Waves and Instabilities in Collisionless Shocks

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 whistlermode 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. Upstream of the shock electron plasma oscillations, ion-acoustic waves, and intense ULF (ultra-low-frequency) magnetohydrodynamic waves are frequently observed. The electron plasma oscillations occur in the electron foreshock and are driven by suprathermal electrons escaping into the region upstream of the shock. Both the ion-acoustic and ULF waves occur in the ion foreshock and are associated with ions streaming into the region upstream of