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Magnetic Helicity Generation and Ejection by the Sun
C. Richard DeVore
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
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