upper and lower values are set as associated error bars. The Chapman fits in this work were conducted with Equation 1 where $1/\cos\chi$ was replaced by $ch^*$ calculated at 130 km above the areoid. The calculation of $ch^*$ for the Chapman fit requires a scale height $H$. $H$ is determined by i.) calculating a preliminary $ch^*$ with $H=11$km, and ii.) conducting a Chapman fit with the preliminary $ch^*$. This preliminary Chapman fit provides an improved estimate for $H$, which is then used in the calculation of $ch^*$ for the final Chapman fit. The distribution of $H$ in MaRS<sub>data</sub> derived from the final Chapman fits are shown in Figure A1 Panel (c) and (d) in the Appendix. While the fits with $1/\cos\chi$ still shows a residual dependence of $h_{M2}^0$ on $\chi$ for $\chi>80^\circ$, the use of $ch^*$ removes most of this effect (see Appendix Figure A1a, b).

**Solar radiation proxies:** Solar EUV and X-ray radiation are known to strongly affect the TEC and electron density of the M2 and M1 regions (e.g. Withers (2009), Peter et al. (2014)), respectively. The integrated solar radiation in the Extreme UltraViolet (EUV) from 10 – 95 nm ($\Phi_{EUV}$) and X-ray < 10 nm ($\Phi_X$) are therefore derived as solar radiation proxies for each MaRS observation (Figure 2c, d). The integrated solar radiation < 190 nm ($\Phi_{190}$, Figure 2b) is used here as proxy for the variability of the total solar radiation at the Mars position.

The derivation of the solar radiation proxies is based on two spectral solar irradiation models FISM-M and FISM-V2. FISM-M is based on observation of the Extreme Ultraviolet Monitor (EUVM), which is a sensor on the Langmuir Probes and Waves instrument onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft at Mars. EUVM provides the input for the FISM-M spectral model, which covers solar radiation between 0.5 to 189.5 nm with 1 data set per day (Thiemann et al. 2017) since the begin of the observations in October 2014. The second model used for the derivation of the solar radiation proxies is the Flare Irradiance Spectral Model version 2 (FISM-V2, Chamberlin et al. (2020)), which is an empirical model of the solar radiation in the wavelength range between 0 and 190 nm for Earth with 1 data set per day in "daily" mode. The temporal and spatial calibration of the FISM-V2 solar flux data to the Mars position are based on the method described in Peter et al.
272 (2014). However, instead of using one FISM-V2 data set per MaRS observation as described in Peter
273 et al. (2014), an additional data set observed 27.27 days (one mean solar rotation) earlier/later,
274 depending on the planetary constellation at the time of the MaRS observation, is extracted. φ₁₉₀, φEUV
275 and φₓ are then determined by a linear interpolation to the time of the MaRS observation and
276 spatially calibrated from the Sun-Earth distance dEarth to the Sun-Mars distance dMars by (dEarth/dMars)².
277 This approach has been shown to add a modest ~7% error, which depends on the Mars-Sun-Earth
278 angle (Thiemann et al. 2021). MaRS observations in this work cover 2004 – 2020, while the preferred
279 FISM-M output is available only after the arrival of MAVEN in 2014. φ₁₉₀, φEUV and φₓ are therefore
280 derived from FISM-M after October 2014, otherwise FISM-V2 solar flux data are used. The low (φ*,L),
281 moderate (φ*,M) and high (φ*,H) solar flux proxy ranges for *=190/EUV/X are determined from the
282 solar fluxes in MaRSdata and are therefore relative and not absolute solar flux boundaries. The low
283 and high solar proxy boundaries are determined by the ±1σ standard deviation from the mean
284 (Table 1).

285 **Proxies for local and global atmospheric dust content:** The proxies for the local (CDODL) and global (CDODG) atmospheric dust level of the Mars atmosphere are based on the multiannual climatology of airborne dust maps of Montabone et al. (2015, 2020). The longitude/latitude dust maps provide the 286 9.3 µm absorption column-integrated dust optical depth (CDOD) on a daily basis from Mars Year 24 287 to 36 (April 1999 to December 2022) and are based on the observations of the Thermal Emission 288 Spectrometer (Christensen et al. 2001) aboard Mars Global Surveyor, the Thermal Emission Imaging 289 System (Christensen et al. 2004) aboard Mars Odyssey and the Mars Climate Sounder (McCleese et 290 al. 2007) aboard Mars Reconnaissance Orbiter. Available are two kinds of databases: i.) solely 291 observation-based and therefore irregularly gridded maps and ii.) observation based but regularly 292 kriged maps of airborne dust, which provide full global coverage for each Martian Sol and are well 293 suited for modelling purposes. 294 295

296 The dust proxies for the local (CDODL) and global (CDODG) dust content of the Martian atmosphere
297 are calculated from the observation based irregularly gridded Column Dust Optical Depth at 9.3 µm
298 normalized to 610 Pa to avoid any potential bias from assumptions made to produce the kriged dust
299 maps. The CDODL proxy is a single CDOD value which describes the local dust content of the Mars
300 atmosphere at the time (translated into Mars Year and Ls) and lon/lat coordinates of the observation
301 when MaRS is sounding 130 km altitude above the areoid. CDODL is derived by selecting the
302 appropriate MY/Ls gridded dust map and an interpolation to the required lon/lat coordinates of the
303 observation. CDODL values are available from the irregularly gridded dust maps for 158 of the 361
304 observations in MaRSdata. The global dust proxy (CDODG) describes the average dust content of the
305 Mars atmosphere at the time (MY/Ls) of the MaRS observation. CDODG values are derived by
<figure><img src="image_6.png" /><figcaption>Figure 4: Zonally averaged irregularly gridded column dust optical depth in absorption at 9.3 µm (normalised to 610 Pa) for Mars Year 27 to 35. White areas indicate regions where no gridded CDOD database entries are available for a certain $L_s/\text{lat}$ combination. Red (white) dots indicate MaRS observation with (without) valid CDOD$_L$ coverage at the specific $L_s/\text{lon/lat}$ combination of the individual observation.</figcaption></figure>

306 extracting the dust map for the specific MY/$L_s$ combination of the individual MaRS observation and 307 calculating the mean from all available $\text{lon/lat}$ CDOD values. A derived CDOD$_G$ value is assumed to be 308 valid, if the $\text{lon/lat}$ coverage of the selected irregularly gridded dust map exceeds 80% surface 309 coverage. The CDOD\* low and high dust proxy boundaries are determined by the ±0.5·σ standard 310 deviation from the mean of the MaRS observations without a global dust storm (non-GDS), 311 respectively (Table 1). The distribution of MaRS$_\text{data}$ in dependence of the zonally averaged 312 atmospheric dust content is illustrated in Figure 4.
<figure><img src="image_7.png" /></figure>

Figure 5: MaRS observed ionospheric parameters (a) TEC, (b) M1 TEC, (c) M2 maximum electron density and (d) M1 maximum electron density in dependence of solar zenith angle. Each MaRS parameter set is divided into low, moderate and high solar flux proxy observations. For TEC<sub>All</sub> (Panel a) and n<sub>M2</sub> (Panel c) *=EUV, for TEC<sub>M1</sub> (Panel b) and n<sub>M1</sub> (Panel c) *=X. The associated solar flux boundaries are given in Table 1. Mean and standard deviation for the individual data subsets are calculated for each 5° solar zenith angle bin if the number of data points in the interval is ≥ 2 and ≥ 4, respectively.

## 3 Results

Figure 5 illustrates the relation between the solar radiation at the top of the Mars atmosphere and the extent of the ionospheric M2 and the M1 regions. TEC<sub>All</sub>, n<sub>M2</sub>, TEC<sub>M1</sub>, n<sub>M1</sub> decrease for increasing solar zenith angle as expected from simple Chapman theory. The correlation of TEC<sub>All</sub> and n<sub>M2</sub> with the Φ<sub>EUV</sub> proxy as well as TEC<sub>M1</sub> and n<sub>M1</sub> with the Φ<sub>X</sub> proxy is in agreement with earlier MaRS investigations (e.g. Peter et al. (2014), Peter et al. (2021)).

The M2 peak altitude $h_{M2}$ in MaRS<sub>data</sub> (Figure 6) varies over a wide altitude range for all shown solar zenith angles. Sources for those variations include a solar zenith angle dependence as described by Chapman theory, tidal effects (e.g. Bougher et al. (2001, 2004)) and seasonal variability, which combines the effects of variations in total insolation caused by the orbital eccentricity with those of the planetary dust cycle. Tidal effects cannot be isolated in MaRS<sub>data</sub> because the dataset does not contain a sufficient amount of observations with appropriately close ranges of $L_s$, lat and local time to allow the identification of those processes. The fit of the Chapman relation on $h_{M2}$ for moderate Mars-Sun distances in Figure 6 shows that the dependence of $h_{M2}$ on χ becomes increasingly
<figure><img src="image_8.png" /><figcaption>Figure 6: MaRS M2 peak altitude in dependence of solar zenith angle. The $h_{M2}$ data set is divided into low, moderate and high Mars-Sun distance observations (boundaries in Table 1). Mean and standard deviation of the individual data subsets are calculated similar to those in Figure 5. The fit of the Chapman relation $h_m = h_0 - H \ln(\cos \chi)$ on $D_M$ yields $H = 9.1$ km.</figcaption></figure>

340 indicate that the lifting effect shown in Figure 6 is not constrained to the ionospheric peak region, but affects the whole photochemical region from the ionospheric base at $h_1$ up to $h_3$ above the main peak (summary in Table 2). No effect of the orbital seasons is seen on the highest ionospheric altitudes ($h_4$), where other drivers dominate the local electron density distribution. The observations conducted during the 2018 global dust storm are also strongly elevated for $h_1, h_{M1}, h_{M2}$, and $h_2$. The lifting effect on the whole photochemically dominated region of the ionosphere is even stronger than that seen by MaRS from orbital season variability during other Mars years.

347 The relation between the extracted ionospheric altitudes and the global dust proxy $CDOD_G$ is illustrated in Panel (c) and (d) of Figure 7-9. The distribution for the low, moderate and high $CDOD_G$ intervals closely resembles that for $D_H, D_M$ and $D_L$. This is explained by the close relation between $CDOD_G$ and $D$ in MaRS<sub>all</sub> (Figure 10a). The average $CDOD_G$ values increase for decreasing $D$ in MaRS<sub>data</sub>, which is expected due to the temporal coincidence of Mars dusty season ($L_s$ approx. between 135° and 360°) and Mars pericenter ($L_s=251°$). A similar relation is, as expected, found between $\Phi_{190}$ and $D$ (Figure 10c). The increase in solar insolation for decreasing $D$ in MaRS<sub>data</sub> is therefore on average accompanied by an increase in global atmospheric dust content. Both investigated phenomena contribute to the observed rise of the Mars dayside ionosphere from apocenter to pericenter for comparable $\chi$. Individual contributions of insolation or global atmospheric dust content can, however, not be separated in MaRS<sub>data</sub>.

358 Panel (e) and (f) of Figure 7-9 show the parametrized ionospheric altitudes in dependence of $CDOD_L$. The lifting effect on the photochemically dominated region of the ionosphere is recognizable, 359

significant for higher solar zenith angles, but cannot explain the large variation in $h_{M2}$ at lower $\chi$. At similar solar zenith angles $h_{M2}$ is higher when the Mars-Sun distance $D$ is small. Small $D$ coincide with increased total insolation due to the large orbit eccentricity of Mars (Figure 2a,b), but also with increased global levels of atmospheric dust (Figure 2f, 10a).

339 Panel (a) and (b) of Figure 7-9
<table><caption>Table 2: Average altitude distribution of $h_1$ to $h_3$ for $D_L$, $D_H$ and the 2018 global dust storm</caption><thead><tr><th>Solar zenith angle</th><th>52.5</th><th>57.5</th><th>62.5</th><th>67.5</th><th>72.5</th><th>77.5</th><th>82.5</th><th>87.5</th><th>mean</th></tr></thead><tbody><tr><td>h<sub>1</sub> D<sub>L</sub></td><td>94</td><td>91.2</td><td>90.5</td><td>97.1</td><td>97.5</td><td>99.5</td><td>99.4</td><td>104.9</td><td>96.8</td></tr><tr><td>D<sub>H</sub></td><td>--</td><td>--</td><td>105.3</td><td>110.8</td><td>109.6</td><td>113.9</td><td>110.7</td><td>109.8</td><td>110</td></tr><tr><td>2018 GDS</td><td>112.6</td><td>113</td><td>--</td><td>115.8</td><td>118.7</td><td>--</td><td>--</td><td>--</td><td>115</td></tr><tr><td>h<sub>M1</sub> D<sub>L</sub></td><td>102.2</td><td>99.3</td><td>98.3</td><td>105.8</td><td>106.9</td><td>105.3</td><td>106.4</td><td>114</td><td>104.8</td></tr><tr><td>D<sub>H</sub></td><td>--</td><td>--</td><td>115.7</td><td>117.2</td><td>116.9</td><td>116.9</td><td>116</td><td>119.1</td><td>117</td></tr><tr><td>2018 GDS</td><td>121.1</td><td>121</td><td>--</td><td>123.8</td><td>126.7</td><td>--</td><td>--</td><td>--</td><td>123.2</td></tr><tr><td>h<sub>M2</sub> D<sub>L</sub></td><td>125.7</td><td>122.2</td><td>124.4</td><td>130.6</td><td>129.8</td><td>127.9</td><td>130.9</td><td>141.3</td><td>129.1</td></tr><tr><td>D<sub>H</sub></td><td>--</td><td>--</td><td>141.6</td><td>139.8</td><td>142</td><td>141.3</td><td>145.2</td><td>153.1</td><td>143.8</td></tr><tr><td>2018 GDS</td><td>145.7</td><td>146.6</td><td>--</td><td>147.6</td><td>151.3</td><td>--</td><td>--</td><td>--</td><td>147.8</td></tr><tr><td>h<sub>2</sub> D<sub>L</sub></td><td>151.4</td><td>147.4</td><td>151.8</td><td>159.4</td><td>157.1</td><td>155.7</td><td>160</td><td>165.1</td><td>156</td></tr><tr><td>D<sub>H</sub></td><td>--</td><td>--</td><td>169.7</td><td>171.3</td><td>172.7</td><td>173.8</td><td>184.5</td><td>185.6</td><td>176.3</td></tr><tr><td>2018 GDS</td><td>173</td><td>172.9</td><td>--</td><td>179</td><td>184.6</td><td>--</td><td>--</td><td>--</td><td>177.4</td></tr><tr><td>h<sub>3</sub> D<sub>L</sub></td><td>182.2</td><td>175.1</td><td>185.7</td><td>188.4</td><td>188.7</td><td>181.8</td><td>183.4</td><td>187.3</td><td>184.1</td></tr><tr><td>D<sub>H</sub></td><td>--</td><td>--</td><td>204.2</td><td>200.9</td><td>208</td><td>199.3</td><td>213.3</td><td>211.2</td><td>206.2</td></tr><tr><td>2018 GDS</td><td>202.7</td><td>204.9</td><td>--</td><td>209.9</td><td>213.5</td><td>--</td><td>--</td><td>--</td><td>207.8</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>Figure 7: Parametrized ionospheric altitudes $h_1$ (a) and $h_{M1}$ (b) in dependence of the Mars-Sun distance $D$, $h_1$ (c) and $h_{M1}$ (d) in dependence of the global dust proxy CDOD$_G$ and $h_1$ (e) and $h_{M1}$ (f) in dependence of the local dust proxy CDOD$_L$. Associated interval boundaries are given in Table 1. The 2018 global dust storm data are highlighted by yellow dots. Mean and standard deviation of the individual data subsets are calculated similar to those in Figure 5 for the three given intervals and the GDS data.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

Figure 8: Parametrized ionospheric altitudes $h_{M2}$ (a) and $h_2$ (b) in dependence of D, $h_{M2}$ (c) and $h_2$ (d) in dependence of CDOD$_G$ and $h_{M2}$ (e) and $h_2$ (f) in dependence of CDOD$_L$. Individual colors, markings and the calculation of the mean and standard deviation are similar to those in Figure 7. Black arrows in Panel (a) indicate the rise of the ionospheric peak from DoY 280 to DoY 282 (magenta square) and its return to the original peak altitude on DoY 285 and DoY 287.

but less clear than that seen for the *D* and *CDOD*$_G$ parameters. While a slight increase in average *CDOD*$_L$ is seen for decreasing *D* in Figure 10b, observations with low as well as high *CDOD*$_L$ proxy values are found close to Mars pericenter. This is explained by the irregular distribution and high temporal variability of the atmospheric dust during Mars dusty season (Figure 4). Therefore high local dust aerosol levels do not automatically coincide with high levels of total insolation in *MaRS*$_{data}$ and a separation of the contributions of local atmospheric dust and solar flux (Figure 10d) to ionospheric lifting is possible.

Figure 5 shows that the *TEC*$_{All}$ and $n_{M2}$ parameters are strongly affected by the available solar EUV at the planet's position, while *TEC*$_{M1}$ and $n_{M1}$ correlate with the available solar X-ray irradiation. The
<figure><img src="image_11.png" /><figcaption>Figure 9: Parametrized ionospheric altitudes $h_3$ (a) and $h_4$ (b) in dependence of D, $h_3$ (c) and $h_4$ (d) in dependence of CDOD$_G$ and $h_3$ (e) and $h_4$ (f) in dependence of CDOD$_L$. Description of individual colors, markings and the calculation of the mean and standard deviation are similar to those in Figure 7.</figcaption></figure>

investigation of the effect of D, CDOD$_G$ and CDOD$_L$ on those ionospheric parameters is therefore
constraint to observations with moderate solar flux ($\Phi_{EUV,M}$ for TEC$_{AII}$ and $n_{M2}$, $\Phi_{X,M}$ for TEC$_{M1}$ and $n_{M1}$)
to reduce the impact of EUV and X-ray variability on the selected data set. No significant effect of D,
CDOD$_G$ or CDOD$_L$ on TEC$_{AII}$, $n_{M2}$, TEC$_{M1}$, $n_{M1}$ and the width of main and secondary ionospheric regions
is found (see Appendix Figure A2-A4).

Figure 7-9 indicate that insolation as well as atmospheric dust aerosols contribute to the variability of the Mars dayside ionosphere. The effect of the solar zenith angle on the ionospheric characteristics (Figure 5), however, complicates the separation of insolation and dust effects. The dependence of $h_{M2}$ on $\chi$ is therefore removed to allow an easier separation of the individual contributions of insolation and dust aerosols to the variability of the ionospheric main peak altitude. The ionospheric
<figure><img src="image_12.png" /></figure>

Figure 10: The distribution of CDOD<sub>G</sub> and CDOD<sub>L</sub> in MaRS<sub>data</sub> is shown in dependence of the Mars-Sun distance for 2018 GDS and non-storm data (Panel (a), (b)) and in dependence of Φ<sub>190</sub> (Panel (c), (d)), respectively. The relation between the M2 altitude at χ=0° and the CDOD<sub>G</sub> and Φ<sub>190</sub> parameters is illustrated in (e), the relation between CDOD<sub>L</sub> and Φ<sub>190</sub> in (f) for MaRS observations with solar zenith angles ≤80°. Error bars for h<sup>0</sup><sub>M2</sub> are shown if their total widths exceed 2 km. Mean and standard deviation are calculated for each 0.05 bin if the number in the interval is ≥2 and ≥4, respectively.

380 main peak altitude at $\chi=0°$ ($h_{M2}^0$) is derived by fitting Equation 1 to each observation in MaRS<sub>data</sub>.

381 Despite the applied correction for the curvature of the atmosphere $ch^*$ a weak dependence between

382 $h_{M2}^0$ and $\chi$ remains for higher solar zenith angles (Appendix Figure A1a,b). Therefore only

383 observations with $\chi \le 80°$ are taken into account. Figure 10e illustrates the relation between $h_{M2}^0$,

384 the global dust proxy CDOD<sub>G</sub> and Φ<sub>190</sub> as proxy for changes in total insolation at the Mars position.

385 The average $h_{M2}^0$ value rises from low to high CDOD<sub>G</sub>, but at the same time from Φ<sub>190,L</sub> to Φ<sub>190,H</sub> due to

386 the correlation of both parameters in MaRS<sub>data</sub> (Figure 10a,c). The highest $h_{M2}^0$ values are found for

387 the observations conducted during the global dust storm in 2018, which occurred close to Mars

388 pericenter with high Φ<sub>190</sub> values. While the combined effect of increased insolation and global
<figure><img src="image_13.png" /></figure>

Figure 11: Derived MaRS<sub>data</sub> M2 peak altitudes at $\chi=0^\circ$ in dependence of $L_s$. Observations are shown if the original solar zenith angle $\le 80^\circ$ and the latitude is either found above $30^\circ$ (northern hemisphere) or below $-30^\circ$ (southern hemisphere). Dashed lines indicate the $L_s$ of Mars apocenter and pericenter, respectively.

atmospheric dust on $h_{M2}^0$ is clearly visible, individual contributions of both parameters can therefore not be separated in MaRS<sub>data</sub>. Figure 10f illustrates the relation between $h_{M2}^0$, CDOD<sub>L</sub> and $\Phi_{I90}$. In contrast to the global dust proxy, increased levels of local atmospheric dust are not necessarily accompanied by increased levels of insolation in MaRS<sub>data</sub> (Figure 10b,d). Therefore the individual effects of local atmospheric dust and insolation can be separated. For a fixed CDOD<sub>L</sub> level in Panel 10f, the average $h_{M2}^0$ values rise from $\Phi_{I90,L}$ to $\Phi_{I90,H}$, which indicates an ionospheric lifting effect from the increase

of total insolation (e.g. for a fixed CDOD<sub>L</sub> value of 0.125, the average $h_{M2}^0$ rises from ~118 km for $\Phi_{I90,L}$ to ~128 km for $\Phi_{I90,H}$). For a fixed $\Phi_{I90}$ interval the average $h_{M2}^0$ values rise for increasing CDOD<sub>L</sub> levels until a maximum elevation of the ionospheric peak is reached. This maximum altitude depends on the available insolation at the top of the planetary atmosphere. Further increases of the local atmospheric dust levels do not lead to a further rise of the average ionospheric peak altitude in MaRS<sub>data</sub>. This indicates that the available insolation at the top of the Mars atmosphere provides an upper limit for the warming/expansion of the lower neutral atmosphere and the consecutive lifting of the ionosphere. This effect is also seen in the MaRS observations conducted during the declining phase of the 2018 global dust storm. The observations were done during the highest local/global atmospheric dust condition available in MaRS<sub>data</sub> and close to pericenter where $\Phi_{I90}$ is large. The derived $h_{M2}^0$ parameters are found on average well above those derived during non-GDS conditions and show an almost constant altitude distribution despite increasing CDOD<sub>L</sub> levels. This supports the hypothesis that the warming/expansion of the lower neutral atmosphere during high levels of atmospheric dust is limited by the available insolation at the top of the Mars atmosphere.

Figure 11 illustrates the variation of $h_{M2}^0$ with solar longitude. Due to the marginal overlap of observations for both hemispheres in $L_s$, the potential for the investigation of Mars obliquity season effects in MaRS<sub>data</sub> are limited. Overall variations of $h_{M2}^0$ with $L_s$ are in good agreement with earlier investigations of Martian season effects on the ionospheric peak altitude (see e.g. Figure 7 of Gonzalez-Galindo et al. 2021). Lowest $h_{M2}^0$ values are found close to apocenter, while highest $h_{M2}^0$ values are found close to Mars pericenter. The 2018 global dust storm observations occurred close to
Mars pericenter and show the largest rise of $h_{M2}^0$ observed by MaRS so far. Obliquity variations are small on the northern hemisphere compared to those the south hemisphere, where a strong increase in $h_{M2}^0$ is found from southern winter to southern summer. The origin of this difference in hemispheric behavior are the effects of orbital and obliquity seasons, which are aligned on the southern and disaligned on the northern hemisphere.

## 4 Discussion

The ionospheric peak altitudes derived from MaRS<sub>data</sub> show a strong dependence on the Mars orbital seasons. For comparable solar zenith angles the peak altitudes observed closer to Mars pericenter are on average found to be elevated above those observed closer to Mars apocenter. This is in agreement with the findings of Morgan et al. (2008), Němec et al. (2011) and González-Galindo et al. (2021) who determined from MARSIS or/and MaRS observations that ionospheric peak altitudes at the subsolar point are elevated when Mars is close to orbit pericenter. In addition to the ionospheric main peak information, MaRS observations also indicate that the orbital seasons strongly affect the whole photochemically dominated region of the dayside ionosphere. While the individual shape of each MaRS electron density profile is affected by atmospheric waves, tides and other local and global transport and circulation effects, the average photochemically dominated region of the ionosphere at a certain solar zenith angle rises by several kilometers from apocenter to pericenter. This is in agreement with the results of the general circulation model developed by the Laboratoire de Météorologie Dynamique which predicts a rise of the whole subsolar ionosphere from Mars apocenter to pericenter (González-Galindo et al. 2013).

MaRS observations of the 2018 global dust storm were conducted on the southern hemisphere between August and October 2018 when Mars was close to orbit pericenter. The average increase in ionospheric main peak altitude $h_{M2}$ during this event exceeds that seen from orbital variability at all available solar zenith angles. This is in agreement with the Mariner 9 radio science observations during the great 1971/1972 global dust storm, where ionospheric main peak altitudes were found to be highly elevated above those observed by earlier missions (e.g. Kliore et al. (1973), Hantsch and Bauer (1990), Wang and Nielsen (2003)). Felici et al. (2020) report MAVEN ROSE dayside occultation egress observations on the southern hemisphere between June and July 2018. Their derived ionospheric peak altitudes (solar zenith angle between 70° and 80°) are in good agreement with those observed by MaRS few weeks later. The effect of larger dust events on the Mars ionosphere below the main peak region has not yet been statistically investigated on an observational base. MaRS observations indicate that the rise of the ionospheric main peak region observed during the 2018 global dust storm affects the whole photochemically dominated region of the dayside
457 ionosphere. The observed ionospheric behavior is confirmed by several investigations of the state of
458 the neutral atmosphere at ionospheric heights during dust events. Observations of the neutral
459 atmospheric density made during MGS aerobraking indicate a rise of the thermospheric reference
460 pressure by ~8 km in coincidence with a regional dust storm (Keating et al. (1998), Bougher et al.
461 (1999)). Withers and Pratt (2013) used stellar occultations of the SPICAM UV spectrometer on
462 Mars Express to determine a gradual increase of the atmospheric densities between 60 and 120 km
463 altitude by minimum a factor 2 between 10° Ls during a regional dust storm. They also used MGS
464 accelerometer aerobraking observations to determine an expansion of the neutral atmospheric
density between 130 and 160 km during a regional dust storm. A rise of the whole dayside
465 ionosphere during dust events is also predicted by an 1-D photochemical equilibrium model of Wang
466 and Nielsen (2003) and a global magnetohydrodynamic model of Fang et al. (2020).
467

468 Withers et al. (2018) and Felici et al. (2020) observe a sudden increase of the ionospheric main peak
469 altitude in MAVEN ROSE observations by 9 and 10-15 km related to the onset of dust storm events.
470 The only comparable sudden increase in ionospheric peak altitude in MaRS observations of the 2018
471 global dust storm is found on DoY 282 (magenta squares in Figure 8a). The altitude of the ionospheric
472 main peak rises from 146 km on DoY 280 to 159 km on DoY 282 and decreases again to 142 km on
473 DoY 285, where it remains for the MaRS observations on DoY 287 and 288 (black arrows in Figure
474 8a). The ionospheric peak altitude on DoY 282 exceeds even the hM2 altitudes observed close to the
475 terminator. This extraordinary lifting effect is observed throughout the photochemically dominated
476 ionospheric region, while the overall profile shows no sign of increased ionospheric disturbances. The
477 CDODL value of ~0.22 is slightly lower than that of the other four close-by observations and a detailed
478 investigation of the global dust distribution in the data base for DoY 282 shows no anomalously
479 strong dust features close to the position of the observation in the southern hemisphere. The
480 magnitude of the crustal magnetic field in 400 km altitude (based on the observations of the Mars
481 Global Surveyor MAG/ER instrument, Connerney et al. (2001)) is ~17 nT. The other observations are
482 conducted over regions where the crustal magnetic field magnitudes are below 10 nT. There is also
483 no publication available which indicates an increase in ionospheric main peak altitudes above crustal
484 magnetic fields. The small latitudinal extent of roughly 3° in combination with the longitudinal
485 distribution of the MaRS observations on DoY 280 (lon=31°), DoY 282 (lon=139°), DoY 285 (lon=301°),
486 DoY 287 (lon=307°) and DoY 288 (lon=259°) indicates, however, that enhanced circulation or tide
487 related effects are a possible cause of the observed ionospheric lifting. Unfortunately, this extreme
488 behavior is observed only once by MaRS during the 2018 global dust storm.
489

489 The comparison of the derived ionospheric peak altitudes for χ=0° with changes in solar insolation
490 and locally suspended atmospheric dust indicates that an increase in insolation as well as in
atmospheric dust aerosols contributes to the variability of the ionospheric peak altitude. This is in agreement with the conclusions drawn by Mukundan et al. (2022) based on the analysis of MAVEN ROSE observations during a regional dust storm in 2016. The average MaRS ionospheric peak altitude at the subsolar point rises for increasing levels of local atmospheric dust until a maximum elevation is reached. This maximum depends on the available insolation at the top of the planetary atmosphere. Further increases of the local atmospheric dust levels do not lead to a further rise of the average ionospheric peak altitude in the investigated data set. This indicates a limit for the local warming/expansion of the lower neutral atmosphere and the consecutive lifting of the ionosphere based on the available insolation and explains why regional dust storms can cause a similar lifting of the ionospheric main peak region as global dust storms as observed by Felici et al. (2020) with MAVEN ROSE radio science.

**Acknowledgments** The Mars Express Radio Science Experiment (MaRS) was funded by BMWi Berlin via the Deutsches Zentrum für Luft- und Raumfahrt (DLR) under the Grant 50QM1802. Support for Mars Express Radio Science at Stanford University was provided by NASA through a JPL Contract. Support for the Multimission Radio Science Support Team was provided by NASA/JPL. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. We thank everyone involved in the Mars Express project at ESTEC, ESOC, ESAC, JPL, and the ESTRACK and DSN ground stations for their continuous support. K. P., M. P. and S. T. acknowledge funding for this project by the Deutsche Forschungsgemeinschaft (DFG) under Grant PE 3225/2-1, PA 525/25-1 and TE 664/4-1. The FISM V2 dataset and ReadMe can be found at: https://lasp.colorado.edu/lisird/. FISM-M data are provided courtesy of E. Thiemann. The multi-annual gridded dust map product used in this study is publicly available and downloadable from the Mars Climate Database project webpage (at the current URL http://www.mars.lmd.jussieu.fr/mars/dust_climatology/).

515
<figure><img src="image_14.png" /></figure>

Figure A1: Atmospheric parameters determined from a Chapman function fit to the individual ionospheric main peak regions of the observations in MaRS<sub>data</sub>. Results shown in (a), (c) and (e) are determined from Eq. 1, while results shown in (b), (d) and (f) are based on a modified Chapman function where 1/cosχ is replaced by ch*. Panel (a) and (b) show the M2 altitudes at χ=0° in dependence of solar zenith angle and the local column dust optical depths. Panel (c) and (d) illustrate the relation between the neutral scale height H, χ and the solar flux proxy Φ<sub>190</sub>. Panel (e) and (f) show the effect of the ch* modification on the h<sup>0</sup><sub>M2</sub> results. Error bars for h<sup>0</sup><sub>M2</sub> in Panel (e) and (f) are shown if their total widths exceed 2 km. Mean and standard deviation of each data subset in Panel (e) and (f) are calculated for 0.05 CDOD<sub>L</sub> intervals if the number of available data points in the interval is ≥2 and ≥4, respectively.
<figure><img src="image_15.png" /></figure>

Figure A2: Derived ionospheric parameters in dependence of the Mars-Sun distance D. Only MaRS<sub>data</sub> observations for moderate solar activity (Φ<sub>EUV,M</sub> for TEC<sub>All</sub>, n<sub>M2</sub>, width of M2 and Φ<sub>X,M</sub> for TEC<sub>M1</sub>, n<sub>M1</sub> and width of M1) are used to reduce the known impact of solar radiation on these ionospheric parameters. The M2 (M1) widths are the altitude difference between 0.9·n<sub>M2</sub> (0.5·n<sub>M1</sub>) above and below h<sub>M2</sub> (h<sub>M1</sub> in the M1 residuals).

521

522

523

524

525

526
<figure><img src="image_16.png" /></figure>

Figure A3: Derived ionospheric parameters in dependence of the global dust proxy CDOD<sub>G</sub>. Only MaRS<sub>data</sub> observations for moderate solar activity (Φ<sub>EUV,M</sub> for TEC<sub>All</sub>, n<sub>M2</sub>, width of M2 and Φ<sub>X,M</sub> for TEC<sub>M1</sub>, n<sub>M1</sub> and width of M1) are used to reduce the known impact of solar radiation on these ionospheric parameters. M2/M1 widths are calculated according to the description in Figure A2.

527

528

529

530

531

532
<figure><img src="image_17.png" /></figure>

Figure A4: Derived ionospheric parameters in dependence of the local dust proxy CDOD<sub>L</sub>. Only MaRS<sub>data</sub> observations for moderate solar activity (Φ<sub>EUV,M</sub> for TEC<sub>All</sub>, n<sub>M2</sub>, width of M2 and Φ<sub>X,M</sub> for TEC<sub>M1</sub>, n<sub>M1</sub> and width of M1) are used to reduce the known impact of solar radiation on these ionospheric parameters. M2/M1 widths are calculated according to the description in Figure A2.

533 Literature

534 Bougher et al. (1999): Mars Global Surveyor aerobraking. Atmospheric trends and model interpretation. In:

535 *Advances in Space Research* 23 (11), pp. 1887–1897. DOI: 10.1016/S0273-1177(99)00272-0.

536 Bougher et al. (1997): Dust storm impacts on the Mars upper atmosphere. In: *Advances in Space Research* 19

537 (8), pp. 1255–1260. DOI: 10.1016/S0273-1177(97)00278-0.

538 Bougher et al. (2001): Mars Global Surveyor radio science electron density profiles. Neutral atmosphere

539 implications. In: *Geophys. Res. Lett.* 28 (16), pp. 3091–3094. DOI: 10.1029/2001GL012884.

540 Bougher et al. (2004): MGS Radio Science electron density profiles: Interannual variability and implications for

541 the Martian neutral atmosphere. In: *JGR* 109 (E3). DOI: 10.1029/2003JE002154.

542 Chamberlin et al. 2020): The Flare Irradiance Spectral Model-Version 2 (FISM2). In: *Space Weather* 18 (12). DOI:

543 10.1029/2020SW002588.
544 Chapman, S. (1931a): The absorption and dissociative or ionizing effect of monochromatic radiation in an
545 atmosphere on a rotating earth. In: *Proc. Phys. Soc.* 43 (1), pp. 26–45. DOI: 10.1088/0959-5309/43/1/305.

546 Chapman, S. (1931b): The absorption and dissociative or ionizing effect of monochromatic radiation in an
547 atmosphere on a rotating earth part II. Grazing incidence. In: *Proc. Phys. Soc.* 43 (5), pp. 483–501. DOI:
548 10.1088/0959-5309/43/5/302.

549 Christensen et al. (2001): Mars Global Surveyor Thermal Emission Spectrometer experiment. Investigation
550 description and surface science results. In: *JGR* 106 (E10), pp. 23823–23871. DOI: 10.1029/2000JE001370.

551 Christensen et al. (2004): The Thermal Emission Imaging System (Themis) for the Mars 2001 Odyssey Mission.
552 In: Christopher T. Russell (Hg.): 2001 Mars Odyssey. Dordrecht: Springer Netherlands, pp. 85–130.

553 Clancy et al. (2000): An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric
554 temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the
555 global Mars atmosphere. In: *JGR Planets* 105 (E4), pp. 9553–9571.

556 Clancy et al. (2010): Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm.
557 MGS TES limb observations. In: *Icarus* 207 (1), pp. 98–109. DOI: 10.1016/j.icarus.2009.10.011.

558 Connerney et al. (2001): The global magnetic field of Mars and implications for crustal evolution. In: *GRL* 28
559 (21), pp 4015–4018. DOI: 10.1029/2001GL013619.

560 Crismani et al. (2017): Detection of a persistent meteoric metal layer in the Martian atmosphere. In: *Nature*
561 *Geoscience* 10, pp. 401–404.

562 Fang et al. (2020): Mars Dust Storm Effects in the Ionosphere and Magnetosphere and Implications for
563 Atmospheric Carbon Loss. In: *JGR* 125 (3), e2019JE006111. DOI: 10.1029/2019JA026838.

564 Felici et al. (2020): MAVEN ROSE Observations of the Response of the Martian Ionosphere to Dust Storms. In:
565 *JGR* 125 (6). DOI: 10.1029/2019JA027083.

566 Fjeldbo and Eshleman (1965): The bistatic radar-occultation method for the study of planetary atmospheres. In:
567 *JGR* 70 (13), pp. 3217–3225. DOI: 10.1029/JZ070i013p03217.

568 Fjeldbo et al. (1971): The Neutral Atmosphere of Venus as Studied with the Mariner V Radio Occultation
569 Experiments. In: *The Astronomical Journal* 76, pp. 123–140. DOI: 10.1086/111096.

570 Fox et al. (1996): The Martian thermosphere/ionosphere at high and low solar activities. In: *Advances in Space
571 Research* 17 (11).

572 Girazian et al. (2020): Variations in the Ionospheric Peak Altitude at Mars in Response to Dust Storms. 13 Years
573 of Observations From the Mars Express Radar Sounder. In: *JGR* 125 (5), e2019JE006092. DOI:
574 10.1029/2019JE006092.

575 González-Galindo et al. (2013): 3D Martian Ionosphere model: I. The photochemical ionosphere below 180 km.
576 In: *JGR* 118, pp. 2105-2123, DOI: 10.1002/jgre.20150.

577 González-Galindo et al. (2021): Seasonal and Geographical Variability of the Martian Ionosphere From Mars
578 Express Observations. In: *JGR* 126 (2). DOI: 10.1029/2020JE006661.

579 Greenley and Iversen (1985): Wind as a Geological Process. On Earth, Mars, Venus and Titan: Cambr. Univ. Pr.

580 Greenley et al. (2006): Active dust devils in Gusev crater, Mars. Observations from the Mars Exploration Rover
581 Spirit. In: *JGR Planets* 111 (E12). DOI: 10.1029/2006JE002743.

582 Gurnett et al. (2008): An overview of radar soundings of the martian ionosphere from the Mars Express
583 spacecraft. In: *Advances in Space Research* 41 (9), pp. 1335–1346. DOI: 10.1016/j.asr.2007.01.062.

584 Hantsch and Bauer (1990): Solar control of the Mars ionosphere. In: *Planetary and Space Science* 38 (4), pp.
585 539–542. DOI: 10.1016/0032-0633(90)90146-H.

586 Heavens et al. (2011): Structure and dynamics of the Martian lower and middle atmosphere as observed by the
587 Mars Climate Sounder. 2. Implications of the thermal structure and aerosol distributions for the mean
588 meridional circulation. In: *JGR* 116 (E1). DOI: 10.1029/2010JE003713.

589 Kahre et al. (2017): The Mars Dust Cycle. In: *The Atmosphere and Climate of Mars: Cambridge University Press*,
590 pp. 295–337.

591 Kass et al. (2020): Mars Climate Sounder Observation of Mars' 2018 Global Dust Storm. In: *GRL* 47 (23),
592 e2019GL083931. DOI: 10.1029/2019GL083931.
593 Keating et al. (1998): The Structure of the Upper Atmosphere of Mars: In Situ Accelerometer Measurements
594 from Mars Global Surveyor. In: *Science* 279 (5357), pp. 1672–1676. DOI: 10.1126/science.279.5357.1672.

595 Kliore et al. (1973): S band radio occultation measurements of the atmosphere and topography of Mars with
596 Mariner 9. Extended mission coverage of polar and intermediate latitudes. In: *JGR* 78, pp. 4331–4351.

597 Kopf et al. (2008): Transient layers in the topside ionosphere of Mars. In: *GRL* 35 (17). DOI:
598 10.1029/2008GL034948.

599 Kopf et al. (2017): The transient topside layer and associated current sheet in the ionosphere of Mars. In: *JGR*
600 122 (5), pp. 5579–5590. DOI: 10.1002/2016JA023591.

601 Mayyasi et al. (2018): A Sporadic Topside Layer in the Ionosphere of Mars From Analysis of MGS Radio
602 Occultation Data. In: *JGR* 123 (1), pp. 883–900. DOI: 10.1002/2017JA024938.

603 McCleese et al. (2007): Mars Climate Sounder. An investigation of thermal and water vapor structure, dust and
604 condensate distributions in the atmosphere, and energy balance of the polar regions. In: *JGR* 112 (E5). DOI:
605 10.1029/2006JE002790.

606 Medvedev et al. (2011): Influence of dust on the dynamics of the martian atmosphere above the first scale
607 height. In: *Aeolian Research* 3 (2), pp. 145–156. DOI: 10.1016/j.aeolia.2011.05.001.

608 Molina-Cuberos et al. (2003): Meteoric ions in the atmosphere of Mars. In: *Planetary and Space Science* 51 (3),
609 pp. 239–249. DOI: 10.1016/S0032-0633(02)00197-6.

610 Montabone et al. (2015): Eight-year climatology of dust optical depth on Mars. In: *Icarus* 251, pp. 65–95. DOI:
611 10.1016/j.icarus.2014.12.034.

612 Montabone et al. (2020): Martian Year 34 Column Dust Climatology from Mars Climate Sounder Observations.
613 Reconstructed Maps and Model Simulations. In: *JGR* 125 (8), e2019JE006111. DOI: 10.1029/2019JE006111.

614 Morgan et al. (2008): Variation of the Martian ionospheric electron density from Mars Express radar soundings.
615 In: *JGR* 113 (A9). DOI: 10.1029/2008JA013313.

616 Mukundan et al. (2022): Atypically Intense and Delayed Response of the Martian Ionosphere to the Regional
617 Dust Storm of 2016: A Study Using MAVEN Observations and Models. In: *JGR* 127 (12), e2022JE007645. DOI:
618 10.1029/2022JE007645.

619 Némec et al. (2011): Dayside ionosphere of Mars. Empirical model based on data from the MARSIS instrument.
620 In: *JGR* 116 (E7), pp. 1424. DOI: 10.1029/2010JE003789.

621 Pätzold et al. (2004): MaRS: Mars Express Orbiter Radio Science. In: Andrew Wilson (Hg.): Mars Express. The
622 scientific payload. Noordwijk: ESA Publications Division (1240), pp. 140–163.

623 Pätzold et al. (2005): A Sporadic Third Layer in the Ionosphere of Mars. In: *Science* 310, pp. 837–839.

624 Pätzold et al. (2009): MaRS: Mars Express Radio Science Experiment. In: Karen Fletcher (Hg.): Mars Express. The
625 scientific investigations. Noordwijk, the Netherlands: ESA Communication Production Office, pp. 217–245.

626 Pätzold et al. (2016): Mars Express 10 years at Mars: Observations by the Mars Express Radio Science
627 Experiment (MaRS). In: *PSS* 127, pp. 44–90.

628 Pesnell and Grebowsky (2000): Meteoric magnesium ions in the Martian atmosphere. In: *JGR* 105 (E1).

629 Peter et al. (2014): The dayside ionospheres of Mars and Venus. Comparing a one-dimensional photochemical
630 model with MaRS (Mars Express) and VeRa (Venus Express) observations. In: *Icarus* 233, pp. 66–82. DOI:
631 10.1016/j.icarus.2014.01.028.

632 Peter et al. (2021): The lower dayside ionosphere of Mars from 14 years of MaRS radio science observations. In:
633 *Icarus* 359, pp. 114213. DOI: 10.1016/j.icarus.2020.114213.

634 Rishbeth and Mendillo (2004): Ionospheric layers of Mars and Earth. In: *Planetary and Space Science* 52 (9), pp.
635 849–852. DOI: 10.1016/j.pss.2004.02.007.

636 Smith and Smith (1972). Numerical evaluation of Chapman's grazing incidence integral ch(X,x). In: *JGR* 77 (19),
637 pp. 3592–3597. DOI: 10.1029/JA077i019p03592.

638 Thiemann et al. (2017): The MAVEN EUVM model of solar spectral irradiance variability at Mars. Algorithms
639 and results. In: *JGR* 122 (3), pp. 2748–2767. DOI: 10.1002/2016JA023512.

640 Thiemann et al. (2021): Solar Extreme Ultraviolet Irradiance Uncertainties for Planetary Studies. *Journal of
641 Geophysical Research: Space Physics*, 126(1), e2020JA028184.
642 Wang and Richardson (2015): The origin, evolution, and trajectory of large dust storms on Mars during Mars

643 years 24–30 (1999–2011). In: *Icarus* 251, pp. 112–127. DOI: 10.1016/j.icarus.2013.10.033.

644 Wang and Nielsen (2003): Behavior of the Martian dayside electron density peak during global dust storms. In:

645 *Planetary and Space Science* 51 (4-5), pp. 329–338. DOI: 10.1016/S0032-0633(03)00015-1.

646 Whalley and Plane (2010): Meteoric ion layers in the Martian atmosphere. In: *Faraday Discuss.* 147, pp. 349–

647 368. DOI: 10.1039/c003726e.

648 Withers (2009): A review of observed variability in the dayside ionosphere of Mars. In: *Advances in Space*

649 *Research* 44 (3), pp. 277–307. DOI: 10.1016/j.asr.2009.04.027.

650 Withers et al. (2018): First Ionospheric Results From the MAVEN Radio Occultation Science Experiment (ROSE).

651 In: *JGR* 123 (5), pp. 4171–4180. DOI: 10.1029/2018JA025182.

652 Withers and Pratt (2013): An observational study of the response of the upper atmosphere of Mars to lower

653 atmospheric dust storms. In: *Icarus* 225 (1), pp. 378–389. DOI: 10.1016/j.icarus.2013.02.032.

654 Wolff et al. (2017): Radiative Process. Techniques and Applications. In: Robert M. Haberle, R. Todd Clancy,

655 François Forget, Michael D. Smith und Richard W. Editors Zurek (Hg.): The Atmosphere and Climate of Mars:

656 Cambridge University Press (Cambridge Planetary Science), pp. 106–171.

657 Wolkenberg et al. (2018): Characterization of dust activity on Mars from MY27 to MY32 by PFS-MEX

658 observations. In: *Icarus* 310, pp. 32–47. DOI: 10.1016/j.icarus.2017.10.045.

659 Wu et al. (2021): Abnormal Phase Structure of Thermal Tides During Major Dust Storms on Mars. Implications

660 for the Excitation Source of High-altitude Water Ice Clouds. In: *JGR* 126 (4), e2020JE006758. DOI:

661 10.1029/2020JE006758.

662 Zou et al. (2005): Effect of the seasonal variations in the lower atmosphere on the altitude of the ionospheric

663 main peak at Mars. In: *JGR* 110 (A9), pp. 373. DOI: 10.1029/2004JA010963.