Determining the Source of Coronal Helicity through Measurements of Braiding and Spin Helicity Fluxes in Active Regions

Abstract

Magnetic helicity has become a valuable tool for understanding the energetics and dynamics of coronal magnetic fields. Recently, long time sequences of magnetograms have been used to measure the flux of helicity into active region coronae. We demonstrate how this helicity flux can be usefully decomposed into contributions of differing origin, called ‘‘spin’’ helicity and ‘‘braiding’’ helicity. These contributions could be envisioned to come at the expense of twist and writhe helicity, respectively, of a subphotospheric flux tube anchored to the regions. In order to effect this decomposition, each magnetogram is partitioned into a set of unipolar regions. We present a method of defining such regions so that they persist through the sequences and track the photospheric flow. The spin helicity of a given region quantifies the mean rotation rate of motions internal to that region, while braiding helicity is injected by the motions of whole regions about one another. Applying the method to six active regions shows cases where either spin or braiding dominates, and where they have the same signs and opposite signs. Thus, it would seem that no general statement can be made regarding the dominance of twist or writhe in supplying helicity to the corona. In one particular case, spin and braiding helicity follow different time histories but inject equal and opposite net helicities. This suggests that the spinning and braiding are driven by a kink instability in the submerged flux tube.