Anomalous transport in discrete arcs and simulation of double layers in a model auroral circuit

Abstract

The evolution and long-time stability of a double layer (DL) in a discrete auroral arc requires that the parallel current in the arc, which may be considered uniform at the source, be diverted within the arc to charge the flanks of the U-shaped double layer potential structure. A simple model is presented in which this current redistribution is effected by anomalous transport based on electrostatic lower hybrid waves driven by the flank structure itself. This process provides the limiting constraint on the double layer potential. The flank charging may be represented as that of a nonlinear transmission line. A simplified model circuit, in which the transmission line is represented by a nonlinear impedance in parallel with a variable resistor, is incorporated in a one-dimensional simulation model to give the current density at the DL boundaries. Results are presented for the scaling of the DL potential as a function of the width of the arc and the saturation efficiency of the lower hybrid instability mechanism.

Auroral arc thicknesses as predicted by various theories

“…Fourth, the power delivered to the region to drive the lower-hybrid waves must be balanced with the power leaving the region owing to wave propagation in order to obtain estimates of the lower-hybrid wave levels. Pertinent to the third and fourth issues, the wave amplitudes have been estimated [Smith, 1986a[Smith, , 1986b[Smith, , 1987…”

“…Note that the obtained arc-width value is fairly insensitive to the choice of the reflection coefficient R. Mapping this width along the dipole magnetic field lines from r -2.5 Rs to the ionosphere according to expression (10) yields W•o, --Wm•g/4.34 • 6.9 km as the width of the arc.3.1oe. Lower-Hybrid-Wave Broadened dc ElectrostaticStructureIn a model presented bySmith [1986aSmith [ , 1986bSmith [ , 1987, the width of a current channel within an electrostatic-potential structure in the auroral zone is determined by cross-field currents that arise from lower-hybrid-wave turbulence. The model can be broken into five steps: (1) in the auroral-arc acceleration region Ex •: 0, and there is a relative drift between ion and electron guiding centers owing to finite-Larmor-radius effects in the F, x B drift; (2) by means ofthe modified-two-stream instability, lower-hybrid waves are driven by this relative drift; (3) the driven waves result in a viscosity between the ion and electron guiding-center drifts, resulting in equal and opposite forces on the electrons and the ions, which give rise to force-cross-B drifts of plasma across the magnetic field toward the outside of the arc potential structure; (4) since the plasma is not charge neutral in regions where Ex •: 0, there is a cross-field convection current associated with the force-cr•B drift of the plasma; and (5) this cross-field component of current, when added to the field-aligned current, acts to deflect the arc current pattern outward away from the center of the arc, widening the current channel of the arc.…”