1. Geomagnetism Program, U.S. Geological Survey, Golden, Colorado 80401, USA, and Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Denver, Colorado 80225, USA
2. Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Denver, Colorado 80225, USA
3. Geomagnetism Program, U.S. Geological Survey, Golden, Colorado 80401, USA