Introduction

There are many individual physical processes in cloud and ground flashes. Each of these processes has associated with it characteristic electric and magnetic fields. Lightning is known to emit significant electromagnetic energy in the frequency range from below 1 Hz to near 300 MHz, with a peak in the frequency spectrum near 5 to 10 kHz for lightning at distances beyond 50 km or so. As noted in Section 1.3, at frequencies higher than that of the spectrum peak, the spectral amplitude varies roughly in inverse proportion to the frequency up to 10 MHz or so and in inverse proportion to the square root of frequency from about 10 MHz to 10 GHz. Further, electromagnetic radiation from lightning is detectable at even higher frequencies, for example, in the microwave, 300 MHz to 300 GHz and, obviously, in visible light, roughly 1014 to 1015 Hz. In addition to electromagnetic radiation, lightning produces the acoustic radiation discussed in Chapter 11. In general, any observable signal from a lightning source can be used to detect and locate the lightning process that produced it. Acoustic locating techniques, including acoustic-signal time of arrival and acoustic ray tracing, are described in Section 11.5 and hence are not discussed further here.