1. Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
2. Laboratoire des Sciences du Climat et de l’Environnement, CEA-Orme des Merisiers, F-91191 Gif-sur-Yvette CEDEX, France
3. Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA