We acknowledge support from INTAS Program Foundation (Grant No. 99-348), NATO Collaborative Linkage Grant PST.CLG 974737 and RFBR grant 00-05-64062a. DS is grateful to the Royal Society for financial support and to a special fund of the Faculty of Engineering of the Ben-Gurion University of the Negev for visiting senior scientists. We thank A.Brandenburg for helpful comments.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000