Interpretation of the Altitudinal Variation in the Martian Ionosphere Longitudinal Wave-3 Structure

Using the electron density profiles from Mars Global Surveyor Radio Occultation observations, the ionospheric longitudinal wave-3 (WN3) structure with different frequencies is investigated. The semidiurnal component is the dominant component for the entire Mars ionosphere profile at northern high latitudes. Both diurnal and semidiurnal components manifest double peaks in altitudinal variations in amplitude, which are 5-15 km lower than the ionospheric M1 and M2 peaks. The phase generally increases with altitude and shows a rapid rate of increase around the M2 peak. To interpret the altitudinal variation in amplitude and phase, we then derive the tidal coupling equation to elucidate the impact of atmospheric tides on the Mars ionosphere. Under the assumption of photochemical equilibrium, the tidal coupling equation indicates that the log density gradient of electrons (kN ${k}_{N}$) critically modulates the ionospheric response to atmospheric tides. In the observations, a positive correlation between the kN ${k}_{N}$ profile and the amplitude of the WN3 structure at altitude is found. The solar longitude variation in the kN ${k}_{N}$ peaks also agrees with the vertical movement of the amplitude peaks of the WN3 structure along the solar longitude. The rapid phase increase corresponds to the abrupt decrease in kN ${k}_{N}$ around the M2 peak. These observational results agree with the tidal coupling equation prediction, thereby supporting that the interpretation of kN ${k}_{N}$ in modulating the altitudinal variation in the WN3 structure is reasonable.