Long-term evolution of an AR and its CME activity

VAN DRIEL-GESZTELYI, L.; MANDRINI, C.H.; THOMPSON, B.; PLUNKETT, S.P.; AULANIER, G.; DÉMOULIN, P.; SCHMIEDER, B.; DE FOREST, C.

Caputh

Congreso; 3rd Advances in Solar Physics Euroconferences, Magnetic Fields and Oscillations; 1998

European Solar Physics Division

Using SOHO/MDI full-disc magnetic maps, we follow the magnetic evolution of a solar active region for several months in the period of July-November 1996. We extrapolate the photospheric magnetic fields in the linear force-free approximation and match the modelled field lines with the soft X-ray loops observed with the Yohkoh/SXT in order to diagnose the coronal magnetic shear. We find that while the turbulent motions diffuse the flux, the differential rotation, and possibly twisted flux emergence, increase the magnetic shear. Flares are observed during the first three rotations, while CME events (observed by SOHO/EIT and LASCO) originate from this AR from its emergence throughout its decay. Several early CMEs, while none of the late CMEs, are related to flare events above the GOES B1 level. We find that the late CMEs occur when the magnetic shear, after accumulating for four rotations, reaches a high level and saturates. We propose that CME activity serves as a valve through which the AR could get rid of excess shear and helicity.