Long-Term Trends and Seasonal Variations in Geomagnetic Storms from Data of St. Petersburg Observatories (1878-1954)

The annual number of magnetic storms N recorded at St. Petersburg observatories (Pavlovsk/Slutsk and Voyeykovo) in 1878-1954 is studied. The analysis shows that N has increased since similar to 1900 for different storm types (storms with sudden commencement Ssc and storms with gradual Sg commencement; moderate, strong and very strong); however, the number of Ssc storms increased more rapidly than the number of Sg storms. The percentage of Ssc storms doubled for the first half of the 20th century, while the number of Sg storms decreased by 1.5 times. The Ssc storms are driven by coronal mass ejections from closed magnetic structures on the Sun, and Sg storms are driven by corotating fluxes from open magnetic structures and coronal holes. These results are apparently evidence of an increase in the activity of both types of solar magnetic structures in the first half of the 20th century and a more rapid increase in the activity of fields with closed lines of forces. A semiannual variation with maxima in the periods of vernal and autumnal equinoxes is clearly pronounced for Sg and moderate storms. The tendency to have two equinoctial maxima is pronounced in the total number of storms N for both even and odd cycles; however, maxima that differ from the arithmetic mean by more than a standard deviation are observed only in September in even cycles and in March in odd cycles.