SPECTROSCOPIC EVIDENCE FOR SUPRATHERMAL ELECTRONS IN ENHANCED AURORAS

Auroral arcs frequently exhibit bands of enhanced intensity along the bottom edge. These bands have sharp upper borders and are referred to as enhanced lower borders or auroral hems. They are thought to be rela ted to thin layers of enhanced luminosity observed both in auroral arc s and in pulsating auroras. These anomalous distributions of intensity as a function of altitude are referred to collectively as enhanced au rora (EA) and are of interest because they suggest a more complex proc ess for producing auroral ionization and luminosity than the collision al degradation of precipitating electrons. Spectra of eight auroral he ms were obtained with an imaging spectrograph and showed consistently that within the hem, there is an enhanced ratio of N-2 1st positive em issions (red) to N-2(+) 1st negative emissions (blue). The enhancement in the red/blue ratio ranged from 10% to 100%. There was a generally smaller and less consistent enhancement in the ratio of the O-2(+) 1st negative band at 5275 Angstrom to the N-2(+) 1st negative. Observatio ns of the far red spectra of several hems showed no enhancement in the ratio of N-2 Ist positive to N-2(+) Meinel emissions. There were no s imultaneous measurements of the Meinel and the N-2(+) 1st negatives. I t is argued that the enhanced spectral ratios are indicative of a popu lation of suprathermal electrons with a sharp energy cutoff near the i onization potential of N-2 and that the electrons are energized in sit u through wave-particle interactions or possibly by de electric fields . It is also noted that some but not all of the hems are type b red lo wer borders.