1. European Space Agency, ESAC, Villanueva de la Cañada, E-28692 Madrid, Spain
2. Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
3. Université Côte d'Azur, CNRS, Observatoire de la Côte d'Azur, Laboratoire J. L. Lagrange, Boulevard de l'Observatoire, CS 34229, 06304 Nice Cedex 4, France
4. Dipartimento di Fisica, Università della Calabria, I-87036 Cosenza, Italy