The Semiannual Variation of Great Geomagnetic Storms
Abstract
The occurrence frequency of the largest geomagnetic storms as measured by the aa index from 1868-present exhibits a well-defined semiannual modulation with 2.4 times as many great storms occurring during equinoctial months than at the solstices. We show that most, but not all, of this variation can be attributed to an equinoctial hypothesis whereby energy transfer from the solar wind to the magnetosphere is modulated by psi, the angle between the solar wind flow direction and Earth's dipole axis. After normalizing aa for the seasonal/daily variation of psi, the imbalance in great storm counts between equinoctial and solstitial months is reduced to a factor of 1.4.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMSM31B0778S
- Keywords:
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- 2111 Ejecta;
- driver gases;
- and magnetic clouds;
- 2784 Solar wind/magnetosphere interactions;
- 2788 Storms and substorms