The physics of double layers and their role in astrophysics

Abstract

Research on electrostatic double layers (DLs) is surveyed and astrophysical applications are reviewed. Strong highly relativistic DLs directly accelerate equal numbers of ions and electrons, whereas in the non-relativistic limit electron acceleration predominates. The existence criteria for DLs are discussed. There is a close relationship between the Bohm criteria, the plasma dispersion relation and the Penrose stability condition. Current-driven instabilities can lead to DL formation, and account for the fluctuations associated with strong DLs. A class of weak DLs may be described using the modified Korteweg-de Vries equation, and the conditions for the emergence of a DL from arbitrary initial conditions can be specified. Simulation studies of strong DL formation reveal important dynamical features. Weak DLs are found in simulations of ion-acoustic instabilities. Laboratory DLs have been studied in great detail. There is a strong coupling between a DL and the global current system. The results of theoretical, simulation and observational investigations of magnetospheric DLs are presented. High-energy observations of solar flares agree well with a DL model. DLs may be a feature of many high-energy astrophysical phenomena.