A. V. Konstantinova

The paper presents a review of variations in ionospheric parameters in the prestorm period obtained from an analysis of particular magnetic storms (case studies). It is demonstrated that, as a result of such analysis, deviations of these parameters (primarily, foF2 and TEC) are often observed on the days preceding the storm commencement (SC). Many authors pay attention to these facts and discuss their relation to space weather parameters, season, local time, and spatial distribution, and even offer suggestions on possible mechanisms of their formation. It is demonstrated that the number of such publications increased substantially in the preceding 4–5 years. It is emphasized that changes in the ionospheric state on prestorm days are detected not only in the “classical” parameters (foF2 and TEC), but also in other ionospheric characteristics. Publications dedicated directly to the problem of ionospheric precursors, their relation to space weather, and their possible role in forecasting coming magnetic storm are considered in detail.

The F2-layer critical frequency foF2 is analyzed for three days preceding a magnetic storm based on Juliusruh station data. A total of 272 storms over the period 1976–2010 are considered, and 2682 events (foF2 deviations from quiet conditions) are found. There were 2007 deviations with positive signs and 675 deviations with negative signs. It was found that the aforementioned deviations demonstrate a well-pronounced and statistically significant dependence on the magnetic-storm intensity (the minimal value of the Dst index), the solar-activity level (the F10.7 index), and the season. The highest probability of the occurrence of the indicated deviations is observed 4–12 and 22–23 h prior to the moment of storm onset (SO). Moreover, the occurrence of these deviations is most probable when the SO moment occurs at 0400, 0700–0800, 1200–1300, and 2200 LT. The results agree well with authors’ previously published results with the Slough data and indicate that the discovered deviations are not casual foF2 fluctuations (Q disturbances) but are related to the coming magnetic storm, i.e., they are its precursors.

Trends in foF2 are analyzed according to the data of Juliusruh and Boulder ionospheric stations to 2018. It is shown that the use of the traditional solar-activity index, F10.7, leads to an unexplainable foF2 trend if the data for the 24th solar-activity cycle are included in the analysis. It is suggested that the F10.7 index does not correctly describe the solar ultraviolet-radiation variation in that cycle. Correction of that index with the Rz (sunspot number) and Ly-α (intensity of the Lyman-alpha line in the solar spectrum) indices is performed. It is shown that reasonable values of the foF2 trends are then obtained. A similar analysis for hmF2 trends leads to the same results.

The article reviews the main publications from the last eight years on long-term trends in the parameters of the middle atmosphere, thermosphere, and ionosphere. It is shown that the negative temperature trends in the middle atmosphere have been reliably established based on observations by various methods. An analysis of satellite orbits in the thermosphere showed negative density trends at heights of ~400 km. Unexpectedly high negative trends in ion temperature Ti above ~220 km obtained with the incoherent-scatter method. The same measurements high positive trends in Ti at the heights of the F1 layer. There are discrepancies in the estimates of the absolute value of the negative trends in the critical frequency and height of the F2 layer; however, these trends are high in the opinion of the authors.

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.

— The problem of the occurrence of disturbances of the ionospheric F2-layer parameters prior to the beginning of a geomagnetic storm is discussed. It is shown that disturbances (substantial...