The connection between auroral acceleration and auroral morphology

dAuroral acceleration is usually conceived in terms of measured electron en ergy-spectra and pitch-angle distributions. But another historical thread d raws upon studies of auroral morphology. By 1970 it had been discovered tha t the rays in active rayed arcs are actually arrays of vortices similar to those observed in fluid shear and in laboratory experiments involving magne tized sheet electron beams (Kelvin-Helmholtz instability). The apparent she ar flow implied that rays drift at the E X B velocity in a convergent elect ric field of the order of 1 v/m. But ionospheric electric fields rarely exc eeded 100 mV/m. A suggested solution, that the electric fields existed in t he source region but did not map down to the lower ionosphere, required tha t there be an upward electric field in the center of the are. This field wo uld 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 are. The AMICIST payload provided such an opportunity and the inferred fields were consistent. Morphology studies also show that clo ckwise spirals, associated with upward currents and counter-clockwise curls , associated with charge sheets, have scale-sizes differing by two orders o f magnitude. This suggests that current sheets have thickness of order 50 k m (multiple are) while negative charge tends to be concentrated in thin lay ers of order 500m (are 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 a cceleration region rather than in the conjugate ionosphere. This raises the question of how electrons are forced into the regions of high negative pot ential. (C) 2001 Elsevier Science Ltd. All rights reserved.