[1] Univ Leicester, Dept Phys & Astron, Leicester LE1 7RH, Leics, England
[2] Univ Coll London, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
[3] Univ London Imperial Coll Sci Technol & Med, Dept Phys, London SW7 2AK, England
[4] Univ Versailles St Quentin, CNRS, IPSL, LATMOS, F-78280 Guyancourt, France
[5] Max Planck Inst Sonnensyst Forsch, D-37077 Gottingen, Germany
[6] Univ Paris Diderot, Univ Paris 06, CNRS, LESIA,Observ Paris, Meudon, France
[7] UPMC, Ecole Polytech, CNRS, LPP, F-92120 Palaiseau, France