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 0 N , 37.4 0 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 ( 1 SFU = 10 - 22 Wm - 2 Hz - 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 hrs. Also of note was that this trend was generally followed after midnight by a significant short-lived 630-nm brightness enhancement (five-fold). 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.