Acknowledgements: This research was supported by NSF Grant ATM9872064 at the University of Alaska and by NSF Grant ATM-9507333 at Auburn University. The work was also supported in part by a grant of HPC resources from the Arctic Region Supercomputing Center. Mark Conde is responsible for maintaining the web site and photometer database which was used to prepare Figure 17. We also thank R. V. Reddy of the Indian Institute of Geomagnetism for carefully reviewing the code used in the simulations and identifying a troublesome coding bug.