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Session 17 - Magnetic Fields II.
Oral session, Tuesday, July 01
Ballroom A, Chair: Paul Charbonneau

[17.05] On the Origin of Helicity in Active Region Magnetic Fields

R. C. Canfield, A. A. Pevtsov (Department of Physics, Montana State University)

The magnetic helicity of flux tubes consists of twist and writhe. If flux bundles at the base of the convection zone are simple axisymmetric toroids, and rise in omega-shaped loops through the convection zone, they acquire writhe through the effect of the Coriolis force on flows within them. The tilt of active regions with respect to the equator is an observable manifestation of this writhe, at photospheric levels. As a consequence of helicity conservation, we expect active regions to acquire twist to compensate for this writhe. The non-zero curl of the large-scale magnetic fields in active-region vector magnetograms is the observable manifestation of twist. Recent observations have revealed many interesting properties of the helicity of solar magnetic fields. Hemispheric preferences exist: active region fields predominantly have left-handed topology in the Northern hemisphere. Prominence structures also predominantly have left-handed writhe in the Northern hemisphere. Large-scale structures have been observed to exist in the distribution of both twist and writhe with longitude and latitude, and to persist for many solar rotations. To study the physical origin of these properties, we have used a dataset of about 100 active regions for which vector magnetograms were obtained at Mees Solar Observatory. We have measured both the overall tilt and the overall twist of these active regions. The dataset clearly shows Joy's law, the well-known dependence of tilt on latitude, as well as the hemispheric dependence of twist. However, our analysis shows that twist and tilt are not related as they should be if both twist and tilt are a consequence of the Coriolis force. Hence, we conclude that the twist seen in active regions is the consequence of a deep-seated phenomenon, presumably that of the solar dynamo itself.

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