Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
PHYSICS OF LIGHTNING 32 Figure 2.2 Sketch of the luminous processes that form the stepped-leader and the first return stroke in acloud-to- ground lightning flash. is probably on the order of 108 watts per meter of channel (Guo and Krider, 1982), and the peak channel temperature is at least 30,000 K (Orville, 1968). The last few steps of the stepped-leader, the onset of a connecting discharge, and the beginning of a return stroke are illustrated in Figure 2.3. Here, the distance between the object that is about to be struck and the tip of the leader when the connecting discharge begins is called the striking distance (SD) and is an important concept in lightning protection. The distance to the actual junction (J) between the leader and the connecting discharge is often assumed to be about half the striking distance. After a pause of 40 to 80 milliseconds, most cloud-to-ground flashes produce a new leader, the dart leader, which propagates without stepping down the previous return-stroke channel and initiates a subsequent return stroke. Most flashes contain two to four return strokes, and each of these affects a different volume of cloud charge (see also Chapter 8, this volume). Figure 2.4 shows a sketch of a dart leader and the subsequent return stroke. Visually, lightning often appears to flicker because the human eye can just resolve the time intervals between different strokes. In 20 to 40 percent of all cloud-to-ground flashes, the dart leader propagates down just a portion of the previous return-stroke channel and then forges a different path to ground. In these cases, the flash actually strikes the ground in two places, and the channel has a characteristic forked appearance that can be seen in many photographs. (See the left two flashes in Figure 2.1.) IMPORTANT RESULTS OF RECENT RESEARCH Three types of research have recently provided new information about the physics of lightning. These are described briefly, and their importance is indicated. First, we discuss how remote measurements of electric and magnetic fields can be used to infer properties of lightning currents, including some implications of recent measurements. Next, we describe how the sources of radio-frequency (rf) noise can be used to trace the geometrical development of lightning channels as a function of time and to determine other properties of lightning. Finally, we discuss how small rockets can be used to trigger lightning artificially and give some applications of this technique. Time-Domain Fields and Lightning Currents Recently, it has become clear that the electric and magnetic fields that are radiated by different lightning
PHYSICS OF LIGHTNING 33 Figure 2.3 Sketch of the luminous processes that occur during attachment of a lightning stepped-leader to an object on the ground. Figure 2.4 The development of a lightning dart-leader and a return stroke subsequent to the first in a cloud-to- ground lightning flash. processes have different but characteristic signatures that are reproduced from flash to flash. For example, Figure 2.5 shows three of the many impulses that were radiated by a typical cloud-to-ground flash at a distance of about 50 km. These particular signatures were recorded using a broadband antenna system and an oscilloscope that covered all frequencies from about 1 kHz to 2 MHz. Trace (a) shows a cloud impulse that was radiated during the preliminary breakdown; trace (b) shows the waveform that was radiated by the first return stroke; and trace (c) shows a subsequent return stroke. The small pulses that precede the first return stroke in trace (b) were radiated by individual steps of the stepped-leader just before the attachment occurred (see Figure 2.3). The characteristics of these newly measured signatures have been put to use in the detection and location of cloud-to-ground lightning. For example, there are now large networks of magnetic direction-finders that can discriminate between the shapes of the return- stroke fields and other processes and that can provide accurate locations of the ground-strike points (see Chapters 1 and 5, this volume; Krider et al., 1980). In a series of recent papers, Uman and co-workers have developed a theoretical model that describes the shapes of the electric and magnetic fields that are produced by return strokes at various distances (Uman et al., 1975; Master et al., 1981; Uman and Krider, 1982). One particularly important result of this work is the prediction that during the first few microseconds of the stroke, i.e., just after the attachment process has been
PHYSICS OF LIGHTNING 34 completed, the waveform of the distant or radiation field is proportional to the channel current, Figure 2.5 Examples of electric-field impulses that were produced by a cloud-to-ground flash at a distance of about 50 km. Trace (a) was radiated during the preliminary breakdown, trace (b) is due to the first return stroke, and trace (c) is due to a subsequent return stroke. where E is the vertical electric field that is measured at the ground at time t, Âµ0 the permeability of free space, Ï the return stroke velocity, c the speed of light, and D the horizontal distance to the flash. A typical first return stroke will produce a peak field of about 8 V/m at a distance of 100 km (Lin et al., 1979). The stroke velocity near the ground is typically on the order of 108 m/sec (Idone and Orville, 1982). With these values, Eq. (2.1) predicts a peak current of about 40 kA, a value that is in good agreement with currents that have been measured in direct strikes to instrumented towers (Berger et al., 1975; Garbagnati et al., 1981). Recently, Weidman and Krider (1978, 1980) examined the microsecond and submicrosecond structure of return- stroke E-field and field derivative, dE/dt, signatures. These investigators found that a typical first stroke produces an electric field "front" that rises in 2-8 Âµsec to about half of the peak-field amplitude. This front is followed by a fast transition to peak whose mean 10-90 percent rise time is about 90 nsec (see Figure 2.6). Subsequent stroke fields have fast transitions similar to first strokes, but fronts that last only 0.5 to 1 Âµsec and that rise to only about 20 percent of the peak field. This fine structure in the initial return-stroke field is illustrated by the waveform shown in Figure 2.7. Unfortunately, the origin of fronts in return-stroke fields is still not well understood, particularly for first strokes (Weidman and Krider, 1978). If fronts are produced by upward connecting discharges, then these discharges must have lengths in excess of 100 m and peak currents of 10 kA or more. A front may be produced by a slow surge of current in the leader channel prior to the fast transition, but then this surge must contain currents on the order of 10kA or more, and the associated channel length must be at least 1 km. To date, the available optical data are not adequate to determine whether either of these processes (or both) does actually occur. Clearly, more research will be needed before we shall understand the physics of the important striking process. Figure 2.6 Histogram of the 10 to 90 percent rise times of the fast portions of return stroke fields over seawater.
PHYSICS OF LIGHTNING 35 Figure 2.7 The initial portion of a return-stroke field recorded with fast time resolution. The same signal is shown on both traces, and the peaks coincide in time. The rise times of the fast field components that are radiated by return strokes are summarized in Figure 2.6. Note that the mean 10 to 90 percent value is about 90 nsec. These submicrosecond components in the field must be caused by submicrosecond components in the current, but few of the currents that have been measured during direct strikes to instrumented towers show components as fast as 90 nsec (Berger et al., 1975; Garbagnati et al., 1981). It is possible that an upward discharge from a tall tower or the electrical characteristics of the tower itself will reduce the rise time that is measured from the value that would actually be present in a strike to normal terrain. Therefore, new measurements of lightning currents and fields with fast time-resolution will be necessary before we can understand the true current rise times. The actual current rise time is important for the design of lightning protection systems (see Chapter 5, this volume). For example, if a 100-nsec current interacts with a resistive load, the voltage rise time on that load will be 100 nsec. Most of the standard surge waveforms that are used to verify the performance of protectors on power and telecommunications circuits specify that open-circuit voltage should have a rise time of 0.5, 1.2, or 10 Âµsec and that the short-circuit current should have a rise time of 8 or 10 Âµsec (see IEEE Standard 587-80 and FCC Docket 19528, Part 68). These values are substantially slower than those shown in Figure 2.6; therefore, it is probable that the degree of protection that is provided by devices that have been tested to the above standards will not be adequate for direct lightning surges. Measurements of the maximum dE/dts that are radiated by return strokes striking seawater are summarized in Figure 2.8. Here, the average maximum dE/dt is about 33 V/m/Âµsec when the values are range-normalized to 100 km using an inverse distance relation. To Figure 2.8 Histogram of the maximum dE/dt during the initial portion of return-stroke fields. All data have been range-normalized to 100 km using an inverse distance relation.