1. Faculty of Infor-mation Science and Engineering, Ningbo University, Ningbo, China
2. School of Computer Science and Mathematics, Fujian University of Technology, Fuzhou, China
3. Key Laboratory of Solar Activity, National Astro-nomical Observatories (CAS), Beijing, China
4. School of Information and Communications, Gwangju Institute of Science and Technology (GIST), Gwangju, Korea