Auroral arcs as current transformers

Abstract

We treat the effects of an electric field tangential to an auroral arc on the basis of an analytical model according to which auroral arcs are energized by the release of magnetic shear stresses and associated internal energy (Haerendel, 2007). If the tangential field exceeds a certain (low) threshold, the ionosphere acts as a current generator, or better transformer, and the magnetosphere as the load. The reason is that energy supplied by a longitudinal Type I current system, as defined by Boström in 1964, is transformed into energy of the latitudinal Type II current system attached to the arc and is mostly consumed by the auroral acceleration process. Closure of the transformer current in the upper magnetosphere and energy deposition in form of internal energy and shear stresses, although small, are crucial for the operation of this process. It can be visualized as a dragging of magnetic field and plasma along the arc in the direction of higher magnetospheric pressure enforced by the electric polarization field. This energy deposition from below is superimposed on the release of free energy supplied by the magnetospheric generator. Formulation of this scenario leads to a set of algebraic equations, whose solution provides quantitative answers on the interdependence of current closure, energy deposition, and enhanced auroral energy flux in relation to the imposed tangential electric field. Contrary to earlier expectations, the polarization field appears not as a consequence of current blockage, but as manifestation of its continuation by field‐aligned currents and their closure in the magnetosphere. Another consequence of the energy deposition from below is a small modification of the arc's proper motion in the ambient plasma frame. A procedure for evaluating applicable measurements in terms of the underlying physics is given and a concrete example added.