Spacecraft observations around Mars show that ionospheric irregularities occur frequently in the Martian ionosphere. In this study, Mars Atmosphere and Volatile Evolution data (the region is below ∼200 km) during 2015 January to 2021 March were used to revisit the statistical characteristics of ionospheric irregularities and the comparison of irregularities in Martian years with higher or lower solar activity phase of solar cycle. Results show that the characteristics of the irregularities with a larger length scale associated with the magnetic field and solar zenith angle are similar to the previous studies. Moreover, our results show that the occurrence rate of irregularities exhibits dawn and dusk asymmetry, and the occurrence rate at dusk is higher than that at dawn. In addition, results demonstrate that the occurrence rate of irregularities is higher in Martian years with higher solar activity than Martian years with lower solar activity, which means that the solar cycle might play an important role in the formation of irregularity events. Further studies show that the solar zenith angle (SZA) and altitude at the maximum occurrence rate depend on the level of solar activity. The SZA and altitude of the maximum occurrence rate are smaller in the Martian year with higher solar activity than the lower. We also found that the rate of events is lower during the day than the terminator in the ionospheric dynamo region. By contrast, in the regions where both electrons and ions are magnetized, events have a higher rate during the day than the terminator. Furthermore, the seasonal variation of the irregularity events was also presented in this study. Results show that the occurrence rate in the dynamo region with 80° < SZA < 150° in MY34 and MY35 show an incremental trend from spring to winter, but this trend is not obvious in MY33.

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Many Mars missions, e.g., Mariner, Viking, Mars Global Surveyor (MGS), Mars Express (MEX), and Mars Atmosphere and Volatile Evolution (MAVEN), have been launched to study the Martian atmosphere. These observations have improved our understanding of the ionosphere of Mars. Observations show that the Martian ionosphere could be divided into an M2 layer (at ~140 km altitude) and an M1 layer (at ~110 km altitude), ionized by EUV and X-ray solar radiation, respectively. However, there are still many scientific questions about the Martian ionosphere. A transient topsider layer (also called the M3 layer, at ~160–~220 km) can frequently be measured above the M2 layer in the Martian ionosphere. The statistical characteristics of the M3 layer show that it is not irradiated by solar incident radiation. Many candidate mechanisms have been suggested to explain the formation of the M3 layer. However, the method of describing or modeling the M3 layer is still one of the many open scientific questions about the Martian ionosphere. This study used a one-dimensional model, including photochemical production, loss, and dynamic transport processes to simulate the transient topside layer in the Martian ionosphere. The M3 layer was reproduced by a perturbed vertical plasma drift in this study. The enhancement of the electron temperature induced by the dynamic process of plasma could facilitate the formation of the M3 layer. Our results show that the vertical transport process of plasma might be more crucial in producing the M3 layer in the topside Martian ionosphere.

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