Anomalous Responses of the F-2 Layer Over the Brazilian Equatorial Sector During a Counter Electrojet Event: A Case Study

In this work, we report the ionospheric F-layer responses over the Brazilian equatorial sector to a counter electrojet event that occurred during the solar minimum period of June 2009. The data collected by the Digisonde over Sao Luis (2.33 degrees S; 44 degrees W; I: -4751 degrees; Dip. Lat. 238 degrees S) showed a strong modification in the ionospheric F-2 layer trace, which in this case appeared to be "broken in half." In this process, the first part of the F-2 layer (lower frequency) was thrown down whilst the upper part remained at higher altitudes. Such characteristics occurred simultaneously with an abrupt decrease in the strength of equatorial electrojet and with intensification in the auroral activity. The origin of this phenomenon seems to have a local nature and seems not to be connected to any magnetic disturbance since similar responses were not observed in other longitudinal and latitudinal sectors. Excluding this possibility, we assume that the strong changes observed in the F layer over Sao Luis had been caused probably by the gravity wave (GW) propagation, as seen in the downward phase propagation of the altitude contours with time over Sao Luis and Fortaleza and the remarkable signatures in ionograms over both regions, such as the forking traces that are typically caused by GWs.