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C. Z. Cheng, G. S. Choe (Princeton Plasma Physics Laboratory)
A model describing physical processes of solar flares and their homologous behavior is proposed based on resistive MHD simulations of magnetic arcade evolution. The individual flaring process encompasses magnetic reconnection of arcade field, generation of magnetic islands, and coalescence of magnetic islands. When the magnetic shear of the arcade field is increased over a threshold (via footpoint motion or flux emergence or both), a current sheet is formed and magnetic reconnection takes place to form a magnetic island. In the underlying arcade below the magnetic island, a new reconnection process can be triggered by a continuing increase of magnetic shear to create a new island. The newborn island rises faster than the preceding island and merges with it to form one island. Before the island merging process 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 of the upward motion of X-ray plasma ejecta by Ohyama and Shibata [1998]. The first phase, in which reconnection of line-tied fields in the underlying arcade is important, is considered to be the preflare phase. In the second phase, the island coalescence process creates an elongated current sheet in the underlying arcade and enhances the reconnection of line-tied arcade fields. This phase is considered as the impulsive phase or the flash phase of flares. The reconnection electric field during the second phase is large enough to accelerate electrons to an energy level greater than 10 keV necessary for hard X-ray production. After the island merging is completed, magnetic reconnection continues in the current sheet under the integrated island for a longer period which is considered as the main phase of flares. The sequence of all these processes is repeated with some time interval while the magnetic shear is increased. A series of these flaring processes is considered to constitute a set of homologous flares. The time interval between successive flaring events depends on the magnetic energy input rate into the system, which is governed by the nature of magnetic shear increase and the plasma dissipation rate.
The author(s) of this abstract have provided an email address for comments about the abstract: fcheng@pppl.gov