Waves and Instabilities in Collisionless Shocks

Abstract

Abstract : By scattering particles and causing dissipation, waves in a collisionless shock play a role similar to collisions in an ordinary gas. In describing the waves and instability that occur near a shock three regions must be considered: (1) the transition region, (2) the upstream region, and (3) the downstream region. The transition region is usually characterized by an abrupt broadband burst of electrostatic noise extending to frequencies well above the ion plasma frequency, and by a broadband burst of whistler-mode electromagnetic noise at frequencies below the electron cyclotron frequency. The burst of electrostatic noise is believed to be the primary mechanism by which heating and dissipation takes place at the shock. This noise is believed to be ion-acoustic noise driven either by a current or an electron beam in the shock. Electrostatic lower-hybrid waves are also sometimes observed in the transition region. These waves are excited by ions reflected by the shock, and can be very effective at heating both ion and electrons. This paper reviews all of the above waves and comments on the similarities between waves observed near the bow shocks of earth, Venus, Jupiter and Saturn, and interplanetary shocks. Possible mechanisms for generating each type of wave are also considered.