Characteristic Timescales for the Dayside Martian Ionosphere: Chemistry, Diffusion, and Magnetization

Different boundaries could be defined in a planetary ionosphere where the dominant process in function switches from one to the other. Identifying these boundaries and understanding their variations are hence crucial for disentangling the complexity of the ionosphere. Focusing on Mars, we perform a data-driven analysis of various boundaries and the associated time constants based on the multi-instrument measurements made by the Mars Atmosphere and Volatile Evolution mission during six campaigns that sample broadly different internal and external conditions. The boundaries we investigate include the photochemical equilibrium (PCE) boundary, the magnetic frozen boundary, the ion collision boundary, and the ion gyration boundary. Our analysis reveals systematic solar cycle and diurnal variations in that all boundaries tend to be elevated at enhanced solar activity and on the dayside and duskside of Mars. The variations with the magnetic environment are not observed for all boundaries except for the PCE boundary that exhibits an obvious elevation in strongly magnetized regions. Finally, our analysis suggests interesting species-dependent variations of different boundaries. In particularly, the PCE boundary shows the largest variability among all, with reduced boundary locations for all terminal species (NO ^+ , HCO ^+ , ${{\rm{O}}}_{2}^{+}$ , and H _3 O ^+ ) and one extra nonterminal species (CO _2 ^+ ) owing to different chemical properties rendered by different ions.