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Very-Low-Frequency Radiation Spectra of Lightning Discharges W Journal of Research of the National Bureau of Sta ndards-D. Radio Propaga tion Vol. 63D, No.2, September- October 1959 Very-Low-Frequency Radiation Spectra of Lightning Discharges w. L. Taylor and A. G. Jean (A pril 9, 1959) Spectral a nalyses are given of t he groundwa ve port ion of 33 sferic waveforms recorded from cloud-to-ground light ning discharges which occu rred at distances ranging between about 150 and 600 kilometers from Boulder, Colo. Frequencies of peak energy li e between 5 and 20 kilocycles p er second, which agree fa vorably wi t h other publi shed results. The average value of energy calculated from the groundwave pulses was found to be 26,600 joules, which is lower than va lu es derived from ot her experiments. Va rious param eters, such as the peak a mplitud e and duratio n of t he first half-cycl e, a re related to the radiated energy of the stroke. 1. Introduction 2. Equipment and Collection of Data Appleton et al. (1 92 6) were probably the first Sta tions at Boulder , Colo .; Salt La ke City, U tah ; workers to utilize the cathode-ray oscilloscope in and P alo Alto, Calif. ; were eq uippecl to record the stud ying the electromagnetic energy emitted from directions of arrival of atmospheric pulses, the lightning discharges. Following the developmen t pulse waveforms, and timing m arks from which the of radio communication techniques, it became times of arrival of individual pulses could be deter­ evid ent that the lightning discharge was the source mined. Atmospheric waveforms used in the spectral of interference to radio communication circuits . analyses were recorded from ver tical antennas. The In recent ~T ea r s, the radio signals emitted from overall amplitude response of the ver tical an tenna ligh tning disc harges, called atmospherics, or sferics, channel was constant within ± 1 db over the band­ have been utilized as a source of signals in propaga­ pass and sloped to 3-db cutoff points at frequencies tion studies at very-low radiofrequencies . of 1 and 100 kc. The phase response of this channel Various workers in England h ave been particularly closely approximated a linear function of frequency active in the u tili zation of sferics in propagation wi thin t he bandpass. studies, and it would be difFi ('.ul t and exhaustive A pair of vertical electrostatically shiclded loop to properly credi t all of tht lr accomplishments. an tennas, arranged at right angles to each other, However, as the work continued, it was learned were used in a direction-finding system a t each tha t some atmospheric waveforms could be in ter­ station. The overall bandpass of each loop-an tenna preted in te rms of the recep tion of a seri es of pulse channel extended from 1 to 100 kc, and closely ap­ due to successive refl ections between the ea r th proximated t hat of the vertical-an tenna channel. and th.e ionosphere. I t became eviden t that the The direction-Ending indications were ob tained charaeter of the atmospheric waveform observed using the en tire bandwidth of the loop-an tenna at great distances was materially altered by the channels. It was determined in earlier experi­ propagation eff ec ts, while at shorter ranges, the men ts [2] with wideb and direc tion finders, that in waveform was more represen tative of the source many cases the groundwave componen t of the fun ction. atmospheric could be discerned from the Sky wclve At the present time the use of atmospherics in components by virtue of differences in polarization propagation studies at the very-low radiofrequencies and times of arrival of the groundwave and skywave is common. NBS Boulder Laboratories havc es­ pulses.2 A time resolution of approximately 10 tablished a network: of stations [1]1 for this purpose, Msec is afforded by the wideband direction-finder , and the material presented in this paper resulted which is adequate to resolve the groundwave and from t he simultaneous observation of atmospherics skywave pulses. The direction of arrival of the at the Bureau recording sites. atmospheric, as indicated by the ver tically polan zed The obj ective of this paper is to present data groundwave component, should be virtually free on Fourier spectra of the electric fi eld strength of from error due to the presence of horizon tally polar­ signals radiated from return-stroke lightning di s­ ized componen ts in the atmospheric. charges. The radi ation spectra of lightning dis­ Timing marks derived from a secondary frequency charges were determined from observations of the standard, synchronized with the standard timing groundwave portion of the atmospheric. The pre­ emissions from station WvVV, were recorded on the cautions taken in identifying and utilizing ground­ atmosp heric waveform records. Using these timing wave pulses for this purpose arc described in this 2 E xcellent discussions regarding the lo cation of sforic sources through tho usc paper. o[ radio direetion.flllding have appeared in the literature: Ilorner [31. in particu­ lar, has discussed the probable sources of error in dircction.(jnding, in cluding that resulting from the reception of hori ~onta ll y polarized components of iono­ 1 Figures in braCkets indicate the literature references at the end o[ this paper. spheri C waves. 199 ----' marks, the timc of arrival of atmospherics could be the induction field at frequencies near 1 kc. It was determined at each station to an accuracy of about desirable to limit the maximum observation range ± 1 msec. Atmospheric waveforms resulting from in order to minimize the distortion of the ground­ a particular lightning discharge were locat.ed on the wave pulse by propagation and to reduce the inter­ photographic records made at each station by virtue ference caused in the groundwave by the first-hop of their times of reception and directions of arrival. skywave pulse. The sweep of the oscilloscopes used in recording the The waveforms of two sferics, representative of atmospherics was activated by signals having those used in this analysis, are reproduced in figure amplitudes of about 50 mv/m or greater. Signals of la. The range to the source is approximately 165 smaller amplitude did not activate the sweep and km from both sferics 1 and 2. The groundwave therefore were not recorded. This feature provided portion of sf eric 1 is believed to comprise the first some discrimination against atmospherics of rela­ 110 I"sec of the approximate 450-.usec time base. tively low amplitude; such as those of distant origin The earliest evidence of the arrival of the first-hop as well as some local cloud-to-cloud and precursory skywave pulse in sferic 1 occurs at 147 I"sec. Note discharges. To minimize loss of early detail in the that the initial half cycle of the skywave pulse is waveform, a 24··l"sec delay line was used to delay negative, or opposite to the polarity of the first recording the signal un til after the sweep was initi­ half-cycle of the groundwave pulse. This reversal ated. The dynamic range of the recording equipment, in polarity of the initial part of the first-hop skywave in terms of the strength of a vertically polarized wave pulses evident in figures la and Ib is a consequence incident at the monopole antenna, extended from 0.2 of the pseudo Brewster angle in ionospheric reflec­ to 6 vim. Most of the analyzed atmospherics had tion. This phenomenon was discussed in an earlier peak field amplitudes approaching 1 vim. paper [1]. The locations of lightning discharges were deter­ The height of the ionospheric reflection was mined by triangulation, using the direction of arrival calculated to be about 65 km, corresponding to a indicated at each station. Due to the geometric range of 165 km and a 147-l"sec time interval between arrangement of the 3-station network, the locations the reception of the groundwave and the first-hop sky­ of lightning discharges which occurred near a record­ wave pulses. In this calculation, the ionosphere ing station are relatively inaccurate; however, for was assumed to be a homogeneous ionized medium distances greater than about 150 km, the error is with a sharp lower boundary [4]. estimated to be equal or less than 10 percent of the Note that sferic 2 is similar in character to sferic range. 1, but the groundwave is of larger amplitude and It was desired to identify waveforms on the photo­ longer duration. The first-hop skywave pulse was graphic records that res ulted from vertical lightning received before the entire groundwave pulse was discharges. Two tests that were used in an attempt recorded. It was necessary in such cases to complete to achieve this selection are described below. The the groundwave pulse as indicated by the dotted line relative amplitude of the vertical component of the before undertaking the spectral calculations. Since a electric field which would be registered at the appro­ very small fraction of the total area enclosed by the priate ranges from a vertical discharge was calculated waveform is contained under the sketched-in por­ using range information available from the direction­ tion, the error introduced into the spectral calcula­ finding "fixes," and from groundwave propagation tions is small. curves in the literature [15]. The source was con­ Waveforms of sferics 17 and 18/ which were re­ sidered to be vertically polarized if the relative ceived at ranges of 460 km under nighttime condi­ values of the observed and calculated pulse ampli­ tions, are given in figure lb.
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