Double layers and electrostatic shocks in space

In recent years, much evidence has been accumulated which supports the existence of spatially limited regions of strong electric fields (>100 mV/m) parallel to the magnetic field. Evidence which will be reviewed comes from rocket measurements, satellite observations, and plasma cloud releases above the polar ionosphere. The most plausible explanation of such structures is provided by the concept of double layers or electrostatic shocks, which are double layers moving in relation to the plasma. We also review some recent laboratory investigations of double layers which show that double layers can exist for long periods of time. The production of these structures requires large currents. The theory of double layers has so far concentrated on the nature of stationary double layers, and theoretical proof is now available that the Vlasov equation allows for stationary double layer solutions in one or two dimensions. A full three‐dimensional treatment is not available. Necessary conditions for the existence of double layers (generalized Bohm criterion), relations between thickness and strength of the layer, and the field signature are studied. Comparison with observations reveals many similarities, but major problems (e.g., lack of magnetic field alignment of accelerated particles and particle heating in contrast to free acceleration) remain. All double layers observed so far are parts of a large‐scale electric circuit carrying a current driven by a generator and dissipated in a load. Thus the whole circuit must be analyzed in order to understand properly the generation and nature of double layers.