Variations in the critical frequency of the ionospheric F-region during magnetic storms in 2008–2012 at auroral latitudes

Abstract

Fifty-one magnetic storms occurred during the last solar half-cycle of transition from the epoch minimum to the epoch maximum are considered. Ionospheric (foF2) and magnetic (X component) data from Sodankyla observatory, Finland, were used for the analysis, as well as values of the ΣKp indices of magnetic activity. The dependence of variations in the critical frequency foF2 was studied before, during, and after each storm. It has been revealed that a major effect (ME) takes place for all of the storms analyzed. It consists in the following: the first maximum in foF2 values occurs several days before the onset of the active phase of a storm, then foF2 attains its minimum during the active phase, and the second maximum occurred after the active phase. Five principals, the most frequent types of variation in foF2 during a storm, have been revealed. However, special cases (30%) in which an ME exists but shifts rightward several days along the time axis are observable. Ionospheric “memory” (inertia) from 8–9 h to 2 days has been revealed. It has been ascertained that the occurrence of the first ME maximum can be considered a magnetic storm precursor. Such a precursor potentially can be used for forecasting the beginning of magnetic storm development, which is important for space weather problems.