1. National Center for Computational Sciences, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, USA
2. Laboratory for Atmospheric and Space Physics, CU, Boulder, Colarado 80309-256, USA
3. NCAR, P.O. Box 3000, Boulder, Colorado 80307-3000, USA
4. Space Sciences Laboratory, University of California, 7 Gauss way, Berkeley 94720, California, USA
5. Institute for Chemical-Physical Processes-IPCF/CNR, Rende (CS) 87036, Italy
6. Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires and IFIBA, CONICET, Ciudad Universitaria, 1428 Buenos Aires, Argentina