**4.2 Sudden change of coronal magnetic field structure and flare initiation**

During the period of quasi-steady evolution of the corona, the plasmas in the corona are in quasi-equilibrium state, and the topological structure of coronal magnetic field evolves continuously. 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 [1, 3, 11]. The plasmas at these sites lost equilibrium and are ejected from their original positions. The magnetic reconnections occur beneath the erupted plasmoid, and then the flares are initiated. After the flare eruptions, the corona returns to equilibrium state and continues its quasi-equilibrium evolution [3, 11].

During the period of flare eruptions, the plasmas are in dynamic state, and the force-free condition is not satisfied. Out of the flare eruption period, the force-free field model is a well approximation to the coronal magnetic field [11].

Because the plasmoid ejection associated with the flare initiation needs magnetic force and the magnetic force (Lorentz force) only acts on the electric current, the sites of flare initiation are always located in the areas with strong electric current which in turn concentrates around the PILs of ARs. That is why the flare phenomenon is connected with the nonpotentiality (indicated by the electric current) of solar ARs, and flares always occur above the PILs.
