Topside of the martian ionosphere near the terminator: Variations with season and solar zenith angle and implications for the origin of the transient layers
In this paper, the morphological variations of the M2 layer of the martian ionosphere with the martian seasons and solar zenith angle (SZA) at the terminator are investigated. The data used are the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) measurements (approximately 5000 ionograms) that were acquired from 2005 to 2012, which have a SZA ⩾ 85° and detect the topside transient layers. A simple, effective data inversion method is developed for the situation in which the upper portion of the height profile is non-monotonic and the observed data are insufficient for adequate reduction. The inverted parameters are subsequently explored using a statistical approach. The results reveal that the main body of the M2 layer (approximately 10 km below the first topside layer) can be well-characterized as a Chapman layer near the terminator (SZA = 85-98°), notwithstanding the high SZA and the presence of the topside layers. The height of the first topside layer tends to be concentrated approximately 60 km (with a standard deviation of ∼20 km) above the main density peak. The peak density and height of the first topside layer are positively correlated to the density and height of the main peak, respectively. The density and height of the first topside layer appear to be independent of the SZA, but possess seasonal variations that are similar to those of the main layer. The height of the topside layer is greater (by ∼10 km on average) in the southern spring and summer than in the southern autumn and winter, coinciding with the observation that, in the southern spring and summer, the underlying atmosphere is warmer due to dust heating (e.g., Smith, M.D. . Icarus 167, 148-165). The statistical regularities of the parameters suggest a possibility that the formation of the topside layers are closely related to the processes of photoionization and diffusion that occur on the topside of the M2 layer. We propose that development of beam-plasma instabilities in the transitional region (between the lower Chapman region and the upper transport-dominating region) is possibly a mechanism that is responsible for the occurrences of the topside layers.