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Evolution of Solar Magnetic Arcades. II. Effect of Resistivity and Solar Eruptive Processes  

G S Choe   Submitted: 2000-02-03 01:01

The resistive evolution of pre-sheared magnetic arcades in a 2-D Cartesian geometry is investigated. We find that there is a critical value of shear, over which magnetic reconnection can take place in a magnetic arcade to create a magnetic island. The diffusion of the toroidal field eventually leads to thinning of the current layer for any amount of shear whereas the diffusion of the poloidal field brings about current layer thinning only for shears above the critical value. The reconnection profiles are found to depend on spatial resistivity patterns. A fast reconnection with small shock angles can be achieved only when the resistivity is confined to a small volume. In this case, high speed reconnection outflows can tear the magnetic island into a pair. The fast-moving island system creates a fast shock or a steepened fast mode structure, which resembles an observed CME frontal loop.

Authors: G. S. Choe and L. C. Lee
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Publication Status: ApJ (published)
Last Modified: 2000-02-03 01:01
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A Model of Solar Flares and Their Homologous Behavior  

G S Choe   Submitted: 2000-02-03 00:05

A model describing physical processes of solar flares and their homologous behavior is presented based on resistive MHD simulations of magnetic arcade evolution subject to continuous shear-increasing footpoint motions. It is proposed in our model that the individual flaring process encompasses magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared, a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create another island in the underlying arcade below the magnetic island. The newborn island rises faster than the preceding island and merges with it to form one island. Before merging with the upper island is completed, the newborn island exhibits two different phases of rising motion: the first phase with a slower rising speed and the second phase with a faster rising speed. This is consistent with the Yohkoh observation by Ohyama and Shibata (1998) of X-ray plasma ejecta motion. The first phase, in which reconnection of line-tied field in the underlying arcade is important, can be regarded to be related with the preflare phase. In the second phase, the island coalescence takes place, which creates an elongated current sheet below and enhances the reconnection rate of the line-tied arcade field. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed X-ray emissions. After merging of the islands is completed, magnetic reconnection continues in the current sheet under the integrated island for rather a long period, which can be considered as the main phase of flares. The sequence of all these processes is repeated with some time interval while a shear-increasing motion continues. We propose that a series of these flaring processes constitutes a set of homologous flares. The time interval between successive flaring events depends on the energy input rate into the system, which is governed by the nature of the footpoint motion and the flux reconnecting rate. We have also investigated the destruction of a magnetic island in a system undergoing a decrease of magnetic shear. The result suggests that there is a critical value of magnetic shear for existence of a magnetic island in an arcade-like field configuration.

Authors: G. S. Choe and C. Z. Cheng
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Publication Status: ApJ (submitted)
Last Modified: 2000-02-03 00:05
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Evolution of Solar Magnetic Arcades. II. Effect of Resistivity and Solar Eruptive Processes
A Model of Solar Flares and Their Homologous Behavior

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