In this paper we study resistive processes in the preflare phase of eruptive flares by means of the 2.5-dimensional MHD numerical simulation. According to many detailed observations of solar flares, their evolution is characterized by several phases, each of which has a distinct nature. In the first phase, some kinds of radiation begin to be enhanced gradually, which implies the occurrence of the preflare heating. Then, at a certain time, that gradual energy-release phase is replaced by the violent energy-release phase in which a huge amount of energy is released in various forms. So far, the nature of this violent energy-release phase has been well studied by using a flare model based on the fast magnetic reconnection, although those problems of the preflare heating and the transition from the gradual energy-release phase to the violent one have not been sufficiently discussed yet. In this paper, in order to tackle these problems, we start with a 2.5-dimensional force-free current sheet under a uniformly distributed resistivity, which is subject to a very small random velocity perturbation. At first the evolution enters on the linear stage of tearing instability and later a sufficient amount of thermal energy is produced in the nonlinear stage, which is considered to have a relation with the preflare heating. In this nonlinear stage, the component of magnetic fields perpendicular to the sheet (perpendicular magnetic fields) flows away from X-points formed in the sheet and eventually the current sheet collapses at these points. This collapse strongly reduces the thickness of the sheet if the magnetic Reynolds number is quite large and the plasma beta is quite low. Since the formation of thin current sheet leads to the occurrence of locally enhanced resistivity (anomalous resistivity), the transition from the gradual energy-release phase under a uniformly distributed resistivity to the violent one under a locally enhanced anomalous resistivity can be accomplished, which causes the fast magnetic reconnection responsible for various explosive phenomena in the Sun.
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