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NO Copyright 2006 by the American Geophysical Union. This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Atlantic Innovation Fund (AIF), and project CGL2005-06097 of the Spanish MEC. Part of this work was carried out while JFGR was a James Chair Professor at STFX. NO Natural Sciences and Engineering Research Council of Canada (NSERC) NO Atlantic Innovation Fund (AIF) NO Ministerio de Educación y Ciencia (MEC), España DS Docta Complutense RD 7 may 2024