1. Instituto de Física, Universidade de São Paulo, 05508-090 São Paulo, Brazil
2. Francis Bitter Magnet Lab and Plasma Science and Fusion Center, Massachusetts Institute of Technology, 77 Mass Ave., Cambridge, Massachusetts 02139, USA
3. Department of Physics, Massachusetts Institute of Technology, 77 Mass Ave., Cambridge, Massachusetts 02139, USA
4. LAS-INPE, Av. dos Astronautas, 1758-Jd. Granja, 12227-010 São José dos Campos, Brazil