Concerning Certain Collisionless Plasma-Shock Wave Models

A shock-wave model proposed by both Kahn and Parker for the collision of two interstellar gas clouds or of the solar wind with the ionized gas around the earth is re-evaluated in the light of recent developments in plasma stability theory. It is shown that the instability which was supposed to arrest the counter-streaming of the ions and to transfer much of their kinetic energy to the electrons does not occur in many important examples. Specifically, it is absent if the initial ion thermal energy exceeds 4.2 x 10 (sup -4) times their translational energy. While other plausible shock structures are known which yield supra-thermal electrons, they depend on magnetic fields, leaving no satisfactory theory in the limit of zero magnetic field.