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During Mars dust storms, atmospheric heating and expansion moves the ionospheric peak upward. Typically, peak altitude increases by no more than 10 km, and this increase occurs simultaneously with the expansion of the dust storm. However, Felici et al. (2020), https://doi.org/10.1029/2019JA027083, using the Mars Atmosphere Volatile EvolutioN (MAVEN) Radio Occultation Science Experiment (ROSE), reported an unusually large increase of ∼20 km at southern latitudes in early October 2016 during a modest dust storm. Here, we investigate why the ionospheric peak altitude increased so much in these observations. We extend the time series of ionospheric peak altitude values beyond the limited extent of the ROSE observations by applying a one‐dimensional photochemical model, in which neutral atmospheric conditions are based on in situ MAVEN Neutral Gas Ion Mass Spectrometer observations at similar latitudes and solar zenith angles to those observed by ROSE. We find that the ionospheric peak altitude was highest throughout October 2016 yet both the local and global atmospheric dust loading were greatest 1 month earlier. We hypothesize that (a) a portion of the unusually large 20 km enhancement in peak altitude and (b) the unusual delay between the greatest dust loading and the highest peak altitude were both associated with the occurrence of perihelion, which maximizes solar heating of the atmosphere, in late October 2016. Plain Language Summary: Dust storms always cause the top of the Martian ionosphere, a weakly ionized region in the upper atmosphere, to move to higher heights, typically by ∼10 km, due to the heating and expansion of the atmosphere caused by the storm. However, during a moderate regional dust storm in 2016, the ionospheric peak height elevated by ∼20 km. This study examines the reasons for this unusual increase. Simulations using a photochemical model showed that the ionospheric peak remained elevated for the whole month of October. However, the dust storm peaked in September, roughly a month before maximum ionospheric height. This is quite unusual as the ionospheric peak usually occurs at the highest heights during the peak of storms as maximum atmospheric heating and expansion occur during this period. Thus, this time delay suggests that the dust storm event does not solely cause the atypical enhancement in the ionospheric peak altitude. We suggest that the occurrence of Martian perihelion, Mars's closest point to the Sun in its orbit, and associated enhancement in solar heating in October 2016 might have contributed significantly to the thermal expansion of the atmosphere and hence toward the observed enhancement in the ionospheric peak height. Key Points: The ionospheric peak altitudes in the southern latitudes enhanced by ∼20 km during the regional dust storm of 2016Dust loading maximized during mid‐September but ionospheric peak heights reached their maximum only in October, suggesting a time delayThe occurrence of perihelion in October 2016 might have caused the time delay and also could have contributed to the unusual enhancement [ABSTRACT FROM AUTHOR]

We utilize data from the MAVEN Radio Occultation Science Experiment (Withers et al., 2020) - with unprecedented coverage in solar zenith angle - to isolate the effects that local time and season induce on the photochemical ionosphere of Mars around solar minimum, leading to solar maximum. 185 out of the 1228 electron density profiles of the Martian undisturbed ionosphere collected by MAVEN ROSE between July 2016 and December 2022 show a distinct M1 layer below the M2 layer. We define undisturbed here as conditions when there are no solar events or dust storms to influence the ionosphere. This allowed us to study the behavior of both the M2 and M1 peak densities and altitudes as a function of solar zenith angle, and, for the first time, to be able to separate these trends by dusk and dawn local time, as well as by southern spring and summer versus southern fall and winter. We find that the M1 layer at small SZA can occur at altitudes lower than 100 km; that the peak altitudes and densities of both the M2 and M1 layers at dawn change more with season than they do at dusk; and that the M2 peak density decreases at a faster rate than the M1 with SZA. • There is more seasonal variability in the dawn ionosphere than the dusk ionosphere. • The M1 layer is present at small SZA and, when present, is located ∼ 20 km below the M2 layer. • The ratio of M1 over M2 peak densities increases with SZA, and the vertical distance between the two layers increases. [ABSTRACT FROM AUTHOR]

We use 62 electron density profiles collected by the Radio Occultation Science Experiment (ROSE), on the Mars Atmosphere and Volatile EvolutioN (MAVEN), when Mars was hit by CIRs and ICMEs close to aphelion (April 2021) and during two dust storms (June/July 2022) to examine the response of the Martian ionosphere to solar events and to solar events hitting during dust storms. We do so through three proxies – variation in total electron content between 80 and 300 km altitude, peak density, and peak altitude – of the aforementioned 62 ROSE electron density profiles, relative to a characterisation of the ionosphere through solar minimum leading to solar maximum, specific to local time sector and season, presented in the companion manuscript by Segale et al. (2024). We observe an increased Total Electron Content (TEC) between 80 and 300 km altitude up to ≃ 2. 5 × 1 0 15 m−2 in April 2021 and up to ≃ 5 × 1 0 15 m−2 in June/July 2022 compared to the baseline photochemically produced ionosphere. This increase in TEC corresponds mainly to increases in the solar energetic particles flux (detected by MAVEN Solar Energetic Particle, SEP) and electron fluxes (detected by the MAVEN Solar Wind Electron Analyzer, SWEA). In addition to solar events, in June/July 2022, an A storm and a B storm were occurring and merging on the surface of Mars. We observe a raise in peak altitude in general lower than expected during dust storms, possibly due to high values of solar wind dynamic pressure (derived from MAVEN Solar Wind Ion Analyzer, SWIA). From 31 ROSE profiles collected in this time period that showed both the M2 and M1 layer, we observe that, on average, M1 and M2 peak altitudes raise the same amount, suggesting that the thermosphere might loft as a unit during dust storms. During this time period, several proton aurora events of variable brightness were detected with MAVEN Imaging Ultraviolet Spectrograph (IUVS), underlining the complex and multifaceted impact of dust activity and extreme solar activity on the Martian ionosphere. • Increase in TEC up to 2. 5 × 1 0 15 m−2 between 80 and 300 km following an ICME/CIR impact. • Increase in TEC up to 5 × 1 0 15 m−2 between 80 and 300 km following ICME/CIR impacts during dust storms. • During dust storms, the peak altitudes of the M1 and M2 layers loft of the same amount. [ABSTRACT FROM AUTHOR]

The Radio Occultation Science Experiment (ROSE; Withers et al., 2018, 2020) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite has produced over 400 electron density profiles from July 2016 to November 2019. These N e (h) profiles occurred over a large range of solar zenith angles (54o- 130o) and solar flux conditions (24–54 solar flux units at Mars). One of the goals of the MAVEN mission is to characterize the status of the topside ionosphere at Mars as a reservoir of possible escaping plasma. Here we evaluate how ROSE topside ionospheric measurements, made predominantly under solar minimum conditions, compare with the only empirical model of the topside ionosphere (Němec et al., 2019). To assess congruence between the model and the observations, a deviation factor (DF) is calculated for each predicted versus observed N e (h) profile. Diurnally, low DFs (and thus higher agreement) occur for occultations with solar zenith angles (SZA) < 82o. On a longer time scale, lower solar fluxes tend to have higher DF values. Correlations of the maximum electron density (N max) of each profile (predicted and observed) have an overall correlation coefficient (CC) of 0.96. Similarly, the observed total electron content of the topside ionosphere (TTEC) agrees with predictions (CC = 0.86). The model and the predictions differ most for the altitude of peak density (CC = 0.62 for SZA < 90o). • Topside ionosphere model from MEX validated successfully using new MAVEN data. • Good model-data agreement except for high solar zenith angles and low solar fluxes. • Solar minimum ionospheric variability at Mars greater than found at Earth. [ABSTRACT FROM AUTHOR]