Latitudinal structure of the longitudinal effect in the nighttime ionosphere during the summer and winter solstice

The paper reports the basic morphological characteristics of the longitudinal variations of the electron density in the nighttime F region of the ionosphere at different latitudes obtained from data collected by the Intercosmos-19 satellite and from the results of calculations within the framework of the Global Self-Consistent Model of the Thermosphere, Ionosphere, and Protonosphere (GSM TIP). Based on the Intercosmos-19 satellite data for a high solar activity, spatial distributions of the critical frequency foF2 of the F2 layer for near-midnight hours of the local time are plotted. The study revealed the main features of the mechanisms of the formation of longitudinal features of the nighttime ionosphere at various latitudes during the summer and winter solstices, as well as two reasons for their occurrence. In particular, we consider (1) the mechanisms of the formation of the nighttime peaks at the longitudes of the Yakutsk anomaly and Weddell Sea anomaly, (2) manifestations of longitudinal variations of the main ionospheric trough, and (3) the longitudinal dependence of the disappearance of the equatorial anomaly during the June and December solstices.