Ionospheric Precursors of Magnetic Storms. 2. Analysis of Slough Station Data

Abstract

The behavior of the critical frequency of the F2 layer, foF 2, during the 3 days preceding a magnetic storm is analyzed. A total of 254 storms in 1976–2010 were considered, and 1353 events (deviations of foF2 from quiet conditions) were found: 996 and 357 deviations had a positive and negative signs, respectively. It is found that the indicated deviations demonstrate a clearly pronounced and statistically significant dependence on the magnetic storm intensity (the minimal value of the Dst index), on the solar activity level (the F10.7 index), and on the season. The probability of the occurrence of the indicated deviations has clearly pronounced maxima 4–6, 12–13, and 18–20 h prior to the moment of the storm onset (SO). Moreover, the occurrence of the indicated deviations is most probable when the SO moment occurs at 0800–1000 and 1900–2100 LT. The results indicate that the found deviations are not occasional fluctuations in foF2 (Q disturbances) but are related to the subsequent magnetic storm, i.e., they are its precursors.