In those works presented above (" Loop top source of solar flares" and "Plasmoid dynamics") we focus our attention on the explosive phase of flares during which a huge amount of energy is released quickly in various forms. The reconnection model of solar flares suggests that such a violent energy release is caused by fast magnetic reconnection. It is therefore important to understand how the fast magnetic reconnection occurs in the evolution of solar flares, which is the aim of our study on the preflare phase (Magara & Shibata 1999). Since it has been suggested that a spatially localized resistivity is needed for fast magnetic reconnection, we investigated how the distribution of resistivity changes from a uniform distribution to a localized one inside a current sheet during the preflare phase. We started with a current sheet under a uniform resistivity, which is later subject to the tearing instability. By simulating this process, we found that the current sheet was strongly compressed locally around X-points which were formed by the tearing process. We derived a scaling law about the thickness of the compressed area in order to see how small it might be in the environment of the solar corona. Consequently, we obtained the result that the current sheet is compressed strongly enough to generate an anomalously enhanced resistivity in the corona (Figure 3). This means that coronal current sheets naturally become the candidate to cause energetic explosions.
Magara, T., & Shibata, K. 1999, ApJ, 514, 456