Abstract:
A computer program is developed to automatically identify the geomagnetic storms in Dst index by applying four
selection criteria that minimize non-storm-like fluctuations. The program is used to identify the storms in Kyoto Dst
and USGS Dst in 50 years (1958–2007). The identified storms (DstMin ≤ −50 nT) are used to investigate their seasonal
variations. It is found that the overall seasonal variations of the storm parameters such as occurrence, average intensity
(average DstMin) and average strength (average ⟨DstMP⟩) in both versions of Dst exhibit clear semiannual variations
with equinoctial maxima and solstice minima; and the maxima and minima in intensity and strength (~±17%
each) are less than those in occurrence (~±28%). Wavelet spectra of the storms reveal the existence of distinct semiannual
component in four solar cycles (SCs 20–23) and weak longer and shorter-period components in some SCs. The
semiannual variation observed also in the mean energy input during the main phase (MP) of the storms estimated
from Dst is interpreted in terms of the (1) equinoctial mechanism based on the varying angle between the Earth–Sun
line and Earth’s dipole axis and (2) Russell–Mcpherron effect based on the varying angle between the GSM Z-axis and
GSE Y-axis; and the yearly range of the dipole tilt angle μ (23.2°) involved in the equinoctial mechanism is found larger
than the title angle θ (16.3°) involved in the RM effect.
