Auroral spirals : a consequence of field aligned currents

Abstract

A study of large spiral forms in the aurora (up to 600 km diameter) led to the identification of a convective form of the Kelvin-Helmholtz instability which can result from field-aligned currents and can explain the observed auroral configurations. A current density of ~ 6 x 10^-6 amps/m^2, inferred from the spirals theory, is found to be in reasonable agreement with values obtained by others. The westward traveling surge is identified as a spiral and is estimated to contain ~ 8 x 10^5 amps. The rotational sense (clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere) is consistent with an assumed downward electron flux in both hemispheres. Small-scale curls (~ 2 km diameter), which can result from a related electrostatic instability in a sheet of negative space charge, are observed to occur within the spirals. Although the rotational sense of the curls is opposite to that of the spirals, it is theoretically consistent to expect them both to occur in an electron arc.