1. A. D. Brandon, S. B. Shirey, R. W. Carlson, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC, USA, 20015.
2. R. A. Creaser, Department of Earth and Atmospheric Sciences, University of Alberta, 126 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3.