Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, University of California, San Diego, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0225, USA
Department of Mathematics and Statistics, Queen's University, Jeffery Hall, University Avenue, Kingston, ON K7L 3N6 Canada
Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974, USA
Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974, USA; Center for Solar Terrestrial Research, Department of Physics, New Jersey Institute of Technology, 323 Martin Luther King Boulevard, Newark, NJ 07102, USA