Magnetic Helicity Generation by Solar Differential Rotation

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Abstract
Observations of sunspots, active regions, filaments, coronal arcades, and interplanetary magnetic clouds indicate that the Sun preferentially exhibits left-handed, negative-helicity features in its northern hemisphere and their opposite counterparts in the south, independent of sunspot cycle. We investigate quantitatively the generation of magnetic helicity by solar differential rotation acting on emerged bipolar sources of flux, using analytical and numerical methods. We find that the vast majority of bipoles absorb negative helicity in the northern hemisphere and positive helicity in the south, in accord with observations. After 2-4 solar rotation periods have elapsed, the helicity generated by differential rotation amounts to about 10% of the bipole's squared flux. Thus, each of the approximately 1 × 103 large bipolar regions emergent on the Sun during sunspot cycle 21 entrained about 1 × 1043 Mx2 of helicity in its 1 × 1022 Mx of flux. We show further that the roughly 5 × 103 coronal mass ejections and associated interplanetary magnetic clouds that departed the Sun during the cycle carried off about 5 × 1024 Mx of flux and 1 × 1046 Mx2 of helicity, within a factor of 2 of the estimates for solar production of these quantities. Evidently, differential rotation acting on emerged bipolar sources of flux can account quantitatively for the magnetic helicity balance of the Sun and the heliosphere, as well as for the observed prevalence of negative-helicity magnetic features in the north and positive-helicity features in the south.

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