[1] Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA
[2] West Virginia Univ, Dept Phys & Astron, Morgantown, WV 26506 USA
[3] Univ Delaware, Bartol Res Inst, Dept Phys & Astron, Newark, DE 19716 USA
[4] Victoria Univ Wellington, Sch Chem & Phys Sci, Wellington 6012, New Zealand
[5] Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA
[6] Northumbria Univ, Dept Math Phys & Elect Engn, Newcastle Upon Tyne NE1 8ST, England
[7] Imperial Coll London, Dept Phys, London SW7 2AZ, England