The instability of a tangential discontinuity in a plasma with cosmic rays: the application to the heliopause

Abstract

The effect of the pressure of cosmic rays on the stability of a tangential discontinuity in a flow of a cosmic plasma is studied. The classical result on the unconditional instability of a tangential discontinuity in a gas in the absence of a magnetic field is not changed when the cosmic ray pressure is taken into account. But a new unstable mode appears if the relative plasma velocity is supersonic. The properties of this mode differ qualitatively from those of the classical Kelvin-Helmholtz one. If the low-energy cosmic rays in the outer heliosphere contain almost all the thermal energy of the plasma, then the unstable waves with the maximum growth rates in the heliopause flanks propagate along the direction of the plasma flow. For a classical, highly supersonic flow without cosmic rays, as is well known, the unstable waves propagate almost at right angles to the direction of the flow. In the general case, the wave structure of the heliopause in the regions with supersonic flows strongly depends on the ratio of the cosmic ray and thermal plasma pressures.