We investigated the physical relationship between magnetic helicity and the solar flare onset mechanism, based on the three-dimensional numerical simulations and on the magnetic observations. First, in terms of the new methodology to derive the magnetic helicity flux through the solar surface using the vector magnetogram (Kusano et al. 2002), we found that the both signs of magnetic helicity were simultaneously injected into the coronal magnetic field in active regions. Secondly, it was also revealed for a couple of the flare events, that the initial brightening in TRACE 1600A image was located at a narrow line, where the sign of magnetic shear is sharply reversed on the photosphere. Thirdly, motivated by these observations, we carried out the numerical simulations, in which the magnetic shear pre-loaded into a magnetic arcade is reversed by a slow foot-point motion. The simulations clearly indicated that the helicity reversal can lead to the eruption of large-scale plasmoid through the nonlinear process of the resistive instability growing on the helicity inversion layer. The results strongly suggest that solar flares are a consequence of the magnetohydrodynamic relaxation process driven by the cancellation of magnetic flux along the arcade configuration. From these studies, we propose a new flare model that the annihilation of magnetic helicity may cause the flare onset.

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