1. Department of Terrestrial Magnetism Carnegie Institution of Washington 5241 Broad Branch Road Washington District of Columbia 20015 USA
2. Geophysical Laboratory Carnegie Institution of Washington 5251 Broad Branch Road Washington District of Columbia 20015 USA
3. Advanced Light Source Lawrence Berkeley National Laboratory Berkeley California 94720 USA
4. Department of Earth and Atmospheric Sciences University of Alberta Edmonton AB T6G 2E3 Canada