The origin of magnetic helicity in solar active regions

摘要

The magnetic fields in solar active regions are known to be helical. The current helicity, as measured from magnetograms, is negative on the northern and positive in the southern hemisphere, its normalized mean value being pB·(×B)/B2~10?8 m?1. Recent observations have convincingly demonstrated that the helicity originates in subsurface regions and the flux tubes emerge in an already helical form. One possible contribution to this helicity is the accretion of the weak general poloidal magnetic field of the Sun by the rising toroidal flux loop. For this process to work, the field lines of the background field first need to be wrapped around the rising flux tube, which assumes that freezing-in holds to a good degree of approximation. The wrapped-up field then has to diffuse into the flux tube, giving rise to a twisted magnetic field structure. The conflict between the requirements of freezing-in (for wrapup) and diffusion (for penetration into the tube) is only apparent. The role of diffusion is likely to be limited during most of the rise of the tube, allowing a significant amount of flux to be swept up; then, in the uppermost 20–30% of the tube's path through the convective zone diffusion prevails, allowing penetration of the wrapped up flux.