Mars in the solar wind

Abstract

The basic interaction between Mars and the solar wind is reexamined by using recent spacecraft observations and model calculations. It is found that the particle pressure is not large enough to stand off the solar wind unless the electron temperature is 4 times the ion temperature in the ionosphere. However, the additional pressure is presumably magnetic provided by a planetary magnetic field, although an induced field cannot be excluded. A planetary field corresponding to a surface field of ∼20 gammas and a dipole moment of ∼8×1021 gauss cm3 (0.01% of the earth’s dipole moment) are implied. On the basis of our calculation, 60–70% of the pressure is supplied by the magnetic field, and the remaining 30–40% by the ionosphere. This suggests that the interaction of Mars in the solar wind is unique in that it is both atmospheric (i.e., Venuslike) and magnetospheric (i.e., similar to that at earth and Jupiter). Which mode of interaction is dominant is likely to depend on the external solar wind, the magnetic field interaction and the atmospheric interaction being characteristic of quiet and disturbed conditions, respectively. The implications of an earthlike interaction are reexamined by using magnetospheric scaling laws, and it is found that plasma convection may play a major role in the Martian magnetosphere and ionosphere and that the auroral oval may extend to low latitudes. The implications of a Venuslike interaction are also examined, in particular the heating and removal of atmospheric ions as a result of the direct interaction between the solar wind plasma and the ionosphere. The existence of a planetary field also implies that Mars has a liquid rather than a frozen core.