Evaluation for effects of variable Martian upper atmosphere on ionospheric peak electron density based on the MGS RO observation

The Martian ionospheric peak is dominated by photochemical processes at dayside. Its primary variations can be well described by the Chapman theory; nevertheless, deviation still exists between observation and the theory. Some factors may be responsible for that deviation, in which the variable upper atmosphere is an important one. Ionospheric radio occultation (RO) measurement can be used to investigate the effect of the upper atmosphere on the Martian ionosphere, since concurrent variations of the ionospheric peak and the neutral scale height (H-n) can be estimated from RO electron density profiles. In this study, H-n was estimated from the Mars Global Surveyor (MGS) RO observation to quantitatively evaluate the effect of the upper atmosphere on the variability of the peak electron density of the M-2 layer (NmM2). That effect manifests in the abilities of solar zenith angle functions for describing NmM2 variability and in the NmM2 deviation from the Chapman theory. Compared with the Chapman function, the simple cosine function is sufficiently accurate for describing dayside NmM2 variation owing to the cold Martian upper atmosphere that consists of heavier molecules, which weakens the spherical effect of plan-etary atmosphere. Hn variations, especially the longitudinal variation associated with non-migrating tides, are important factors responsible for the NmM2 deviation from the Chapman theory. Statistically, they can cause similar to 1.4% average deviation for the condition of the MGS RO measurement. An effective modification method based on available Hn was correspondingly suggested to promote the accuracy of NmM2 estimation based on the Chapman theory. It is notable that Hn variations do not show evident contribution to NmM2 deviation from the Chapman theory in the southern hemisphere, where the stronger crustal magnetic fields likely have dominant contribution to the NmM2 deviation as compared with Hn variations.