We would like to thank R. Howe and M. Thompson for many useful discussions regarding the helioseismic observations and for providing us with the time average of the solar rotation. EC is supported by a PPARC fellowship. RT would like to thank R. Brandenberger and R. Unruh for their hospitality during a visit to the University of British Columbia, where some of this work was done.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000