The work was supported by the UK NERC COAPEC program (NER/T/S/2000/00305) and an HEFCE JREI award in 1999 for a supercomputing cluster (NESSC). We are grateful to Isaac Held for an insightful review, which strengthened this study. We thank Vassil Roussenov for assistance with the modeling studies and Richard Greatbatch, Chris Hughes, and David Marshall for useful discussions.
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