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Z. A. Holder, R. A. McMullen, R. C. Canfield (Physics Department, Montana State University), R. F. Howard, A. A. Pevtsov (National Solar Observatory)
We combine two large datasets to study the formation of the overall twist that is present in solar active-region magnetic fields. For purposes of discussion, we adopt a simplified model in which the magnetic flux bundles that form active regions start as axisymmetric toroids, without intrinsic twist, and rise in \Omega-shaped loops through the convection zone, acquiring writhe through the effect of the Coriolis force on the large-scale flows within them, as well as buffeting by turbulent convection. The tilt of active regions with respect to the equator is an observable manifestation of such writhe, at photospheric levels. Owing to magnetic helicity conservation, we expect this tilt to be related to twist of the fields of these same regions.
Using Mees Solar Observatory active-region vector magnetograms and Mount Wilson Observatory full-disk longitudinal magnetograms, we have measured both the tilt (Mount Wilson) and twist (Mees) of their magnetic fields, on active-region scales. This dataset clearly shows two well-known phenomena, Joy's law and the hemispheric handedness rule. In this paper we present the relationship between twist and tilt and estimate the extent to which that relationship is due to a mutual dependence of tilt and twist on latitude. We then compare our observational results to the simplified model.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.