Magnetic Helicity Generation and Ejection by the Sun

C. Richard DeVore

Abstract

Observations of a number of solar magnetic features and of interplanetary magnetic clouds indicate that the Sun preferentially hosts left-handed, negative-helicity structures in its northern hemisphere, and their opposite counterparts in the south, independent of sunspot cycle.  We investigate quantitatively the action of differential rotation on emerged bipolar sources of flux, using analysis and numerical simulation.  The results show that, for the vast majority of bipoles, the nonuniform rotation of the Sun generates negative helicity in the north and positive in the south, in accordance with observations.  Furthermore, the total helicity generated in a nominal bipolar region is about 10% of its squared magnetic flux, or about 1x10^43 Mx^2 for a bipole containing 1x10^22 Mx of flux.  Over an entire sunspot cycle, about 1x10^25 Mx of flux emerges and is projected to absorb about 1x10^46 Mx^2 of helicity from differential rotation. These numbers agree to within factors of two with the total flux and magnetic helicity entrained in interplanetary magnetic clouds associated with coronal mass ejections.  Thus, the Sun's magnetic helicity balance can be accounted for quantitatively, and its patterns of helicity can be understood qualititatively, based on the action of differential rotation on its emerged bipolar sources of flux.

Organization: Laboratory for Computational Physics & Fluid Dynamics Naval Research Laboratory