1. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
2. Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, E-08860 Castelldefels, Spain
3. Institute for Space Studies of Catalonia, c/Gran Capità 2–4, Edif. Nexus 201, E-08034 Barcelona, Spain
4. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
5. South-Western Institute for Astronomy Research, Yunnan University, Kunming, Yunnan 650091, China
6. University of Chinese Academy of Sciences, Beijing 10049, China