THE SOLAR-WIND INTERACTION WITH MARS - MARINER-4, MARS-2, MARS-3, MARS-5, AND PHOBOS-2 OBSERVATIONS OF BOW SHOCK POSITION AND SHAPE

Observations taken by Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 are used to model the shape, position, and variability of the Martian bow shock for the purpose of better understanding the interaction of this planet with the solar wind. Emphasis is placed upon comparisons with the results of similar analyses at Venus, the only planet known to have no significant intrinsic magnetic field. Excellent agreement is found between Mars bow shock models derived from the earlier Mariner-Mars data set (24 crossings in 1964-1974) and the far more extensive observations recently returned by Phobos 2 (94 crossings in 1989). The best fit model to the aggregate data set locates the subsolar bow shock at a planetocentric distance of 1.56 +/- 0.04 R(M). Mapped into the terminator plane, the average distance to the Martian bow shock is 2.66 +/- 0.05 R(M). Compared with Venus, the bow wave at Mars is significantly more distant in the terminator plane, 2.7 R(M) versus 2.4 R(V), and over twice as variable in location with a standard deviation of 0.49 R(M) versus 0.21 R(V) at Venus. The Mars 2, 3, and 5 and Phobos 2 data also contain a small number of very distant dayside shock crossings with inferred subsolar obstacle radii derived from gasdynamic modeling of 2000 to 4000 km. Such distant bow shock occurrences do not appear to take place at Venus and may be associated with the expansion of a small Martian magnetosphere under the influence of unusually low solar wind pressure. Finally, the altitude of the Venus bow shock has a strong solar cycle dependence believed to be due to the effect of solar EUV on the neutral atmosphere and mass loading. Comparison of the Phobos 2 shock observations near solar maximum (R(z) = 141) with the Mariner-Mars measurements taken much farther from solar maximum (R(z) = 59) indicates that the Martian bow shock location is independent of solar cycle phase and, hence, solar EUV flux. These results are interpreted in terms of a hybrid solar wind interaction model in which this planet possesses a weak, but significant, intrinsic magnetic field.