From the photosphere to magnetic clouds: The helicity ejected by AR 7978

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

Córdoba

Congreso; X IAU Reunion Regional Latinoamericana de Astronomia; 2001

Active region (AR) 7978 was observed on the Sun alongseven solar rotations. We present a comprehensive study of its magneticevolution, concentrating mainly in the variations of its magnetic flux andhelicity and their relation to the production of flares and coronal massejections (CMEs).}   NOAA 7978 was the only major active region observed on the Sunduring seven solar rotations, from its birth in July 1996 to its fulldispersion in December 1996.~Solar activity along that periodwas dominated by this region, allowing us to make a direct comparisonbetween theevolution of its magnetic field (mainly its flux and helicity)and its flare and CME productivity. We have determinedthe budget of the relative magnetic helicity during the long lifetime of theAR: its injection into the corona by differentialrotation, its coronal evolution and its ejection in CMEs.   The coronal magnetic field and its magnetic helicity have been computedusing magnetograms obtained with the Michelson Doppler Imager (MDI/SoHO).The parameters for our magnetic model have been determined byfitting the computed magnetic field lines tothe soft X-ray loops observed with the Soft X-ray Telescope(SXT/Yohkoh). Furthermore, MDI magnetograms at each central meridian passagehave been also used to compute the magnetic helicity injected into the coronaby the differential rotation.   We have identified all the CMEs (a total of 26) thatoriginated from this region using the observations ofthe Large Angle Spectroscopic Coronagraph (LASCO/SoHO)and the Extreme Ultarviolet ImagingTelescope (EIT/SoHO). Considering a set of 18 well-observed interplanetarymagnetic clouds for which the magnetic field has been measured and modelledusing a linear force-free approach, we have computed an averagevalue for the magnetic helicity in a cloud.Assuming a one to one correspondence between the CMEs and the magneticclouds originating from them, we have estimated the magnetic helicitythat was shed via CMEs.    We have found that the differential rotation can neither provide therequired magnetic helicity to the coronal field, which isat least a factor 2.5 to 4 larger,~nor to the field ejected to theinterplanetary space,~which is at least a factor4 to 20 larger. On the other hand,~the totalhelicity ejected is equivalent to that of a twisted flux tube havingthe same magnetic flux as the studied AR and a number of turns in theinterval $[0.5,2.0]$, equivalent to the amount of twist which can ensurethat the buoyant flux tube survives its emergence through theconvection zone.  Thus, we suggest that the main sourceof helicity injection into the coronal fieldis the inherent twist of the magnetic flux tube forming the activeregion.  This magnetic helicity is transferred to the corona either bythe continuous, but slow, emergence of the flux tube or bytorsional Alfv\'en waves, for several solar rotations; that is tosay, along a period of time much longer than the classicalemergence phase.