Abstract
The magnetic helicity H of flux tubes can be separated into twist T and writhe W (Moffatt & Ricca 1992). If magnetic flux rises in Ω-shaped loops through the convection zone (Babcock 1961), it acquires writhe through the Coriolis force on internal longitudinal flows (Fan, Fisher, & DeLuca 1993). The tilt θ of active regions with respect to the equator is an observable manifestation of such writhe, at photospheric levels. If H is conserved, as we expect (Taylor 1974), rising flux should acquire equal and opposite values of T and W. We expect W > 0 (θ < 0) in the Northern hemisphere, since the leading polarity of active regions tends to be closer to the equator. Hence, we expect T < 0 there.
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References
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Pevtsov, A.A., Canfield, R.C. (1998). On the Origin of Helicity in Active Region Magnetic Fields. In: Watanabe, T., Kosugi, T., Sterling, A.C. (eds) Observational Plasma Astrophysics: Five Years of Yohkoh and Beyond. Astrophysics and Space Science Library, vol 229. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-5220-4_13
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DOI: https://doi.org/10.1007/978-94-011-5220-4_13
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