Magnetic helicity accumulation behind the CME process

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

Postdam

Workshop; 1st Postdam Thinkshop; 2002

Based on observations and model calculations of the well-studied CME prolific active region NOAA 7978 in July-November 1996 we evaluate the relative importance of different mechanisms for helicity input in this active region. We follow magnetic helicity from the interior of the Sun, where it is mostly generated presumably in the tachocline, to the corona where it is accumulated via twisted magnetic flux emergence and by differential rotation. Finally, we trace helicity into the interplanetary space, where coronal mass ejections bring the twisted magnetic field. We find that the differential rotation plays a subordinate part in the generation of magnetic helicity and can only provide 1/8th of the huge amount of helicity ejected with the 31 CMEs from this active region. Our results suggest that helicity plays a crucial role in the CME process: CMEs erupt when a threshold is reached in the corona. After CMEs helicity is replenished from the sub-photospheric twist of the flux tube forming the active region. This process can continue for several solar rotations until the helicity reserves of the flux tube are exhausted, or the flux tube is destroyed by convective motions. This explains why active regions keep producing CMEs during their decay phase when their flare activity has had long ceased.