Solar Magnetic Fields and Geomagnetic Events
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.
Publication Status: JGR 106, A11, 2591 (2001)
Last Modified: 2003-11-03 11:12