Advection of magnetic energy as a source of power for auroral arcs

Abstract

We examine the energetics of a system wherein large-scale magnetospheric convection causes plasma to flow across two-dimensional sheets of field-aligned current (FAC). This scenario requires that the FACs be held stationary or move slowly relative to the background flow, for example through their connection to conductivity enhancements in the lower ionosphere. The key result is that plasma convection across quasi-static current sheets at speed V d implies cross-field transport of magnetic energy at a rate proportional to V d δB 2 /2μ 0, where δB is the magnetic perturbation associated with the FAC. Poynting's theorem shows that this energy is available to enhance the field-aligned component of Poynting flux, to accelerate particles via electric fields, or both. We show that, for nominal values of field-aligned current (∼10 μA/m 2) and cross-current-sheet convection (∼100 m/s), the net field-aligned energy flux made available through cross-field advection can contribute of the order of 1mW/m 2 to auroral energy fluxes (electromagnetic plus particle), and conceivably can dominate the energy budget in more intense arcs. While our analysis is applied to an idealized, quasi-static, two-dimensional system, the mechanism it illustrates could play an important role in more dynamic and highly-structured auroral forms as well. Copyright © 2011 by the American Geophysical Union.