1. The Johns Hopkins University Applied Physics Laboratory; Laurel Maryland USA
2. Space Science and Engineering Division; Southwest Research Institute; San Antonio Texas USA
3. Department of Physics and Astronomy; University of Texas at San Antonio; San Antonio Texas USA
4. IRAP (Institut de Recherche en Astrophysique et Planétologie); Université Paul Sabatier de Toulouse and CNRS; Toulouse Cedex 4 France