Experimental investigations and simulations of quasi-steady potential drops in plasmas

Magnetic-field-aligned potential drops in the auroral zone have recently be en interpreted in terms of "quasi-steady" states which are evaluated for a fixed, given ion density profile. Here we present experimental studies of s uch states, observed when a voltage drop is suddenly applied to an inhomoge neous plasma column with a homogeneous, axial magnetic field. The quasi-ste ady potential drops have a spatial extension related to the gradient length of the initial ion density and the potential profile steepens slowly to a U-shaped double layer on the ion time scale. They exist only when the appli ed voltage drop does not exceed a critical value which depends on the maxim um difference in ion density. Higher voltage drops concentrate in a cathode sheath. Results from PIC-simulations and from a theoretical model, based o n steady electron motion, are also presented. They agree excellently for ti mes up to about an ion plasma period (omega (-1)(pi)) The theoretical curre nt-voltage characteristic, which is determined by electron reflection at a virtual cathode formed in the plasma, agrees with the experiments up to abo ut one omega (-1)(pi). However, already at about 4 omega (-1)(pi) the measu red currents have dropped much below the theoretical values, and also the m easured average potential profiles are inconsistent with steady acceleratio n of the electrons. The increased resistivity is associated with strong flu ctuations. (C) 2001 Elsevier Science Ltd. All rights reserved.