1. Department of Physics and Astronomy, FI-20014 University of Turku, Finland
2. Department of Astrophysics, St Petersburg State University, Universitetskiy Pr. 28, Peterhof, 198504 St Petersburg, Russia
3. Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden
4. Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia