The solar wind interaction with Mars revisited

Owing to a paucity of observational data, no clear consensus has been reached concerning the general nature of the solar wind interaction with Mars. In particular, the previous analyses are still at odds regarding the existence of a small intrinsic field magnetosphere at Mars as opposed to a Venus-type ionospheric interaction (e.g., Russell, 1978a,b; Dolginov, 1978b,c). This study contributes to the resolution of this question in three ways. First, an improved determination of effective obstacle altitude and shape is obtained from the Mars 2,3, and 5 bow shock encounters through the use of a recently published catalog of gasdynamic flow solutions (Spreiter and Stahara, 1980a,b). Second, building upon the Pioneer Venus findings at a field-free planet (Brace et al., 1980; Elphic et al., 1980), it is shown that the Martian ionosphere cannot support a Venus-type ionopause at the obstacle altitudes inferred through our modeling of the bow wave observations even when maximal induced ionospheric magnetic fields and solar maximum EUV levels are assumed. These results allow an effective Mars magnetic dipole moment of 1.4 (±0.6) × 1022 G-cm3 to be determined that stands off the solar wind over the dayside hemisphere at altitudes ranging from∼500 km at the subsolar point to ∼1000 km near the terminator with no direct aid from the ionosphere under average solar wind/magnetospheric conditions. Third, a search of published Mars and Mariner radio occultation measurements produced no evidence for the existence of an ionopause at Mars in agreement with the Viking study of Lindal et al. (1979). Rather, the electron density altitude profiles appear qualitatively consistent with the Martian ionosphere terminating in a chemopause associated with the effects of magnetospheric convection as first proposed by Bauer and Hartle (1973). After a review of the various arguments in the literature, as supplemented by the results of this study, we conclude that Mars most probably possesses a small intrinsic field magnetosphere.