Passage of the solar current disk and major geomagnetic storms

Abstract

It is shown that major geomagnetic storms (¦Dst¦ > 100γ) 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 Θ (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 ∂¦Dst¦/∂t< 0 during major storms coincides approximately with the period when the solar windmagnetosphere energy coupling function ɛ becomes ≳10¹⁹ 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 ∼ 3 days) or an extended period (∼2 weeks). A relatively thin region of a large IMF B > 10γ 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 Θ. A high value of ɛ is found in the region of a large IMF B near the wavy solar current disk, where Θ has a large negative value.