1. Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
3. Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, San Diego, CA 92093, USA.
4. Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
5. Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA.