Interrelation Among Double Layers, Parallel Electric Fields, and Density Depletions

Abstract

Using numerical simulations of voltage- and current-driven plasmas, we demonstrate an intimate relationship between electric fields parallel to the ambient magnetic field and density depletions (cavities) in collisionless magnetized plasmas. Simulations of double layers (DLs) in plasmas driven by applied potential drops are reviewed, showing that a strong DLs form in density cavities. The dependence of scale length of the DL and the associated density structure on the potential drop and plasma density is discussed and compared with observations. Simulations of current-driven plasmas also show that a density cavity in current-carrying plasmas could charge to large potentials forming DLs. Simulations of plasmas in an auroral-type diverging flux tube show that the DLs form in the region of the lowest density in the form of a cavity extending over a few Debye lengths in the local plasma. We also review the formation of two-dimensional U-or V-shaped double layers with deep density depletions in which the parallel fields dominate. The transverse size of the narrow density depletions and the DL structures is also discussed. This review reveals that voltages, currents, and density cavities in collisionless plasmas are highly coupled; when the plasma is driven by either a voltage drop or a current, they all appear in the plasma and collectively affect the plasma electrodynamics.