Auroral Arcs in a Rice Convection Model Context

Abstract:

Sharp gradients in the entropy parameter PV^5/3 tend to form naturally in Rice Convection Model (RCM) runs. Vasyliunas' equation implies intense sheets of Birkeland current at these sharp gradients, and we know that intense upward currents are associated with field-aligned potential drops and auroral arcs. The situation represents a local breakdown of the usual RCM assumption of perfectly conducting field lines and also implies a local decoupling of magnetospheric and ionospheric motions. We have had difficulty representing this decoupling in the RCM, because of difficulties resolving the thin structures on the large-scale grid. As a first step toward understanding the large-scale effects of the decoupling, we have found two highly idealized cases in which the RCM equations can be reduced to linear one-dimensional differential equations, which can easily be solved to high accuracy. Both cases assume steady state conditions and uniform ionospheric conductance, and the field-aligned potential drop is assumed to be proportional to the density of field-aligned current. One case involves region-2 current flowing on the inner edge of the plasma sheet, with convection considered as a small perturbation. The other considers a plasma boundary that makes an arbitrary angle with the gradient of flux tube volume, with the field-aligned potential considered as a small perturbation. In both cases, the main effects of field-aligned potential drops are: (i) to create zonal flows on the two sides of the arc, in the magnetosphere but not in the ionosphere; (ii) to reduce the overall arc-associated Birkeland current.