Magnetic Helicity Generation by Solar Differential Rotation

Abstract

Observations of sunspots, active regions, Ðlaments, 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 di †erential rotation acting on emerged bipolar sources of Ñux, using analytical and numerical methods. We Ðnd that the vast majority of bipoles absorb negative helicity in the northern hemisphere and positive helicity in the south, in accord with observations. After 2È4 solar rotation periods have elapsed, the helicity generated by di †erential rotation amounts to about 10% of the bipoleÏs squared Ñux. 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 Ñux. We show further that the roughly 5 ] 103 coronal mass ejections and associated interplanetary magnetic clouds that departed the Sun during the cycle carried o † about 5 ] 1024 Mx of Ñux and 1 ] 1046 Mx2 of helicity, within a factor of 2 of the estimates for solar production of these quantities. Evidently, di †erential rotation acting on emerged bipolar sources of Ñux 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.