Airglow characteristics of equatorial plasma depletions

A low light level, all-sky 6300-A airglow imaging system developed for ground-based studies of the optical signatures of equatorial plasma depletions incorporates narrowband interference filters with interchangeable 180 and 60 deg lenses. Images are recorded photographically on 35 mm film. Photographs obtained with the system have been analyzed for broad morphological characteristics by means of overlay techniques augmented with digital image processing to extract more quantitative detail, and have shown that airglow depletions most often occur during the 2030-2330 LT period. The depletions often cover 1/4 of the visible sky, extending from the magnetic equator to beyond the crest of the equatorial anomaly, and are not usually aligned with magnetic meridians. Several cases of apparently twisting, overlapping and bifurcating depletions were observed. Airglow intensities within depletions were typically 40-50% of the background levels