Effects on the equatorial and low latitude thermosphere and ionosphere during the 19\textendash22 December 2015 geomagnetic storm period

In this paper, the critical frequency of F2 layer (foF2) and their peak height (hmF2) recorded at equatorial and low latitude stations of the South American sector were analyzed during the 19\textendash22 December 2015 geomagnetic storm period. We have observed initially decreases of foF2 at equatorial and low latitudes over the North crest of the equatorial ionization anomaly (EIA) and almost no effect over the South crest. These initial negative storm effects can be fully explained by the penetration of a westward electric field to low and equatorial latitudes. The positive storm effects that replaced the negative storm effects during the development of the main phase may have occurred possibly because of an intensification of the \textquotedblleftfountain effect\textquotedblright due to an increase of the upward vertical drift at equatorial latitudes. Changes in the O/N2 ratio (increases or decreases) may be the most probable cause for the positive and negative storm effects observed during the recovery.