GEOPHYSICAL RESEARCH LETTERS, VOL. 8, NOo 8, PAGES 931-934, AUGUST 1981 LIGHTNING AMPLITUDE SPECTRA IN THE INTERVAL FROM 100kHz TO 20 MHz C. D. Weidman and E. P. Krider Institute ofAtmospheric Physics, TheUniversity ofArizona, Tucson, Arizona 85721 M. A. Uman Department ofElectrical Engineering, University ofFlorida, Gainesville, Florida 32611 Abstract. The electricradiation fields produced by lightning returnstrokes, stepped leaders, and intracloud discharge proc- esses have been Fourier-analyzed to determine amplitude spectra forthese processes from about 100 kHz to20MHz. The fields were recorded under conditions where the lightning loca- tions were known andwherethe fieldpropagation from the lightning sources to therecording site was entirely over salt water.The spectra forreturnstrokes show anf- • frequency dependence from 100 kHz to2 MHz, an f-•'dependence between 2 and10MHz, andanf-s decrease above 10MHz. In the I to 20MHz range, the spectra oftheinitial fast transition in return strokes, theinitial fast-rising portion ofleader steps and thefast transitions in positive intracloud pulses aresurprisingly similar. Introduction The excitation of electromagnetic signals in conducting struc- tures such as aircraft, missiles, ships, and buildings ismost effi- cient at frequencies which correspond to thenatural resonances of these structures [e.g., Hess,1978; Taylor,1978]. These reso- nant oscillations, when coupled totheinterior ofthestructure via, forexample, apertures, can upset orpermanently damage sensi- tive solid-state electronic systems [e.g., Corbin, 1979]. Lightning discharges area principal cause ofsuch deleterious excitations and hence, froma practical point of view, it is important to un- derstand in detail the amplitude spectrum of lightning. Since many structures ofinterest have physical dimensions on the order of 10to 100 m, it is frequencies in the range fromabout 1.5 to 15MHz that are ofprimary concern, that is, frequencies for which half of the associated wavelength equals the structure dimension. Published measurements of lightning amplitude spectra are limited primarily to (1) thereturn-stroke phase and then only for frequencies below about 300 kHz [e.g., Taylor, 1963; Serhan et al., 1980] and (2) spectra assembled fromthe outputs ofnarrow- band receivers [e.g., Kimpara, 1965; Oh, 1969; Oetzel and Pierce, 1969; Pierce, 1977]. Thenarrow-band measurements have not,in general, been used to determine thespectra ofspecific discharge processes. Further, most previous narrow and wide-band deter- minations ofspectra arefordischarges which occurred at dis- tances of 10km or more over land. At theseranges,frequencies above about 100 kHz can be strongly attenuated by a finitely conducting earth[Johler et al., 1969; Wait and Walters, 1965; Uman et al., 1976; Serhanet al., 1980]. In this paper, wepresent Fourier transforms oflightning elec- tricfield signatures which cover thefrequency interval from about100 kHz to 20 MHz. The waveforms were produced by returnstrokes, individual steps ofthe stepped leader, and large amplitude intracloud discharge processes. Tothebest ofour knowledge, these arethefirstaccurate determinations ofthe frequency spectra forreturn strokes in theinterval from about 300 kHz to 20MHz, and thefirst spectra for stepped-leader and intracloud-discharge processes in anyinterval. Measurements The instruments andantennas used to detectandrecord electric-field, E, and field-derivative, dE/dt, signatures and the Copyright 1981 by the AmericanGeophysical Union. Paper number 1L1077. 0094-8276/81/001L-1077501.00 931 calibrations ofthese systems have been described previously by Krider et al. [1977] andby Weidman andKrider [1980]. The dE/dt signals werephotographed directly ona Tektronix 7834 storage oscilloscope whenever theyexceeded a preset positive threshold. E fields were recorded usinga Biomation1010 waveform digitizer that was operated in thepretrigger recording V/m NANOSECONDS 0 500 V/m//us ' ' ' ' ' ' ioo f "'•" 0 '1 -I00 0 8 16 24 52 (c) V/m ,oo[ 50 0 0 500 V/m/•us ' ' ' ' ' ' 0 ,• -I00 (b) I I I I I 0 8 16 i I i I I I V/m 20 0 0 500 V/m//us' ' ' ' ' ' -I00 (a) I I I I I I I I I I I o 8 I• • $• •o MICROSECONDS Fig. 1. Examples of electric field, E, andfieldderivative, dE/dt, waveforms used to deriveamplitude spectra. A positive intra- cloud pulse is shown in (a), a stepped-leader impulse in (b), and a Errst return strokein (c). The bottomtrace in eachexample is E, andthe top traceis the dE/dt signature beginning at the time of the arrow below E.