1. Department of Physics, University of Warwick , Coventry CV4 7AL , UK
2. Department of Physics and Astronomy, University of California , Los Angeles, CA 90095-1562 , USA
3. Institut für Theoretische Physik und Astrophysik, University of Kiel , D-24098 Kiel , Germany
4. Center for Astrophysics and Space Sciences, University of California , San Diego, CA 92093-0424 , USA
5. Department of Earth, Planetary, and Space Sciences, University of California , Los Angeles, CA 90095 , USA
6. Department of Physics and Astronomy, University of Sheffield , Sheffield, S3 7RH , UK
7. Department of Physics, Università degli Studi di Trieste , I-34127 Trieste , Italy
8. Anton Pannekoek Institute for Astronomy, University of Amsterdam , 1090 GE Amsterdam , The Netherlands
9. Earth and Planets Laboratory, Carnegie Institution for Science , 5241 Broad Branch Rd NW, Washington, DC 20015 , USA