[1] Carnegie Inst Sci, Dept Terr Magnetism, 5241 Broad Branch Rd, Washington, DC 20015 USA
[2] Queen Mary Univ London, Sch Phys & Astron, 327 Mile End Rd, London E1 4NS, England
[3] Carnegie Inst Sci, Carnegie Observ, 813 Santa Barbara St, Pasadena, CA 91101 USA
[4] Natl Res Council Canada, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada