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

Through a multi-wavelength and multi-instrument analysis we evaluate the magnetic helicity budget of an isolated active region (NOAA 7978) from its emergence throughout its decay. Using Yohkoh/ SXT images and linear force-free magnetic extrapolations carried out on SOHO/MDI magnetograms, we compute the relative magnetic helicity in the corona. Based on the observed magnetic field distribution we calculate the magnetic helicity injected by differential rotation. Then, using SOHO/LASCO & EIT and SXT images we identify all the 26 coronal mass ejections (CMEs) which originated from this active region during its lifetime and we estimate the amount of helicity which was shed via CMEs. Comparing these three values we find that the differential rotation can neither provide enough helicity to account for the diagnosed coronal helicity 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. © 2002 Elsevier B.V. All rights reserved.