We are very indebted to Dr. B. Dubrulle, the referee, for raising questions that helped us to improve our understanding of this problem. C.M.F. expresses gratitude for the hospitality received as a visiting scientist at Rice University, in the High Energy Astrophysics Group of the Space Physics and Astronomy Department headed by Prof.Edison Liang, and also to the Massachusetts Institute of Technology, Physics of High Energy Plasmas Group of the Research Laboratory of Electronics headed by Prof.Bruno Coppi. He also acknowledges support from Conselho Nacional de Desenvolvimento Cientifico e Tecnologico. This work has been partially supported by NASA grant NAG5-1547.
© European Southern Observatory (ESO) 1997