Latitudinal structure of the equatorial F3 layer based on Intercosmos-19 topside sounding data

Abstract

We studied the equatorial F3 layer using the Intercosmos-19 (IK-19) topside sounding large data set (about 3600 passes across the equator). The IK-19 data were obtained during the 1979–1981 high solar-activity period. They cover all seasons, local time, longitudes and magnetic activity levels. The F3 layer is usually identified as an ionization ledge, i.e., a local enhancement in electron density at heights over the F2 layer maximum. On the topside ionograms this ledge manifests itself as a cusp. IK-19 data show that this enhancement reaches 10–15% in the daytime, and 30% at night. Our data analysis demonstrated that the F3 layer is most often observed in the afternoon and evening hours, less often at night and rarely in the morning. The F3 layer height increases in the course of time and reaches a maximum of ∼900km at 21LT. The F3 layer is usually observed within a geomagnetic latitude range from −10° to 10° around the magnetic equator. The peak of the ledge (F3 layer) and the crest of the equatorial anomaly existed near the same geomagnetic field line. We analyzed the IK-19 data during 10 geomagnetic storm events with Kp≥5. The F3 layer was generally absent during two storm events; it was weakly manifested during five storms and was clearly seen only during three storms. Our data analysis shows that the F3 layer is connected to the intensity of the equatorial anomaly but this relationship is not simple.