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 Sao Jose´ 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.