Sign-Singularity of the Current Helicity in Solar Active Regions

On the basis of vector magnetograms of 20 active regions we analyzed the scaling behavior of the current helicity H_c in the photosphere. We show that H_c possesses well pronounced sign-singularity in the range of scales from more than 10⁴ km up to the resolution limit of observations.     

Analysis of electric current helicity in active regions on the basis of vector magnetograms

The problem of (dc) magnetic field energy build up in the solar atmosphere is addressed. Although large-scale current generation may be due to large-scale shearing motions in the photosphere, recently a new approach was proposed: under the assumption that the magnetic field evolves through a sequence of force-free states, Seehafer (1994) found that the energy of small-scale fluctuations may be transferred into energy of large-scale currents in an AR (the α-effect). The necessary condition for the α-effect is revealed by the presence of a predominant sign of current helicity over the volume under consideration. We studied how frequently such a condition may occur in ARs. On the basis of vector magnetic field measurements we calculated the current helicity B _z · (▽ × B) _z in the photosphere over the whole AR area for 40 active regions and obtained the following results:

1. In 90% of cases there existed significant excess current helicity of some sign over the active region area. So one can suggest that the build up of large-scale currents in an active region due to small-scale fluctuations may be typical in ARs.
2. In 82.5% of cases, the excess current helicity in the northern (southern) hemisphere was negative (positive).

The method proposed can be applied to those ARs where the determination of the predominant sign of current helicity by traditional visual inspection of Hα-patterns is not reliable.     

ELECTRIC CURRENT HELICITY IN 40 ACTIVE REGIONS IN THE MAXIMUM OF SOLAR CYCLE 22

Using vector magnetograms of 40 active regions (ARs) of the maximum of solar cycle 22, we calculated the imbalance _h (over the AR area) of the current helicity h_c B_z ( × B)_z in the photosphere. In 82.5% of the cases the predominant current helicity was negative (_h < 0) in the northern hemisphere and positive (_h > 0) in the southern hemisphere. Thus, the predominance of counter-clockwise (clockwise) vortices in the northern (southern) hemisphere seems to be valid not only for unipolar spots with obvious vortex structure (Hale, 1927; Richardson, 1941; McIntosh, 1979; Ding, Hong, and Wang, 1987) but also for ARs of different types. The forces of rotation of the Sun (Coriolis force and/or differential rotation) seem to take effect in the twisting of various magnetic structures.