Evolution, Dynamics and Heating of Coronal Magnetic Flux Ropes
A. A. van Ballegooijen
SAO
- The coronal magnetic field is anchored in the photosphere and continually evolves in response to changes in the photospheric boundary conditions. At the photosphere the field is concentrated into discrete flux elements that exhibit various random and systematic motions, and these elements frequently split-up or merge with each other. New flux emerges in the form of twisted bipoles that reconnect with preexisting coronal fields, and magnetic flux may be removed from the corona by submerging below photosphere. All of these effects cause electric currents to be induced in the corona, producing a complex magnetic field that deviates significantly from a potential field. The strongest deviations occur near polarity inversion lines where highly sheared, weakly twisted magnetic fields can exist for long periods of time. These "coronal flux ropes" contain large amounts of magnetic free energy and helicity. Occasionally the flux ropes erupt, causing helicity to be ejected into the heliosphere.
- In this talk I describe various models for the structure, dynamics and heating of coronal flux ropes. Global models describe the evolution of the coronal field on time scales of many months, and are important for understanding how the helicity that emerges in active regions is spread over the quiet Sun. Local models describe how coronal flux ropes interact with the dynamic flux elements in the photosphere. I also discuss a new theory for plasma heating in coronal flux ropes. According to this theory, the energy dissipated in flux ropes has two distinct contributions: one from photospheric footpoint motions, and another from the magnetic free energy of the flux rope. It is assumed that flux ropes contain stochastic magnetic fields, and that small-scale reconnection occurs at many sites with the flux rope. The effect of such reconnection on the mean magnetic field is described in terms of hyperdiffusion, a type of magnetic diffusion in which the magnetic helicity of the mean field is conserved (Taylor relaxation). Initial results from modeling the hyperdiffusive heating of coronal flux ropes are presented.
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