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Richard Canfield   Submitted: 2000-05-25 20:00

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 geomagnetic consequences of their eruption. We find that if the CFR model is used, eruptions in sigmoids with a southward leading magnetic field component are associated with stronger geomagnetic storms, and northward leading field, weaker storms. The opposite is true if the FFF model is used. From this we infer that the magnetic structure of individual active regions plays a significant role in geomagnetic events, and no simple cycle-dependent generalization is useful in predicting the geomagnetic effects associated with an individual solar eruption.

Authors: Pevtsov, A. A. and Canfield, R. C.
Projects: None

Publication Status: JGR 106, A11, 2591 (2001)
Last Modified: 2003-11-03 11:12
Go to main E-Print page  Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data  Vector Magnetic Fields, Sub-surface Stresses, and Evolution of Magnetic Helicity  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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