[1] Max Planck Inst Extraterr Phys, Giessenbachstr 1, D-85748 Garching, Germany
[2] CSIC IEEC, Inst Ciencies Espai, Campus UAB,Carrer Can Magrans S-N, Cerdanyola Del Valles 08193, Catalonia, Spain
[3] Univ Fed Minas Gerais, ICEx, Dept Fis, Caixa Postal 702, BR-30123970 Belo Horizonte, MG, Brazil
[4] Chalmers, Dept Earth & Space Sci, Onsala Space Observ, S-43992 Onsala, Sweden
[5] Univ Leeds, Sch Phys & Astron, Leeds LS2 9LN, W Yorkshire, England
[6] Rice Univ, Dept Phys & Astron, Main St, Houston, TX 77005 USA