1. Centre for Plasma Physics, Queen's University Belfast, BT7 1NN Northern Ireland, United Kingdom
2. Physics Department, Faculty of Science, Mansoura University, 35516 Mansoura, Egypt
3. Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, CEP 91501-970, Porto Alegre, RS, Brazil