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Passage of the solar current disk and major geomagnetic storms
Authors:S I Akasofu
Institution:(1) Geophysical Institute, University of Alaska, 99701 Fairbanks, Alaska, U.S.A.
Abstract:It is shown that major geomagnetic storms (¦Dst¦ > 100gamma) tend to develop at about the time of the passage of the solar current sheet or disk at the location of the Earth, provided this passage is associated with (1) a large impulsive increase of the IMF magnitude B, (2) a negative value of the IMF angle THgr (Theta), and (3) an increasing solar wind speed. The passage occurs in association with the 27-day rotation of the warped current disk or a temporal up-down movement of the latter. The period in which part¦Dst¦/partt< 0 during major storms coincides approximately with the period when the solar windmagnetosphere energy coupling function epsiv becomes gap1019 erg s–1. These conclusions do not depend on the phase of the sunspot cycle.These results may be interpreted as follows: A high speed solar wind flow, originating either from flare regions or coronal holes, tends to push the solar current disk to move upward or downward for either a brief period (1 sim 3 days) or an extended period (sim2 weeks). A relatively thin region of a large IMF B > 10gamma is often present near the moving current disk. Waves are also generated on the moving current disk, and some of them cause large changes of THgr. A high value of epsiv is found in the region of a large IMF B near the wavy solar current disk, where THgr has a large negative value.
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