One of the authors (VMN) is grateful to B. Roberts and M.S. Ruderman for a number of stimulating discussions. LO would like to thank the NASA SR&T program (NASW-98004), the NASA SOHO Guest Investigator program (NASW-98024), the NASA Space Physics Theory program, and the ISTP program. The authors are grateful to the referee, U. Torkelsson for valuable comments.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999