Nighttime dynamics of the F region near Arecibo as mapped by airglow features

Abstract

Red line (6300 Å) airglow measurements have been made at Arecibo (29°N geomagnetic latitude) by using a scanning photometer in order to determine the direction and phase velocity of propagation of major airglow enhancements, which incoherent scatter and ionosonde measurements have shown are in turn associated with pronounced descents of the F region. The scans were made in the N‐S direction on 13 nights and in the E‐W direction on five nights. The most common type of descent is observed first to the south and then progresses northward, with an apparent phase velocity of the order of 300 m/s. Neutral winds are apparently the cause of this type of descent, and future models of upper atmospheric diurnal pressure patterns should be consistent with this large‐scale regular nighttime feature. Descents of the equatorial ionosphere were observed from ionogram data to occur 2–3 hours earlier than this type of airglow enhancement at Arecibo. The equatorial descents are caused by electric fields and do not directly drive the descent at Arecibo, but both are ultimately driven by the global neutral pressure gradients and resulting winds. A second type of enhancement, seen on occasion and probably due to traveling ionospheric disturbances, was observed to travel from north to south with comparable velocities. A third type does not seem to correspond to anything previously reported in the literature. Sudden descents lasting an hour or so sometimes occurred nearly simultaneously over regions extending at least 1000 km in the N‐S or E‐W direction.