Evolution and decay of an active region: Magnetic shear, flare and CME activity

Abstract

An activity complex was observed on the solar disk between April, 1996 and February, 1997 that reached its highest level of activity during the birth of AR 7978. Our observations extend over six solar rotations, from the emergence of AR 7978 (July 1996) until the decay and dispersion of its flux (November 1996). Multi-wavelength observations, provided by instruments aboard the Solar and Heliospheric Observatory (SOHO) and the Japanese spacecraft Yohkoh, follow the evolution of the region from the photosphere to the corona. Using full disk magnetograms obtained by the Michelson Doppler Imager (SOHO/MDI) as boundary condition, we calculate the coronal magnetic field and determine its shear by fitting the computed field lines to the observed soft X-ray loops. We discuss the evolution of the coronal field shear and its probable relation to flare and coronal mass ejection activity.