1. Institute of Geophysics and Planetary Physics/ESS, University of California at Los Angeles, Los Angeles, CA 90095, USA.
2. Space Sciences Laboratory, University of California at Berkeley, CA 94720, USA.
3. Code 674, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
4. TUBS, Braunschweig, D-38106, Germany.
5. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4.