Forty simultaneous, submicrosecond time‐resolved measurements of triggered lightning returnstroke current (I), current derivative (dI/dt), and electric field derivative (dE/dt) were made in Florida and France in 1985 and 1986. Peak currents ranged from about 5 to 50 kA, peak dI/dt amplitudes from 60 to 260 kA/μs in 1985 and from 20 to 140 kA/μs in 1986. The mean peak dI/dt values, 111 kA/μs (1985) and 68 kA/μs (1986), are 2–3 times higher than data from instrumented towers, and peak I and dI/dt appear to be positively correlated. The dE/dt and dI/dt waveform pairs have similar shapes, and the peak amplitudes are linearly proportional. Return‐stroke velocity, computed from the ratio of peak dE/dt and dI/dt signal amplitudes using an expression derived from the radiation field term of the transmission line model (TLM), averaged 2.9×108 m/s and 3.0×108 m/s in 1985 and 1986, respectively, which is about 2 times higher than most optical measurements. The TLM velocity may be erroneous because (1) the dE/dt measurement was made only 50 m from the lightning channel, where fields other than the radiation field component, that is near fields, may contribute to the total dE/dt and (2) fine structure on the measured E fields was not consistent with a single upwardly propagating return‐stroke current wave assumed by the TLM (two waves are more consistent).
Average energy spectral densities are presented for the fast transitions in most of the components that produce large radiation field impulses from cloud-to-ground lightning: first and subsequent return strokes; stepped, dart-stepped, and "chaotic" leaders; and "characteristic" cloud pulses. A disagreement in the previous literature about the spectral energy radiated by return strokes at high frequencies is noted and explained. We show that our spectral amplitudes are not seriously distorted by propagation over less than 35 km of seawater, although as much as 45 km of such propagation does appear to produce significant attenuation above about 10 MHz. First and subsequent return strokes produce identical spectra between 0.2 and 20 MHz. The spectra of stepped and dart-stepped leader steps are nearly identical and are very similar to that of characteristic pulses. The spectra of leader steps also match return stroke spectra above 2-3 MHz after the former are increased by about 7 dB. The shapes of individual spectra do not depend on their amplitude, so the shapes of the average spectra are probably not distorted by the trigger thresholds used in the data acquisition. Return strokes are the strongest sources of radiation from cloud-to-ground lightning in the 0.2-to 20-MHz frequency range, although certain intracloud processes are stronger radiators above about 8 MHz. 20,367 20,368 WILLETT ET AL.: LIGHTNING ELECTROMAGNETIC HF SPECTRA been emphasized in the appendix of Nanevicz et al. [ 1987]. It is also likely that there are lightning processes, as yet unstudied by the waveform analysis technique, which radiate strongly enough to help resolve the disagreement. This last probability has been supported by Wi!lett et al. [1989], who demonstrated the existence of waveforms containing unusually large HF energy.In this paper we give some new Fourier transforms of wideband, time domain electric fields (E) recorded in such a way that several different events in each lightning flash could be captured. This method of recording produces less bias toward the first large event in the flash and thus yields a larger sample of a wider variety of lightning processes than available to previous investigators. The results are reliable, composite spectral amplitudes for a number of different processes in cloud-to-ground (CG) lightning over the frequency interval from 0.2 to 20 MHz. EXPERIMENT During the summers of 1985 and 1987, lightning E and dE/dt signals were recorded using a wideband, digital, data acquisition system at a site about 45 m from the Atlantic Ocean at the NASA Kennedy Space Center (KSC), Florida. The electronics were housed inside a grounded metal trailer that had several flat-plate antennae installed on the roof. This station was capable of recording E, dE/dt, and HF radiation from multiple impulsive events (return strokes, leader steps, cloud pulses, etc.) within individual lightning flashes, with submicrosecond time resolution, and of placing these events in the context of the flash as a whole. The equipment and calibr...
The “transmission‐line” model of return‐stroke radiation, proposed by Uman and McLain (1970) and invoked frequently thereafter to deduce peak currents from remote fields or to estimate propagation velocities from measured fields and currents, has never received a thorough experimental test. During the summer of 1985 at the Kennedy Space Center in Florida, we were able to measure peak currents (with a coaxial shunt), two‐dimensional average propagation speeds (with a high‐speed streak camera), and electric field waveforms (at 5.15‐km range) for a number of subsequent return strokes in rocket‐triggered lightning flashes. Because of the temporal ambiguity on the streak‐camera films, it has not been possible to identify individual velocity measurements with particular strokes for which current and field data are available. Three multistroke flashes, however, each yielded a tight cluster of velocity measurements and a group of peak field to peak current ratios, though not necessarily for the same strokes. A further six flashes provided more current and field measurements for which no velocity information was obtained, and velocity measurements only are available for still other flashes. It is shown that these data indicate reasonable agreement between the propagation speeds measured with the streak camera and those deduced from the transmission‐line model. The previously observed difference between current and radiation‐field waveforms suggests a modification of the model, involving two wave fronts traveling upward and downward away from a junction point a short distance above the ground, which substantially improves the agreement between measured and inferred propagation speeds.
An experiment to measure the electric field E and dE/dr signatures that are radiated by the first return stroke in cloud-to-ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during the summer of 1984. At this site, there was minimal distortion in the fields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases that are introduced by a finite threshold in the triggered recording system were kept to a minimum by triggering this system on the output of a wideband RF receiver tuned to 5 MHz. Values of the peak dE/dr during the initial onset of 63 first strokes were found to be normally distributed with a mean and standard deviation of 39 _+ 11 V m-• /xs -• after they were normalized to a range of 100 km using an inverse distance relation. Values of the full width at half maximum (FWHM) of the initial half-cycle of dE/dr in 61 first strokes had a mean and standard deviation of 100 +_ 20 ns and were approximately Gaussian. When these results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawater, the average value of the maximum current derivative, (dI/dt)p, and its standard deviation are inferred to be 115 + 32 kA/•s -•, with a systematic uncertainty of about 30%. The FWHM after correction for propagation is about 75 +_ 15 ns. The inferred values of (dI/dt)p are significantly higher than most previous measurements of natural first strokes during direct strikes to instrumented towers but are in good agreement with direct measurements of dI/dt during subsequent return strokes in rocket-triggered discharges in Florida. 1589 1590 KRIDER ET AL.: SUBMICROSECOND FIELDS RADIATED BY LIGHTNING ET AL.' SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1593 80-70-E 50ß -4.0-'o 313-20-10-0 25 ß ß ß ß ß ß ßß ß ß ß i 315 i i 30 40 45 50 Range (kin) Ip 12 P KRIDER ET AL.' SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1595 KRIDER ET AL.: SUBMICROSECOND FIELDS RADIATED BY LIGHTNING 1597
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