1. a Laboratory for Terrestrial Physics, NASA/Goddard Space Flight Center , Greenbelt, Maryland, 20771, U.S.A
2. b Universities Space Research Association Research Scientist, NASA/Goddard Space Flight Center , Greenbelt, Maryland, 20771, U.S.A
3. c Science Systems Application Inc. and NASA/Goddard Space Flight Center , Greenbelt, Maryland, 20771, U.S.A