Review of auroral currents and auroral arcs.

Discrete auroral arcs involve both magnetic-field-aligned electric fields and currents. Theories agree that the currents close below the arcs through the E region ionosphere. Postulated closures above the arcs include: (1) currents due to pressure gradients; (2) polarization currents; (3) currents in the conjugate ionosphere; (4) currents on the magnetopause. The energization of particles to several keV by parallel electric fields requires an energy input and hence a force doing work in the outer magnetosphere. For the above, this force is identified as: (1) diamagnetic repulsion of the plasma by the earth's field; (2) inertial forces as a flux tube gains and loses momentum due to interaction with the ionosphere; (3) magnetic stresses due to conjugate points not being those required for a minimum energy magnetosphere; (4) discontinuities in the magnetic stresses at the magnetopause arising from either the different histories of flux tubes or topological features such as the cleft. Any of the above result in a shear in the magnetic stresses from one side of the current sheet to the other consistent with the existence of the current sheet. Thus, if the magnetospheric convective flow is decoupled from the ionosphere by a non-zero resistivity, rapid convection occurs in thin sheets on each side of the arc producing: (1) a potential drop along field lines; (2) a net downward component of the Poynting vector to the region of particle acceleration. It is concluded that the process is one of energy transfer from magnetic fields to plasma and the possibility arises that this could occur at any tangential discontinuity in space where the electric current is subject to a non-zero resistivity. The process is compared with merging and possible flows are presented.