Airglow characteristics of equatorial plasma depletions

Abstract

A new low‐light‐level, all‐sky, 6300 Å airglow imaging system has been developed for ground‐based studies of the optical signatures of equatorial plasma depletions. The system was designed for narrowband interference filters (6 Å full width at half power) with interchangeable all‐sky (180°) and narrow‐field (60°) lenses and records images photographically by using a standard 35 mm camera. A field test of the apparatus was conducted on Ascension Island (8.0°S, 14.4°W) from January 24 to February 10, 1981. The photographs were analyzed for broad morphological characteristics by using simple overlay techniques augmented by digital image processing methods to extract more quantitative detail. The initial results to come from these observations show that airglow depletions occur most often during the 2030–2330 LT period, often covering one‐fourth of the visible sky; they extend from the magnetic equator to beyond the crest of the equatorial anomaly, with east‐west cross sections of ∼100 km to several hundred kilometers; they are usually not aligned with magnetic meridians, showing a progressive skewness to the west at further distances from the equator; several cases of apparently twisting, overlapping, and bifurcating depletions were observed; the depletions drifted to the east with speeds that decreased from ∼190 m/s at 2100 LT to ∼80 m/s at 0100 LT; the airglow intensities within depletions were typically 40–50% of the background levels, and the western walls of the depletions usually had a steeper gradient than the corresponding eastern walls.