Using ionograms from the Brazilian equatorial region of São Luís (SL, 2° S; 44° W, I =-3.8°), the relation between the uplift of the intermediated layers (ILs) at sunset and the prereversal enhancement of the zonal electric field (PRE) is investigated. The ILs were studied during periods of maximum (2003) and minimum solar activity (2009). The presence of the ILs during the PRE occurrence time was very low for both years. In 2003, six ILs’ events were observed, being four of them in the summer solstice and one in the March equinox. In 2009, only a single event was registered and occurred during the December month. The results show that depending on the height at which the ILs are located, their upward movement at sunset can be in some way related to the normal F layer rise at sunset due to the PRE. Additionally, the eastward prompt penetration electric fields (PPEF) during weak magnetic storms can also contribute to the IL’s rise. An interesting case of uplift of a sporadic-E layer from ~120 to 290 km of altitude probably due to the PPEF is investigated. The absence of answers of the F layer to the same disturbed electric field reveals the complex nature of the layers located in the ionosphere valley region.

Key Points: • Gravity waves impacts in the anomalous responses of the F layer over Brazil during a counter electrojet event; • Some signatures in ionograms suggest the gravity wave as responsible for the CEJ occurrence in this day. Abstract In this work, we report the ionospheric F-layer responses over the Brazilian equatorial sector to a counter electrojet (CEJ) event that occurred during the solar minimum period of June 2009. The data collected by the Digisonde over São Luis (2.33° S; 44° W; dip in 2009:-5.7°) showed a strong modification in the ionospheric F 2 layer trace, that 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 São Luis had been caused probably by the gravity wave (GWs) propagation, as seen in the downward phase propagation of the altitude contours with time over São Luis and Fortaleza and the remarkable signatures in ionograms over both regions, such as the forking traces that are typically caused by GWs.