Nighttime equatorial 630-nm emission variability over Ethiopia

Abstract

Two winters of nightglow intensity observations of the atomic oxygen 630-nm redline emission at Bahir Dar (11.6°N,37.4°E) were collected from measurements between November 2015 and March 2017 by the selection of 89 nights of quality data. The solar activity trend during the 2015–2017 period was decreasing with the solar flux activity varying within the range of 70 and 140 sfu (1SFU=10-22Wm-2Hz-1). The results have shown that for the first winter (2015–2016) for quiet geomagnetic activity and with the solar flux near the higher end of this range, the 630-nm intensity decreased significantly during the post-sunset period of 3–4 h. Also of note was that this trend was generally followed after midnight by a significant short-lived 630-nm brightness enhancement (fivefold). In the subsequent year of low solar fluxes, a similar significant decrease in the 630-nm intensity is seen starting from the early evening but for the remainder of the night the intensity remained generally constant with no significant sign of the post-midnight 630-nm enhancement of the previous year. During disturbed times the study of the temporal variations of the 630-nm intensity found a double peak structure that persisted beyond one day.