Photospheric Motions and Coronal Mass Ejection Productivity

Abstract

Shearing motions have been frequently used in MHD simulations of coronal mass ejection (CME) initiation but have hardly been reported from observations of CME-producing regions. In this Letter, we investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by such motions or by emerging magnetic flux. We use photospheric magnetic field observations of NOAA Active Region 9165, which is an ideal candidate for such study because (1) it is the site of both new flux emergence and intense horizontal shearing flows; (2) it shows rapid development and rapid decay, and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; and (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shearing motions is probably the largest ever reported; it amounts to about the total helicity that the active region's differential rotation would have injected within three solar rotations. But the CMEs linked to the active region remove at least a factor of 4-64 more helicity than the helicity injected by horizontal shearing motions. Consequently, the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Our study implies that shearing motions, even when they are strong, have little effect in the process of buildup of magnetic free energy that leads to the initiation of CMEs.

Solar Flares: Magnetohydrodynamic Processes

“…Deriving observationally how much magnetic helicity is injected into the atmosphere has widely been done, which is a key to the understanding of the relationship between helicity evolution and the occurrence of active phenomena including flares (Chae, 2001;Moon et al, 2002;Nindos and Zhang, 2002;Kusano et al, 2002;Démoulin and Berger, 2003;Yang et al, 2004;Pariat et al, 2005;Jeong and Chae, 2007;Magara and Tsuneta, 2008). These studies are important in that we can derive the characteristics of helicity evolution and use it to predict the occurrence of active phenomena on the Sun.…”

Interaction between a Fast Rotating Sunspot and Ephemeral Regions as the Origin of the Major Solar Event on 2006 December 13

“…The movement of magnetic footpoints by photospheric flows can lead to the destabilization of the magnetic field and hence to flares by increasing the length of the field lines in the corona (e.g., Somov et al 2002). On the other hand, Nindos & Zhang (2002) have pointed out that shearing motions have little effect in the process of buildup of magnetic free energy that leads to the initiation of coronal mass ejections (CMEs).…”

Self and mutual magnetic helicities in coronal magnetic configurations

“…It is worth noticing that this does not necessary imply the existence of bounds on magnetic helicity which could be the evidence for the trigger of an eruptive event, since it has been shown than constant helicity mechanisms can lead to such disruptions (Amari et al 2003a,b). Several methods have been developed to estimate the magnetic helicity injection rate from photospheric magnetic field measurements (line-of-sight or vector measurements) and for which the flow field on the photosphere is needed (Chae et al 2001;Démoulin et al 2002;Nindos & Zhang 2002;Kusano et al 2002;Moon et al 2002;Démoulin et al 2003;Chae et al 2004). In this Paper, we have a different approach to compute the magnetic helicity and its evolution in the corona: we use the magnetic field vector and the vector potential in the coronal half-space above the photospheric plane of measurement, determined by a force-free extrapolation method.…”