1. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024, USA
2. Department of Mathematical Sciences, Loughborough University, Loughborough LE11 3TU, United Kingdom
3. Air Force Research Laboratory, Kirtland Air Force Base, Albuquerque, New Mexico 87123, USA
4. Center for Space Physics, Boston University, Boston, Massachusetts 02215, USA