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

Magnetic-field-aligned potential drops in the auroral zone have recently been interpreted in terms of “quasi-steady” states which are evaluated for a fixed, given ion density profile. Here we present experimental studies of such states, observed when a voltage drop is suddenly applied to an inhomogeneous plasma column with a homogeneous, axial magnetic field. The quasi-steady 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 applied voltage drop does not exceed a critical value which depends on the maximum difference in ion density. Higher voltage drops concentrate in a cathode sheath. Results from PIC-simulations and from a theoretical model, based on steady electron motion, are also presented. They agree excellently for times up to about an ion plasma period (ω_pi⁻¹). The theoretical current-voltage characteristic, which is determined by electron reflection at a virtual cathode formed in the plasma, agrees with the experiments up to about one ω_pi⁻¹. However, already at about 4ω_pi⁻¹ the measured currents have dropped much below the theoretical values, and also the measured average potential profiles are inconsistent with steady acceleration of the electrons. The increased resistivity is associated with strong fluctuations.