Abstract:
Airglow emissions from the thermosphere act as tracers of the behaviour of the upper atmosphere. OI 630 nm nightglow emissions that originate from an altitude of around 250 km in the thermosphere have been investigated using a large field-of-view ground-based optical spectrograph from Mt. Abu (24.6°N, 72.7°E, 19°N Mag.), a low-latitudinal location in India. The latitudinal movement of the crest of the equatorial ionization anomaly (EIA) in both poleward in the evening and equatorward in the night has been reported using the OI 630 nm nightglow emissions. The EIA crest is found to shift away from the equator after the sunset, which has been shown to be directly related to the strength of the twilight time equatorial electrodynamics. Later in the night, after 20 LT, a clear movement of the crest back towards the equator, known as the reversal of EIA, has been observed. The speeds of the EIA reversals for forty nights in the years 2014 and 2015 have been obtained and are found to be in the range of 10–55 ms−1. As the global equatorial electric fields (E) are irrotational in nature (), simultaneous variations in the daytime electrojet strength over Jicamarca in the American sector have been compared with the nighttime reversal speeds of the EIA over the Indian sector, which show a remarkable relationship with each other. Thus, it is hereby demonstrated that the reversal in EIA as inferred by the OI 630 nm nightglow emissions in our measurements is due to the westward equatorial electric field. It is hereby proposed that the reversal speed derived from optical nightglow measurements can serve as a proxy for the determination of westward electric field over equator for that longitude sector.