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PHYSICS OF LIGHTNING 39 CONCLUSION We have seen that new experimental techniques now provide an opportunity to investigate many of the important questions that remain unanswered about the physics of lightning. Among the more important unknowns are the following: How is lightning initiated within a cloud? Can the initiation of lightning be suppressed or controlled? What are the mechanisms of stepped-and dart-leader propagation? What factors control the geometrical development of lightning? What is the physics of the attachment process? What are the currents in return strokes? How rapidly do return-stroke currents change with time? What physical processes control the propagation of return strokes? What is the energy balance of the various lightning processes? What physical phenomena occur during a cloud discharge? What are the characteristics of the currents in cloud discharge processes? What processes generate HF and VHF radio noise in lightning? Recent spacecraft observations have shown that lightning may be present in the atmospheres of Jupiter, Venus, and Saturn, and the upcoming Galileo probe will carry a lightning detector to Jupiter (Lanzerotti et al., 1983). Perhaps a study of lightning in atmospheres that are radically different from that of Earth will help us to better understand lightning on Earth and offer even more challenging questions for future work. References Berger, K., R. B. Anderson, and H. Kroninger (1975). Parameters of lightning flashes, Electra 80 , 23-37 . Brook, M., and T. Ogawa (1977). The cloud discharge, in Lightning, Vol. 1, R. H. Golde, ed., Academic, New York, pp. 191-230 . Djibari, B., J. Hamelin, C. Leteinturier, and J. Fontain (1981). Comparison between experimental measurements of the electromagnetic field emitted by lightning and different theoretical modelsâ Influence of the upward velocity of the return stroke, Proc. Int. Conf. on EMC , March 1981, Zurich. Fieux, R., and P. Hubert (1976). Triggered lightning hazards, Nature 260 , 188 . Fieux, R., C. Gary, and P. Hubert (1975). Artificially triggered lightning above land, Nature 257 , 212-214 . Fieux, R., C. H. Gary, B. P. Hutzler, A. R. Eybert-Berard, P. L. Hubert, A. C. Meesters, P. H. Perroud, J. H. Hamelin, and J. M. Person (1978). Research on artificially triggered lightning in France, IEEE Trans. Power Appar. Syst. PAS-97 , 725-733 . Garbagnati, E., F. Marinoni, and G. P. Lo Pipero (1981). Parameters of lightning currentsâInterpretation of the results obtained in Italy, Proc. 16th Int. Conf. on Lightning Protection, July 1981 , Szeged, Hungary. Guo, C., and E. P. Krider (1982). The optical and radiation field signatures produced by lightning return strokes, J. Geophys. Res. 87 , 8913-8922 . Hayenga, C. O., and J. A. Warwick (1981). Two-dimensional interferometric positions of VHF lightning sources, J. Geophys. Res. 86 , 7451-7462 . Horii, K. (1982). Experiment of artificial lightning triggered with rockets, Mem. 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Planetary lightning and lightning measurements on the Galileo Probe to Jupiter's atmosphere, in Proceeding in Atmospheric Electricity , J. Latham and L. Ruhnke, eds., Deepak Publ. Co., Hampton, Va., pp. 408-410 . Lin, Y. T., M. A. Uman, J. A. Tiller, R. D. Brantley, E. P. Krider, and C. D. Weidman (1979). Characterization of lightning return stroke electric and magnetic fields from simultaneous two-station measurements, J. Geophys. Res. 84 , 6307-6314 . Master, M. J., M. A. Uman, Y. T. Lin, and R. B. Standler (1981). Calculations of lightning return stroke electric and magnetic fields above ground, J. Geophys. Res. 86 , 12127-12132 . Miyachi, I., and K. Horii (1982). Five years' experiences on artificially triggered lightning in Japan, 7th International Conference on Gas Discharges and Applications , London, pp. 468-471 . Newman, M. M., J. R. Stahman, J. D. Robb, E. A. Lewis, S. G. Martin, and S. V. Zinn (1967). Triggered lightning strokes at very close range, J. Geophys. Res. 72 , 4761-4764 . Orville, R. E. (1968). A high-speed time-resolved spectroscopic study of the lightning return stroke, Pts. 1, 2, 3, J. Atmos. Sci. 25 , 827-856 . Proctor, D. E. (1971). A hyperbolic system for obtaining VHF radio pictures of lightning, J. Geophys. Res. 76 , 1478-1489 . Proctor, D. E. (1981). VHF radio pictures of cloud flashes, J. Geophys. Res. 86 , 4041-4071 . Proctor, D. E. (1983). Lightning and precipitation in a small multicellular thunderstorm, J. Geophys. Res. 88 , 5421-5440 . Rustan, P. L., M. A. Uman, D. G. Childers, W. H. Beasley, and C. L. Lennon, (1980). Lightning source locations from VHF radiation data for a flash at Kennedy Space Center, J. Geophys. Res. 85 , 4893-4903 . Salanave, L. E. (1980). Lightning and Its Spectrum , Univ. of Arizona Press, Tucson. Schonland, B. F. J. (1964). The Flight of Thunderbolts , Clarendon Press, Oxford. St. Privat d'Allier Research Group (1982). Eight years of lightning experiments at St. Privat d'Allier, Rev. Gen. 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PHYSICS OF LIGHTNING 40 Taylor, W. L. (1978). A VHF technique for space-time mapping of lightning discharge processes, J. Geophys. Res. 83 , 3575-3583 . Uman, M. A. (1969). Lightning , McGraw-Hill, New York. Uman, M. A, and E. P. Krider (1982). A review of natural lightning: Experimental data and modeling, IEEE Trans. Electromagn. Compat., EMC-24 , 79-112 . Uman, M. A., D. K. McLain, and E. P. Krider (1975). The electromagnetic radiation from a finite antenna, Am. J. Phys. 43 , 33-38 . Uman, M. A., M. J. Master, and E. P. Krider (1982). A comparison of lightning electromagnetic fields with the nuclear electromagnetic pulse in the frequency range 104-107 Hz, IEEE Trans. Electromagn. Compat., EMC-24 , 410-416 . Waldteufel, P., P. Metzger, J. L. Aouley, P. Laroche, and P. Hubert (1980). Triggered lightning strokes originating in clear air, J. Geophys. Res. 83 , 2861-2868 . Warwick, J. W., C. O. Hayenga, and J. W. Brosnahan (1979). Interferometric directions of lightning sources at 34 MHz, J. Geophys. Res. 84 , 2457-2468 . Weidman, C. D., and E. P. Krider (1978). The fine structure of lightning return stroke wave forms, J. Geophys. Res. 83 , 6239-6247 . Weidman, C. D., and E. P. Krider (1980). Submicrosecond risetimes in lightning return-stroke fields, Geophys. Res. Lett. 7 , 955-958 . [See also C. D. Weidman and E. P. Krider (1982), Correction, J. Geophys. Res. 87, 7351.]