Helicity loading and dissipation: The helicity budget of AR 7978 from the cardle to the grave

VAN DRIEL-GESZTELYI, L.; DÉMOULIN, P.; MANDRINI, C.H.; PLUNKETT, S.P.; THOMPSON, B.; KHOVARI, ZS.; AULANIER, G.; YOUNG, A.; LOPEZ FUENTES, M.; POEDTS, S.

Hawaii

Congreso; Yohkoh 10 Anniversary Meeting; 2001

An isolated active region was observed on the Sun during seven rotations, starting in July 1996. I will present a study of its magnetic field, concentrating on its helicity budget. The photospheric field is extrapolated into the corona in a linear force-free approach, using SOHO/MDI magnetograms and Yohkoh/SXT images, allowing us to compute, in a crude way, the relative coronal magnetic helicity of the active region. Using the observed magnetic field distribution (SOHO/MDI magnetograms) we also calculate the helicity injected by the differential rotation during seven solar rotations. Finally, using SOHO/LASCO and EIT as well as Yohkoh/SXT observations, we identify all the 26 CMEs which originated from this active region during its lifetime and using average values of the field and radius of magnetic clouds, we estimate the helicity which should be shed via CMEs. We compare these three values to evaluate the importance of the differential rotation relative to twisted flux emergence as a source of magnetic helicity. We find that the differential rotation can neither provide enough helicity to account for the diagnosed coronal heicity values, nor for the helicity carried away by CMEs. We suggest that the main source of the magnetic helicity must be the inherent twist of the magnetic flux tube forming the active region. This magnetic helicity is transferred to the corona either by a slow continuous emergence of the flux tube or by torsional Alfven waves, during several solar rotations.