The connection between auroral acceleration and auroral morphology

Abstract

Auroral acceleration is usually conceived in terms of measured electron energy-spectra and pitch-angle distributions. But another historical thread draws upon studies of auroral morphology. By 1970 it had been discovered that the rays in active rayed arcs are actually arrays of vortices similar to those observed in fluid shear and in laboratory experiments involving magnetized sheet electron beams (Kelvin-Helmholtz instability). The apparent shear flow implied that rays drift at the E X B velocity in a convergent electric field of the order of 1 v/m. But ionospheric electric fields rarely exceeded 100 mV/m. A suggested solution, that the electric fields existed in the source region but did not map down to the lower ionosphere, required that there be an upward electric field in the center of the arc. This field would accelerate electrons downward into the ionosphere. To compare electric fields inferred from inverted V energy with those inferred from ray motions requires a fortunate conjunction of a rocket or satellite passing through an overhead rayed arc. The AMICIST payload provided such an opportunity and the inferred fields were consistent. Morphology studies also show that clockwise spirals, associated with upward currents and counter-clockwise curls, associated with charge sheets, have scale-sizes differing by two orders of magnitude. This suggests that current sheets have thickness of order 50 km (multiple arc) while negative charge tends to be concentrated in thin layers of order 500m (arc elements). This difference needs to be considered in relating parallel fields to Birkeland currents. Finally, conjugate studies of auroras suggest that the potential contours close somewhere above the acceleration region rather than in the conjugate ionosphere. This raises the question of how electrons are forced into the regions of high negative potential.