This work was supported by NSF Grant ATM-9812078 and NASA Grant NAG5-8361. Operation of the Northern Hemisphere SuperDARN radars is supported by the national funding agencies of the U.S., Canada, the U.K., and France. We thank D. G. Sibeck, B. U. Ö. Sonnerup, and S. W. H. Cowley for valuable discussions. We gratefully acknowledge the CDAWeb for access to the GEOTAIL, WIND, and IMP8 data. The CANOPUS array is maintained and operated by the Canadian Space Agency.