1. Dipartimento di FisicaUniversità della Calabria Cosenza Italy
2. Now at Center for Mathematical Plasma Astophysics, Departement WiskundeUniversiteit Leuven Leuven Belgium
3. Observatorio Astronómico de QuitoEscuela Politécnica Nacional Quito Ecuador
4. Department of Physics and AstronomyUniversity of Delaware Newark Delaware USA