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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 report the impact of the 2018 Mars global dust storm on the ionosphere, in particular, its peak altitude during the onset and growth phases of the event. A one‐dimensional photochemical model is applied to estimate the ionospheric electron densities below 200 km using the measurements of in situ neutral densities and ionizing solar radiation by the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Our model results show that while the peak density of the ionosphere on the dawn side during the dust storm is hardly impacted, the altitude of the peak layer may be lifted by ∼10 km. The simulation results support the MAVEN radio occultation observations that there is a time delay in the response of the ionospheric peak altitude in the southern hemisphere (SH), compared with the northern hemisphere (NH) during the dust storm development. Although the 2018 global dust storm developed in the northern Martian mid‐latitudes on 1 June and developed into a global dust event on 17 June, our study suggests that the impact on the ionospheric peak altitude near the equatorial and southern mid‐latitudes became important only after 22 June. This suggests that there was a time‐lag of about 3 weeks in the response of the Martian upper atmosphere of the SH to the dust storm that originated in the NH. Plain Language Summary: Mars has the greatest dust storms in the solar system, which occur infrequently but are able to obscure the planet's surface and last for several months. Such Planet encircling dust events affect all layers of the Martian atmosphere, the most notable effect being the expansion of the neutral atmosphere due to intensified solar heating through atmospheric dust loading. As a result, the peak of the ionosphere, where the plasma density is maximized, is elevated to greater heights. Such a global storm event took place on Mars in June 2018 and lasted for 2–3 months. The spacecraft observations of the Martian ionospheric peak during this event were mostly confined to the northern hemisphere where the storm originated and only very limited observations are available in the southern hemisphere (SH). In the present study, we investigate how the peak of the ionosphere in the SH of the planet varied during the onset phase of the global storm in 2018. We found that the peak altitude increased by 7–10 km. In addition, we report that the response was not immediate, but instead took around 22–26 days for the enhancement to appear in the ionospheric peak altitude of the SH. Key Points: Ionospheric electron densities are calculated in the southern hemisphere (SH) during the onset and growth phases of the 2018 global dust stormThe dust storm caused an uplift of the ionospheric peak altitude by ∼10 km in the SHThe uplift of the ionosphere near equatorial and southern mid‐latitudes lagged 22–26 days from the storm's onset in the northern hemisphere [ABSTRACT FROM AUTHOR]