Emergence and long term evolution or an isolated active region

VAN DRIEL-GESZTELYI, L.; DÉMOULIN, P.; MANDRINI, C.H.

Paris, Francia

Congreso; 2nd. Franco-Chinese Meeting on Solar Physics; 2002

We describe results of a comprehensive analysis of thelong-term evolution of an isolated active region (NOAA 7978,July-December 1996), which, due to its undisturbed evolutionduring the last solar minimum and its simple bipolar structure,can be used as a test-case for long-term evolution studies. First, usingSOHO/MDI magnetic data, we follow the main flux emergence phase,when during a few days $10^{22}$ Mx flux surfaced.Then we show the much longer decay phase of theactive region (AR), during which its magnetic flux gradually spreadsover an ever increasing area while being removed by magneticcancellation. Since the magnetic field governs allmanifestations of activity, we study the evolution of differentsignatures of activity in  the AR during six solarrotations. We find that flare activity shows a sharp decrease andceases after three solar rotations, while CME activity remains on arelatively high level (3-4 CMEs per solar rotation) even duringthe decay phase of the AR. Finally, using Yohkoh/SXT and SOHO/EIT,LASCO and MDI data we draw the long-term magnetic helicity budget ofthe active region and find that the main source of magnetichelicity carried away in CMEs is the sub-photospheric twist of theflux tube forming the AR.