Magnetic helicity accumulation and tilt angle evolution of newly emerging active regions

¹ National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China e-mail: yangshb@bao.ac.cn
² Max-Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany

Received: 23 April 2008
Accepted: 23 April 2009

Abstract

Context. It has been known for years that there is a general dominance of negative (positive) helicity of active regions (ARs) in the northern (southern) solar hemisphere. For a better understanding of the role of helicity in the evolution of active regions, it is necessary to know more about the accumulation of helicity during the emergence of active regions. In particular, different conclusions were drawn in the past about the relationship between the accumulated helicity and the writhe of active regions.

Aims. We investigate the accumulation of helicity in newly emerging simple bipolar solar active regions. We also investigate the relation between the accumulated helicity and writhe.

Methods. We obtain helicity accumulation by applying Fast Fourier Transforms (FFT) and local correlation tracking (LCT) to MDI data. We deduce the writhe of the active regions according to the evolution of the tilt angle between the connecting line of the weighting centers of opposite polarities in the ARs.

Results. It is found that the accumulated helicity is proportional to the exponent of magnetic flux in the 58 selected newly emerged simple ARs. 74% of ARs have a negative (positive) helicity when the above defined tilt angle rotates clockwise (counter-clockwise). This means that the accumulated helicity and writhe have the same sign for most of the investigated ARs according to the tilt angle evolution of ARs. We also found that 56% (57.6%) of these ARs in the northern (southern) photosphere provide negative (positive) helicity to the corona in the course of the emergence of magnetic flux.