Effects of Martian crustal magnetic field on its ionosphere

The effect of the Martian crustal magnetic field is one of the hot topics of the study of Martian ionosphere. The studies on this topic are summarized in this paper. Main data of the Martian ionosphere were resulted from radio occultation experiments. According to the observations, the electron density scale height and the peak electron density of the Martian ionosphere are influenced by its crustal magnetic field. The strong horizontal magnetic field prevents the vertical diffusion of the plasma and makes the electron density scale height in the topside ionosphere close to that in the photo equilibrium region. In the cusp-like regions with strong vertical magnetic field, the enhanced vertical diffusion leads to a larger electron density scale height in the diffusion equilibrium region. The observation of radio occultation experiment onboard Mars Global Surveyor (MGS) showed that the averaged peak electron density observed in the southern hemisphere with strong crustal magnetic field was slightly larger than that in the northern hemisphere with weak crustal magnetic field. The Mars advanced radar for subsurface and ionosphere sounding (MARSIS) onboard Mars Express (MEX) was the first topside sounder to be used to observe Martian ionosphere. The MARSIS results confirmed that the enhancement of the peak electron density occurred in cusp-like regions with open field lines, and the amount of the enhancement was much larger than that observed by the radio occultation experiment. There are two possible mechanisms for the peak electron density enhancement in the cusp-like crustal magnetic field regions: One is the precipitation of the energetic particles and the other is the heating by the waves excited by plasma instabilities. It's difficult to determine which one is the key mechanism for the peak electron density enhancement. Based on these studies, several interesting problems on the Martian ionosphere and plasma environment are presented.