[1] Univ Warwick, Dept Phys, Coventry CV4 7AL, England
[2] Univ Calif Los Angeles, Dept Phys & Astron, Los Angeles, CA 90095 USA
[3] Univ Kiel, Inst Theoret Phys & Astrophys, D-24098 Kiel, Germany
[4] Univ Calif San Diego, Ctr Astrophys & Space Sci, San Diego, CA 92093 USA
[5] Univ Calif Los Angeles, Dept Earth Planetary & Space Sci, Los Angeles, CA 90095 USA
[6] Univ Sheffield, Dept Phys & Astron, Sheffield S3 7RH, England
[7] Univ Trieste, Dept Phys, I-34127 Trieste, Italy
[8] Univ Amsterdam, Anton Pannekoek Inst Astron, NL-1090 GE Amsterdam, Netherlands
[9] Carnegie Inst Sci, Earth & Planets Lab, 5241 Broad Branch Rd NW, Washington, DC 20015 USA