1. SUPA School of Physics and Astronomy, University of Glasgow , Glasgow G12 8QQ UK
2. Space Sciences Laboratory, University of California , Berkeley, CA 94720 USA
3. National Solar Observatory , 3665 Discovery Drive, Boulder CO 80303 USA
4. Rosseland Centre for Solar Physics, University of Oslo , PO Box 1029 Blindern, NO-0315 Oslo , Norway
5. Astronomy & Astrophysics Section, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies , Dublin, D15 XR2R , Ireland
6. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences , Nanjing 210034 , China