Abstract :
Despite a number of observations near Mars, the structure and dynamics of the ionosphere of Mars have not been fully understood mainly because of an insufficient amount of magnetic field data within the ionosphere. In 1997, Mars Global Surveyor (MGS) successfully observed magnetic fields within the ionosphere for the first time. MGS discovered fairly strong (B ≤ 1600 nT) but localized magnetic fields in various regions on the Martian surface. Those magnetic fields are interpreted as crustal magnetic anomalies. At the same time, strength of a global intrinsic magnetic field of core origin was found to be smaller than about 5 nT at the surface of Mars. A Venus-like ionospheric magnetic field induced by the solar wind was also seen in the ionosphere of Mars. The results suggest that the Martian ionosphere is controlled both by solar wind interaction and by the crustal magnetic field. Therefore, the nature of the Martian ionosphere is probably different from any other planetary ionospheres, and is likely to be most complicated among the planetary ionospheres. Current understanding of the ionospheres of Mars is reviewed, and outstanding problems with the Martian ionosphere are pointed out.
