Solar Magnetic Fields and Geomagnetic Events.

Recent interplanetary studies conclude that the large-scale solar dipolar field dominates the solar cycle modulation of the magnetic structure of interplanetary clouds. Other studies lead one to expect that the toroidal fields of active regions, described by the Hale-Nicholson polarity law, play an important role. We have studied the ratio of the geomagnetic A p index to the sunspot number for solar cycles 17-22. We find no compelling evidence that either the large-scale dipolar field or active regions uniquely modulate this quantity on solar-cycle time scales. In the period 1991 -- 1998 the large-scale solar dipolar magnetic field pointed southward. During this period we studied geomagnetic storms temporally associated with the eruption of 18 individual coronal X-ray sigmoids observed with the Yohkoh Soft X-Ray Telescope (SXT). We apply two different models -- force-free field (FFF) and coronal flux-rope (CFR) -- to infer the magnetic fields in these sigmoids and the geomagne..