A study on the night time equatorward movement of ionization anomaly using thermospheric airglow imaging technique

• Observations of nighttime equatorward movement of EIA during deep solar minimum.
• Method of extraction of true equatorward drift speed from all-sky airglow images.
• Large day-to-day variabilities in the equatorward drift are identified.
• Optical signatures of EIA crest are identified in airglow upto about 5° dip latitude.

In the night time low latitude ionosphere, the equatorial ionization anomaly (EIA) crest move towards the equator as a result of change in the direction of the zonal electric field from eastward to westward. This is referred to as reverse plasma fountain. On some nights, imaging observations of OI 630.0 nm thermospheric nightglow taken from low latitude Indian station Panhala (16.8°N, 74.1°E; 11.1°N dip latitude) during deep solar minimum period revealed southward movement of a broad enhanced airglow intensity region that is aligned along east–west direction. This could be seen in the meridional keograms and is interpreted as the equatorward passage of the EIA crest. The studies that concentrate on the night time evolution of EIA with the help of airglow observations are sparse. In addition to the equatorward motion, the thickness of the crest region decreased as a result of recombination. This reduction in thickness is pronounced in the earlier part of the night and brings about an apparent drift that is added to the true equatorward drift of the EIA crest. In this work, we describe a method to measure the equatorward drift speed of the EIA crest from airglow imaging observations. The method includes the estimation and correction for the apparent drift caused by recombination. The results show that drift speed varies widely between 28 m/s and 89 m/s (~100 km/h to ~315 km/h) with an average speed of 52 m/s (188 km/h). A part of the variability might be due to variations in thermospheric meridional wind. The drifts observed during magnetically disturbed days were found to be relatively smaller than that on quiet days.