Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations

This paper is focused on unusual nighttime impulsive electron density enhancements that are rarely observed at low latitudes on a wide region of South America, under quiet and medium/high geomagnetic conditions. The phenomenon under investigation is very peculiar because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. The phenomenon was studied and analyzed using both the F2 layer critical frequency (foF2) and the virtual height of the base of the F region (h′F) values recorded at five ionospheric stations widely distributed in space, namely: Jicamarca (−12.0°, −76.8°, magnetic latitude −2.0°), Peru; Sao Luis (−2.6°, −44.2°, magnetic latitude +6.2°), Cachoeira Paulista (−22.4°, −44.6°, magnetic latitude −13.4°), and São José dos Campos (−23.2°, −45.9°, magnetic latitude −14.1°), Brazil; Tucumán (−26.9°, −65.4°, magnetic latitude −16.8°), Argentina. In a more restricted region over Tucumán, the phenomenon was also investigated by the total electron content (TEC) maps computed by using measurements from 12 GPS receivers. A detailed analysis of isoheight ionosonde plots suggests that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could play a significant role in causing the phenomenon both for quiet and for medium/high geomagnetic activity; in the latter case however a recharging of the fountain effect, due to electric fields penetrating from the magnetosphere, joins the TID propagation and plays an as much significant role in causing impulsive electron density enhancements.