Geophysical effects of solar activity: long-term variations in occurrences of magnetic storms with sudden and gradual commencements

Abstract

Magnetic storms with sudden and gradual commencements are related to different agents of solar activity and form two independent populations. Coronal mass ejections which are more frequently related to active regions on the Sun (local magnetic fields with closed configuration) give rise, as a rule, to magnetic storms with sudden commencements. High-speed streams of solar wind related to solar coronal holes (regions of magnetic fields with open configuration) give rise to magnetic storms with gradual commencements. In this work we studied annual occurrences of magnetic storms with sudden and gradual commencements on the base of the data from IZMIRAN and Slutsk (Pavlovsk) magnetic observatories for the period 1878-2015, the correlation and spectral analysis being used. It was found that the occurrences of large and moderate magnetic storms with gradual commencements are characterized by long-term variations at the multidecadal and secular time scales. Wavelet spectra of occurrences of these storms reveal a dominant periodicity of ∼36 years (close to the Brückner cycle) and less pronounced ∼60-year and ∼90-year periodicities on the entire time interval under study. It was found that ∼11-year variations in occurrences of storms with gradual commencements were the most statistically significant in ∼1900-1920 and ∼1980-2000, but they were not observed in ∼1940-1960. The occurrence of magnetic storms with sudden commencements is characterized by a dominant ∼11-year periodicity almost on the entire interval of observations, whereas long-term variations are less pronounced. The obtained results provide evidence for a different time evolution of global and local solar magnetic fields.