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PHYSICS OF LIGHTNING 37 interferometer observations of lightning have also provided interesting results (Warwick et al., 1979; Hayenga and Warwick, 1981), and perhaps in the future interferometric methods will be developed to the point that they can provide unambiguous three-dimensional reconstructions of the discharge processes. Figure 2.9 Geometrical reconstructions of six successive lightning discharges in South Africa (Proctor, 1983). The top panel shows a plane view of the channels from above, and the bottom panel shows an elevation view that is a projection of the same channels on a vertical plane parallel to the x axis. Artificial Triggering of Lightning The last development that we describe is the artificial triggering of lightning by small rockets. This technique is particularly important because it provides, for the first time, the capability of studying both the physics of the discharge process and the interactions of lightning with structures and other objects in a partially controlled environment. Although rockets were first used to study atmospheric electricity in the eighteenth century, the first artificial initiation of lightning was clearly demonstrated by Newman et al. (1967). The technique has subsequently been improved by researchers in France (Fieux et al., 1975; Fieux and Hubert, 1976; St. Privat d' Allier Research Group, 1982) and is now being used to investigate a variety of lightning problems in France, Japan, and the United States. When a thunderstorm is overhead and the electrical conditions are favorable, a small rocket is launched and carries a grounded wire aloft. If the rocket is fired when the surface electric field is 3 to 5 kV/m, then about two thirds of all launches will trigger a lightning discharge (Fieux et al., 1978). Most triggers occur when the rocket is at an altitude of only 100 to 300 m, and the first stroke in the flash usually propagates upward into the cloud. The majority of the subsequent strokes follow the first stroke and the wire to ground; but in about one third of the cases, the subsequent strokes actually forge a different path to ground. These latter events are called ''anomalous triggers" (Fieux et al., 1978). The first stroke in a triggered discharge is not like natural lightning, but subsequent strokes appear to be almost identical to their natural counterparts. An example of lightning that was triggered by Hubert and co-workers is shown in Figure 2.10. The upward branching in this photograph was produced by a leader that propagated upward from the wire, and the bright, straight section of channel near the ground shows the path of the wire just before it exploded as a result of the lightning current. Triggered lightning is now being used to investigate the luminous development of lightning channels, the characteristics of lightning currents, the velocities of return strokes, the relation between currents and fields, the mechanisms of lightning damage, the performance of lightning protection systems, and many other problems (Fieux et al., 1978; Hubert and Fieux, 1981; Horii,
PHYSICS OF LIGHTNING 38 1982; Miyachi and Horii, 1982; St. Privat d'Allier Research Group, 1982; Hubert et al., 1984). Figure 2.10 An example of a rocket-triggered lightning flash in New Mexico. Among the more important results to date have been a direct experimental verification of the existence of submicrosecond fields and currents during return strokes and the general validity of Eq. (2.1) (Fieux et al., 1978; Djibari et al., 1981; Hubert and Fieux, 1981). Waldteufel et al. (1980) also reported a curious case in which a triggered discharge originated everywhere in clear air. The main benefit of the rocket triggering technique is that it can be used to cause lightning to strike a known place at a known time, thus enabling controlled experiments to be performed. The triggering wire guides the lightning current to a point where a variety of sensors can measure the physical properties of the discharge and its deleterious effects directly. All cameras and data-recording equipment can be turned on and be fully operational just before the rocket is fired. In most locations, the total number of triggers is limited to a few tens of events per year by the frequency of overhead storms, but the number and quality of the measurements can be made quite high to compensate for the relatively few events.