1. Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605 Roma, Italy. E-mail: alessandro.piscini@ingv.it
2. COMET, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Parks Road, OX1 3PU Oxford, UK
3. Earth Observation Laboratory, Engineering department Tor Vergata University, Via del Politecnico 1, I-00133 Rome, Italy