**Abstract**

Solar flares are prominent eruptive phenomenon happening in the solar atmosphere. Major flares usually come from solar active regions (ARs) where strong and concentrated bipolar magnetic field exists and manifests as dark sunspots in photosphere. The photospheric magnetic field acts as the bottom boundary of corona system and confines the magnetic structure of the corona. For complex ARs, the coronal magnetic field generally contains electric current around the magnetic polarity inversion lines, which corresponds to the nonpotential magnetic field and manifests as twisted field lines. The coronal magnetic field structure evolves as the response to the variations of the photospheric magnetic field. This coronal evolution can be quasi-steady and approximated by the force-free condition. In some situations, the variations of photospheric magnetic field may cause sudden changes of topological structure of coronal magnetic field at certain sites in the corona. The plasmas at these sites lost equilibrium and are ejected from their original positions. This process is accompanied with magnetic reconnection and leads to the release of magnetic energy in the corona. Part of the released magnetic energy is converted to the electromagnetic emission which manifests as sudden brightening across a broad range of electromagnetic wave spectrum, and hence the flare phenomenon is initiated.

**Keywords:** solar flare, magnetic field, plasma, corona
