SHINE 20051 The Role of Sub-Surface Processes in the Formation of Coronal Magnetic Flux Ropes A. A. van Ballegooijen Smithsonian Astrophysical Observatory.

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Presentation transcript:

SHINE The Role of Sub-Surface Processes in the Formation of Coronal Magnetic Flux Ropes A. A. van Ballegooijen Smithsonian Astrophysical Observatory Cambridge, MA

SHINE Pre-CME Magnetic Structure Sheared photospheric magnetic fields ( Pevtsov, Canfield & Metcalf 1995 ) and S-shaped coronal X-ray structures ( Rust & Kumar 1996 ) suggest presence of a magnetic flux rope in corona: Flux rope must be held down by overlying coronal arcade.

SHINE Formation of Coronal Flux Ropes by Reconnection Pneuman (1983):

SHINE Formation of Coronal Flux Ropes by Reconnection Photospheric flux cancellation in a sheared arcade involves reconnection, creating a helical field: From: van Ballegooijen & Martens (ApJ 343, 971, 1989)

SHINE Formation of Coronal Flux Ropes by Reconnection Mean-field approach to modeling the evolution of the coronal field over many days (e.g., van Ballegooijen et al, ApJ 539, 983, 2000): The coronal field is sum of mean and fluctuating components. The fluctuating field is due to small-scale random footpoint motions, which produce diffusion of the mean field. Only the mean field B(r,θ,φ,t) is actually simulated. Mean field is assumed to be force free (magneto-friction: v ~ j × B). At the photosphere, field is subject to diffusion and differential rotation Surface diffusion of B r results in flux cancellation, but there is no radial diffusion of horizontal field:

SHINE Formation of Coronal Flux Ropes by Reconnection t=0 Example: Evolution of a single bipole. Coronal field evolves in ideal MHD. Various degrees of active-region tilt and helicity were considered. Initial field is weakly sheared (no helix). Photospheric flux is subject to diffusion and differential rotation. Flux cancellation at the neutral line involves magnetic reconnection, produces a flux rope after ~ 3 days. See: Mackay & van Ballegooijen ApJ 560, 445, 2001 ApJ 621, L77, 2005 t=3

SHINE Flux Cancellation Modes for removal of magnetic flux from the photosphere: From: Zwaan (1987) Ann. Rev. A&A 25, 83

SHINE Fibril Structure of Sub-Surface Field At photosphere-corona interface, continuity of mean magnetic stress (M xz,phot =M xz,cor ) and mean vertical field (B z,phot =B z,cor ) implies there is a discontinuity in mean horizontal field (f=filling factor):

SHINE Sub-Surface Structure of Active Regions See: van Ballegooijen (1997) in Synoptic Solar Physics, ASP Conf. Ser. v140, p17. Simple 2D model of active-region decay and repair of toroidal field: Diffusion (600 km 2 s -1 ) and magnetic pumping (-20 m/s) in convection zone. Potential field in corona. Constant filling factor (f=0.1). day 1 day 51 day 101

SHINE Model of Active-Region Evolution Initial state is a toroidal flux rope with left-helical twist: Simulated flux emergence: lat = -60° lat = +30°

SHINE Model of Active-Region Evolution Coronal field lines (left) and photospheric vector field (right) just after emergence: longitude latitude lat = +20°

SHINE Model of Active-Region Evolution Sub-surface evolution with diffusion (600 km 2 s -1 ), differential rotation, and downward pumping (-15 m/s). Ideal-MHD, non-linear force free field in corona. Coronal flux rope forms after only 2 days:

SHINE Model of Active-Region Evolution Surface and sub-surface fields (day 2): longitude radius longitude latitude

SHINE Model of Active-Region Evolution Flux rope lifts off (day 13): phot. radius longitude

SHINE Model of Active-Region Evolution The field opens up after lift-off, and a new flux rope forms (day 20): lat = -60°

SHINE Model of Active-Region Evolution Cancelled flux submerges and “repairs” the toroidal field: day 20 lat = -60° Problem with this simulation: toroidal field has lost its twist.

SHINE Conclusions 1.Magnetic helicity of active regions originates in the convection zone. Initially, the coronal field is only weakly sheared (no helix). 2.Continuity of magnetic stress at photosphere-corona interface requires that the fibril field at top of convection zone is near vertical. This prevents the submergence of sheared (horizontal) fields. 3.During the decay of an active region, magnetic flux submerges, but magnetic helicity stays in the corona. Helical flux ropes are formed by reconnection associated with flux cancellation. The helicity is eventually removed by coronal mass ejections. 4.The submergence of magnetic flux into the convection zone leads to the repair of the toroidal field.