Evolution of coronal helicity in a twisted emerging active region

Abstract

Active-region magnetic fields are believed to be generated near the shear layer of the convection zone by dynamo processes. These magnetic fields are concentrated into fluxtubes, which rise, due to buoyancy, through the convection zone to appear in the form of bipoles at the photosphere. Thin-fluxtube simulations suggest that active regions require twist to emerge. All regions are observed to emerge with some twist; some of them show larger twist than others. A theoretical model [Longcope andWelsch 2000, ApJ, 545, 1089] predicts that an emerging fluxtube injects helicity into the corona for one or two days after its initial emergence through rotation of its footpoints driven by magnetic torque. There have been very few observational studies of helicity injection into the corona by emerging flux. This paper presents a study of helicity during the emergence of active region NOAA 8578. The time history of spin helicity injection, related to footpoint rotation, suggests that an Alfvén wave packet crossed the apex of the emerging fluxtube.