Atypically Intense and Delayed Response of the Martian Ionosphere to the Regional Dust Storm of 2016: A Study Using MAVEN Observations and Models.

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]