National Academies Press: OpenBook

The Earth's Electrical Environment (1986)

Chapter: Tortuosity and the Thunder Signature

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Suggested Citation:"Tortuosity and the Thunder Signature." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
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ACOUSTIC RADIATIONS FROM LIGHTNING 50 wave following the shock front will form and propagate outward faster than predicted by theory. We expect, therefore, that the elevated core temperature associated with sparks and lightning can reasonably produce the shortened wave forms. Figure 4.6 Shock-front overpressure as a function of distance from the spark. The dots represent data obtained with a piezoelectric microphone; the crosses data obtained with a capacitor microphone. The total electric energy per unit length computed from measurement of the spark voltage and current is 5 × 103 J/m. Also shown are theoretical values for cylindrical and spherical shock waves. From Uman et al. (1970). Figure 4.7 Duration of positive part of the shock wave from the long spark. For the same data of Figure 4.6, we see here the length of the positive pressure pulse for the 5 × 103 J/m sparks at various distances. From Uman et al. (1970). The wave shape produced by the shock wave is related to the energy per unit length of the lightning flash; thunder is superposition of many such pulses from the lightning channel; hence, the power spectrum of the thunder, with simplifying assumptions, can be related to the energy per unit length of the channel (Few, 1969). Other properties (tortuosity and attenuation) that influence the spectrum of thunder are discussed later. The assumptions in this theory all affect the thunder spectrum in the same sense; the peak of the theoretical spectrum will occur at higher frequencies than the peak of the real thunder spectrum (Few, 1982). The lightning- channel energy that one estimates from the peak will therefore be an overestimate of the actual lightning-channel energy. Holmes et al. (1971) provided the most complete published thunder spectra to date; these spectra show a lot of variation. Most of the spectra are consistent with the qualitative expectations of thunder produced by multiple-stroke lightning, but a few of them exhibit very-low-frequency (<1 Hz) components that are dominant during portions of the record and appear to be totally inconsistent with the thunder-generation theory from the hot explosive channel. Dessler (1973), Bohannon et al. (1978), and Balachandran (1979) suggested that these lower-frequency components might be electrostatic in origin; Holmes et al. (1971) also considered that this was a possible explanation. Tortuosity and the Thunder Signature With respect to the effects of lightning-channel tortuosity on the thunder signal there is almost unanimous agreement among researchers. Lightning channels are undeniably tortuous and are tortuous apparently on all scales (Few et al., 1970). For convenience in discussing channel tortuosity Few (1969) employed the terms microtortuosity, mesotortuosity, and macrotortuosity relative to the relaxtion radius of the lightning shock wave. For a lightning channel having an internal energy of 105 J/m (see Table 4.1), R c 1/2 m. The microtortuous features smaller than Rc , although optically resolvable, are probably not important to the shock wave as measured at a distance because the high-speed internal waves (3 × 103 m/sec) are capable of rearranging the distribution of internal energy along the channel while the shock remains in the strong-shock regime. At the mesotortuous scale (~ R c ) the outward propagating shock wave decouples from the irregular line source because

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This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

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