Appearance and extension of airglow depletions

Previous observations using all-sky imagers show that the field-aligned airglow depletions propagate poleward in the low-latitude and/or equatorial ionosphere. This paper reports the first observations of 630.0 nm depletions that appear to be extending equatorward during the premidnight hours in the all-sky camera observations at the low-latitude station Kavalur (12.5°N, 78.8°E; 3.9°N geomagnetic), India. The airglow imager and two ionosondes at the geomagnetic and off-equator along the same longitude are used to understand the appearance and extension/development of depletions. The F layer peak height gradient along the meridional direction is obtained using the ratio of simultaneous 777.4 and 630.0 nm airglow intensities, and the observed apparent equatorward propagation of ionospheric depletions is explained in terms of such equatorial dynamics and photochemical processes, consistent with the poleward propagation previously reported.