Magnetic field evolution leading to solar flares I. Cases with low magnetic shear and flux emergence

We picked up five solar flares larger than GOES X-ray class M/H alpha impor tance 1, and studied the magnetic evolution of the corresponding active reg ions. In particular, we investigated the spatial relationships among H alph a flare ribbons, soft X-ray (SXR) hare loops, and magnetic configurations. Our results show that all of these flares are not associated with high magn etic shear, but with emerging flux, indicating that flux emergence is a com mon phenomenon in this kind of solar flare with low magnetic shear. The max imum values of the transverse magnetic fields on the neutral lines responsi ble for these hares are 300-800 G. All of these flares but one have multipl e (more than two) H alpha ribbons, and almost all of these ribbons are loca ted at the footpoints of SXR flare loops. The flare-related shear changes, which can be both positive and negative, may be determined by the balance o f the energy carried by the emerging flux and that required to power the fl are.