PLASMA COMPOSITION MEASUREMENTS OF THE MARTIAN MAGNETOSPHERE MORPHOLOGY

Plasma composition measurements from the Phobos‐2 spacecraft have revealed new data on the dynamics and composition of plasmas in the Martian magnetosphere—a magnetosphere being significantly wider and more dynamic than was anticipated before. The tail width is some 6–9 Mars radii at antisolar distances greater than ten Martian radii and it contains some features that are similar to those observed in the Geotail. The strong dominance of plasma of Martian origin implies a modest access of solar wind particles into the central tail. Heavy ions like O+ are in fact up to ten times more abundant then H+. Some plasma properties, such as the characteristics of the ionospheric ion outflow, the presence of a distinct plasma sheet and the overall width of the tail are arguments for a weak intrinsic magnetic field on Mars. However, yet other plasma characteristics, such as strong ion pick‐up processes, indicates a Venus‐like interaction. Thus, our present view based on primarily plasma measurements is that Mars constitutes a hybrid obstacle to the solar wind. The observation of two distinctly different plasma boundaries suggests that the magnetosphere of Mars is contained whithin an exterior / composition boundary—termed the Mass‐Loading Boundary (MLB), and an interior boundary—a “magnetopause”. MLB marks the boundary for a cometary‐like interaction and the “magnetopause” the limit of solar wind plasma entry—possibly also constituting the boundary for an intrinsic magnetic field. Copyright 1990 by the American Geophysical Union.