9.2 The twist of active region magnetic fields

Vector magnetic field observations on the photosphere show that on average solar active regions have a small but statistically significant mean twist that is left-handed sense in the northern hemisphere and right-handed sense in the southern hemisphere. The origin of this hemispheric dependent twist in solar active regions is not clear. The twist may be due to the current helicity in the dynamo generated, predominantly toroidal magnetic field at the base of the convection zone, from which buoyant flux tubes form, or it may be acquired during the rise of the flux tubes through the convection zone as a result of buffeting by the helical convective motions (called the Σ-effect) and also through the accretion of the mean poloidal magnetic field in the convection zone onto the rising flux tube (Section 5.3). Observational studies of the correlation between active region twist and tilt angles indicate that the Σ-effect cannot be the only source for the twist (Section 5.3).

It is found that in order for the tilt of the Ω-shaped emerging tube to be consistent with Joy’s law of active region mean tilt, the magnitude of the twist in the rising flux tube cannot be too high (Section 5.5). On the other hand, the rise of highly twisted, kink unstable magnetic flux tubes can produce kinked or knotted emerging tubes which may explain the origin of the unusual class of flare productive active regions called δ-sunspots which are observed to be highly twisted and often show polarity order inverted from the Hale polarity rule (Section 5.6).

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