Solar an interplanetary magnetic helicity balance of ARs

MANDRINI, C.H.; DÉMOULIN, P.; VAN DRIEL-GESZTELYI, L.; DASSO, S.; GREEN, L.M.; LÓPEZ FUENTES, M.C.

Sydney

Jornada; XXV th General IAU Assembly - Joint Discussion 03; Magnetic Fields and Helicity in the Sun and the Heliosphere; 2003

IAU

    We have analyzed the long-term evolution of two active regions (ARs)from their emergence through their decay using observations from several instruments on board SOHO (MDI, EIT and LASCO) and Yohkoh/SXT. We have computed the evolution of the relative coronal magnetic helicity combining data from MDI and SXT with a linear force free model of the coronal magnetic field. These computations are affected by the inherent problems of this simplified representation of the field. However, the results we have found agree, in order of magnitude, with the magnetic helicity expected for large solar active regions.    The variation of the coronal magnetic helicity along the long-term evolution of the ARs should be balanced by injection of helicity via footpoint motions parallel (shearing, twisting and braiding) or orthogonal (flux emergence) to the photospheric surface, and ejection of helicity via CMEs or submergence of flux. For these two ARs, we have computed the injection of helicity by surface differential rotation and the ejection by CMEs. In the first case we have used MDI magnetic maps at consecutive central meridian passages and we have evaluated the input of helicity rotation by rotation, as the ARs orientations were changing.    To compute the depletion of helicity we have counted all the CMEs of which these ARs have been source, and we have evaluated their magnetic helicity by assuming a one to one correspondence with magnetic clouds. The magnetic helicity of a magnetic cloud was calculated using averaged values for the magnetic field and radius of a well-observed set of clouds and a linear force-free model in cylindrical geometry. Since the amount of helicity depends on the length of the interplanetary flux rope, we have used two different lengths to evaluate it: a conservative one of 0.5 AU, and a 2 AU length assuming that the rope is still anchored to the solar surface. We show that the computed mean helicity value obtained with a 2 AU length agrees, in order of magnitude, with the one obtained for a particular magnetic cloud that we have fitted using three different models in cylindrical geometry (a linear force-free, a constant twist and constant current model), and an estimated length obtained from observations of bidirectional electron flows.     When these three values (variation of coronal magnetic helicity, injection by differential rotation and ejection via CMEs) are compared, we find that surface differential rotation is a minor contributor since CMEs carry away at least 10 times more helicity than the one the differential rotation can provide. Therefore, the magnetic helicity needed in the global balance should come from localized photospheric motions.   Considering the findings of theoretical models of twisted flux tube emergence, we have obtained that the total helicity carried away in CMEs is equivalent to the end-to-end helicity of the flux tubes forming these ARs. Therefore, we conclude that most of the helicity ejected in CMEs is generated below the photosphere and emerges with the magnetic flux.