Interaction between the solar plasma wind and the geomagnetic cavity

Abstract

If the quiet-time interplanetary magnetic field near the earth is parallel to the solar wind, the flow of this collision-free plasma over the geomagnetic cavity is reducible to an equivalent hypersonic blunt-body problem in ordinary gasdynamics. The shape of the converging portion of the geomagnetic cavity is determined by an equivalent gasdynamic (PrandtlMeyer) expansion along the surface streamline. The length of the tail measured from the earth's center is about 54 earth-radii. At the aft end of the cavity, the converging plasma flow is deflected back to the axial direction across an oblique tail or shock, which decays to an Alfven wave in a distance of some 10-20 cavity diameters downstream, or about 300 earth-radii. The geomagnetic cavity leaves a wake of hot plasma in the solar wind, and a corresponding defect in magnetic field intensity. The effects of a transverse component of the interplanetary magnetic field are illustrated by considering the flow along the stagnation line between the bow shock and the stagnation point on the magnetosphere. The axial velocity and plasma density both vanish at the stagnation point, whereas the transverse magnetic field reaches its maximum value there. This discussion furnishes the necessary first step for the analysis of the flow and magnetic field around the magnetosphere.