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The semiannual variation of great geomagnetic storms
Author(s) -
Svalgaard L.,
Cliver E. W.,
Ling A. G.
Publication year - 2002
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2001gl014145
Subject(s) - solstice , storm , universal time , atmospheric sciences , climatology , geomagnetic storm , earth's magnetic field , environmental science , amplitude , geology , meteorology , geodesy , physics , latitude , quantum mechanics , astronomy , magnetic field
The occurrence frequency of the largest geomagnetic storms from 1868–1998 exhibits a well‐defined semiannual modulation with more than twice as many storms occurring during equinoctial months than at the solstices. To examine the cause of this seasonal imbalance, we empirically obtained a new geomagnetic index aa m that has the same seasonal and Universal Time variation as the am index. In effect, this extends the am index backward in time to 1868. By normalizing the aa m time series for Ψ, the angle between the solar wind flow direction and Earth's dipole, we removed 75% of the amplitude of the six‐month wave in monthly averages of aa m and ∼75% of the seasonal discrepancy in the numbers of great storms. We obtained similar percentages for the (unmodified) am index over the shorter 1959–1998 interval. These results indicate that most, though not all, of the discrepancy in storm counts between the equinoxes and solstices is due to an equinoctial effect.