Unusual nighttime impulsive enhancements of electron density characterizing the low-latitude ionosphere: Phenomenology and possible mechanisms of triggering

Abstract

Unusual nighttime impulsive electron density enhancements that are rarely observed at low latitudes on a wide region of South America are here investigated. These phenomena are very atypical because besides being of brief duration, they are characterized by a pronounced compression of the ionosphere. The events were studied and analyzed using both the F2 layer critical frequency (foF2) and the lowest virtual height of the ordinary trace of the F region (h'F) values recorded at five ionospheric stations widely distributed in space. A careful analysis of isoheight ionosonde plots suggests that traveling ionospheric disturbances (TIDs) caused by atmospheric gravity wave (AGW) propagation could play a significant role in causing these phenomena, both for quiet and for medium-high geomagnetic activity; in the latter case however a nocturnal recharging of the fountain effect, due to electric fields penetrating from the magnetosphere, plays an as much significant role.