1. School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
2. Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70–264, 04510 Ciudad de México, Mexico
3. Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
4. Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, E-08193 Barcelona, Spain
5. Apache Point Observatory, New Mexico State University, PO Box 59, Sunspot, NM 88349, USA
6. McDonald Observatory, The University of Texas at Austin, 1 University Station, Austin, TX 78712, USA
7. Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, UT 84112, USA