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OVERVIEW AND RECOMMENDATIONS 2 and communication lines as well as within the Earth and oceans. The upper atmospheric current systems are highly variable and are strongly related to solar-terrestrial disturbances. Power failures and communication disruptions have occurred during intense geomagnetic storms. It also appears that the electromagnetic transients that are produced by lightning and man-made power systems can affect trapped particle populations in the magnetosphere and cause particle precipitation into the upper atmosphere at low geomagnetic latitudes. The practical needs for understanding many of the basic questions about atmospheric electricity were brought into clear focus on November 14, 1969. Thirty-six seconds after lift-off from the NASA Kennedy Space Center, Apollo 12 was struck by lightning, and 16 seconds later it was struck again. The first discharge disconnected all the fuel cells from the spacecraft power busses, and the second caused the inertial platform in the spacecraft guidance system to tumble. Fortunately, the rocket was still under control of the Saturn V guidance system at the time of the strikes; and, as a result, the astronauts, who had never practiced for such a massive electrical disturbance, were able to reset their circuit breakers, reach Earth orbit, realign their inertial platform, and ultimately land on target on the Moon. Although permanent damage to Apollo 12 was minimal, the potential for disaster of this lightning incident called attention to the important unanswered questions regarding lightning and atmospheric electricity. Research in atmospheric electricity traditionally has been divided into several broad areas: (1) ion physics and chemistry, (2) cloud electrification, (3) lightning, (4) fairweather electrical processes, (5) ionospheric and magnetospheric current systems, and (6) telluric current systems. Most of this research has been pursued independently by scientists and engineers in different disciplines such as meteorology, physics, chemistry, and electrical engineering. This study reviews the recent advances that have been made in these independent research areas, examines the interrelations between them, and projects how new knowledge could be applied for benefits to mankind. The study also indicates needs for new research and for the types of coordinated efforts that will provide significant new advances in basic understanding and in applications over the next few decades. It emphasizes a need to consider the interactions between various atmospheric, ionospheric, and telluric current systems that will be necessary to achieve an overall understanding of global electrical phenomena. Lightning Lightning is a large electric discharge that occurs in the atmosphere of the Earth and other planets and can have a total length of tens of kilometers or more. The continental United States receives about 40 million cloud-to-ground (CG) lightning strikes each year; on average, there are probably 50 to 100 discharges each second throughout the world (Chapter 1). Most lightning is produced by thunderclouds, and well over half of all discharges remain within the clouds. Most of our knowledge about the physics of lightning has come from the study of CG discharges. Most CG flashes effectively lower negative charge to ground, however recent evidence shows that positive charge can also be lowered (see Chapter 3 on positive lightning). Cloud-to-ground lightning kills about a hundred people and causes hundreds of millions of dollars in property damage each year in the United States; it is clearly among the nation's most severe weather hazards. Most CG discharges begin within the cloud where there are large concentrations of positive and negative space charge (see Chapter 8). After several tens of milliseconds, the preliminary cloud breakdown initiates an intermittent, highly branched discharge that propagates horizontally and downward and that is called the stepped-leader (Figure 1). When the tip of any branch of the stepped-leader gets close to the ground, the large electric field that is produced near the surface causes one or more upward propa
OVERVIEW AND RECOMMENDATIONS 3 gating discharges to form. When an upward discharge makes contact with the steppedleader, the first return stroke begins. The return stroke is an intense wave of ionization that starts at or just above the ground and that propagates up the leader channel at about one third the speed of light. The return stroke is typically the brightest phase of lightning. The peak currents in these return strokes can reach several hundred thousand amperes; a typical value is about 40,000 A. The peak electric power that is dissipated by a return stroke is on the order of 100 million watts per meter of channel; and the peak channel temperatures approach 30,000 K. A shock wave is produced by the rapid expansion of the hot, high-pressure channel, and this eventually becomes thunder with its own characteristics that depend on the nature of the discharge and the atmospheric environment (see Chapter 4). Figure 1 The top figure shows the stepped-leader channel just before attachment; the bottom figure shows attachment and the development of the return stroke. The estimated time between the two figures is on the order of 0.001 sec. The currents in the return stroke carry the ground potential upward and effectively neutralize most of the leader channel. After a pause of 40 to 80 milliseconds, most CG flashes produce a new leader, the dart leader, that propagates down the previous return-stroke channel and initiates a subsequent return stroke. Most flashes contain two to four return strokes, with each affecting a different volume of cloud charge (see Chapter 8