1. Hey, J. S., Nature, 190, 1150 (1961).
2. Watkins, C. D., and Sutcliffe, H. K., J. Atmos. Terr. Phys., 27, 309 (1965).
3. Evans, J. V., and Brockelman, R. A., Mass. Inst. Tech., Lincoln Lab. Tech. Rep., 332 (October 1963).
4. Greenhow, J. S., and Watkins, C. D., J. Atmos. Terr. Phys., 26, 539 (1964).
5. Kaiser, T. R., Phil. Mag., Supp., 2, 495 (1953).