THE INSTABILITY OF A TANGENTIAL DISCONTINUITY IN A PLASMA WITH COSMIC-RAYS - THE APPLICATION TO THE HELIOPAUSE

The effect of the pressure of cosmic rays on the stability of a tangen tial discontinuity in a flow of a cosmic plasma is studied. The classi cal result on the unconditional instability of a tangential discontinu ity in a gas in the absence of a magnetic field is not changed when th e cosmic ray pressure is taken into account. But a new unstable mode a ppears if the relative plasma velocity is supersonic. The properties o f this mode differ qualitatively from those of the classical Kelvin-He lmholtz one. If the low-energy cosmic rays in the outer heliosphere co ntain almost all the thermal energy of the plasma, then the unstable w aves with the maximum growth rates in the heliopause flanks propagate along the direction of the plasma flow. For a classical, highly supers onic flow without cosmic rays, as is well known, the unstable waves pr opagate almost at right angles to the direction of the flow. In the ge neral case, the wave structure of the heliopause in the regions with s upersonic flows strongly depends on the ratio of the cosmic ray and th ermal plasma pressures.