THE EFFECT OF MAGNETIC-FIELDS ON THE MACROSCOPIC INSTABILITY OF THE HELIOPAUSE .1. PARALLEL INTERSTELLAR MAGNETIC-FIELDS

Several solutions for stationary models for the plasma flow configurat ion in the interface region between the unperturbed flows of the inter stellar plasma and the supersonic solar wind plasma have been discusse d in the literature. The magnetohydrodynamic form of an interaction of these counterstreaming flows establishes a free pressure equilibrium surface, called the heliopause, with a frontal nose part in the upwind direction and a cylindrical tail extending into the downflow directio n. There is, however, a need to discuss whether or not the proposed he liopause configurations can be expected to be stable with respect to m agnetohydrodynamical or hydrodynamical perturbations caused by time-de pendent or turbulent structures in the counterflowing plasmas. In a fo regoing paper we have studied the stability of the heliopause boundary surface with respect to purely hydrodynamical perturbations. In this paper here, we in addition take into account interstellar magnetic fie lds at the heliopause and investigate their influence on the stability of this layer. Again it turns out that this problem is of a different quality in the nose region and in the off-nose region (the flanks) of the heliopause. However, it is shown here that compared to the purely hydrodynamic case magnetic fields can have a strongly stabilizing eff ect at those heliopause regions where the outer LISM fields are parall el to the shear velocities of the adjacent plasmas. We especially devo te attention to these latter regions and investigate the general forms of MHD Kelvin-Helmholtz instabilities for the prevailing transition c onditions in compressible plasmas. We show that the part in the off-no se region of the heliopause, where the Mach number of the shocked sola r wind flow is smaller than some critical value defined by the undistu rbed flow parameters, is stable with respect to short wavelength distu rbances. In any case, however, this critical Mach number is smaller th an unity. On the other hand, this means that the part of the off-nose heliopause region where the shocked solar wind flow becomes supersonic cannot be stabilized by the interstellar magnetic field, no matter ho w strong it is.