**5. Conclusion**

As a prominent eruptive phenomenon happening in solar atmosphere, solar flares usually come from solar ARs which possess strong and concentrated bipolar magnetic field in the photosphere. Between the two opposite polarities is the PIL of the magnetic field. Around the PIL is the sheared vector magnetic field in photosphere and twisted field lines in the corona, which are the manifestations of electric current distribution around the PIL and the existence of free magnetic energy bundled with the electric current. When a flare happens, the variation of the photospheric magnetic field causes sudden change of topological structure of coronal magnetic field at a site in the strong electric current area. The electric current and associated plasmas at this site lost equilibrium and are ejected from their original position. The magnetic reconnection occurs beneath the erupted plasmoid, and the flare is initiated. Part of the released free magnetic energy is converted to the electromagnetic emission of flares, which manifests as sudden brightening across a broad range of electromagnetic wave spectrum, such as white-light flare in photosphere, optical flare in chromosphere, and soft X-ray flare in the corona. Other released magnetic energy is transferred to CMEs and SEPs associated with flares. Big solar flares and the associated CMEs and SEPs can cause severe disturbances to the space weather condition in the solar-terrestrial space as well as in the whole heliosphere [14, 23–25].

Besides the Sun, flare phenomenon was also observed on the solar-type stars that possess magnetic activity [26–29]. In recent years, owing to the continuous light-curve observations for a large volume of stellar objects by the space missions, such as the Kepler space telescope [30], much more stellar flare samples were obtained and available for analysis [27, 29]. The understandings about the solar flares can provide a good physical framework basis for investigating the eruption mechanism of stellar flares [31, 32].
