AuthorWEIDMAN, CHARLES DAVID.
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PublisherThe University of Arizona.
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AbstractLightning electric field (E) and electric field derivative (dE/dt) signals have been recorded using sensors with 40 ns and 10 ns response times, respectively. Field propagation between the source and the recording antennas was entirely over salt water, so that distortions due to ground wave propagation were minimal below about 20 MHz. The fast-varying, initial portions of return stroke E fields have 10% to 90% risetimes which average 90 ± 40 ns. Peak dE/dt values range from 7 to 71 V/m/μs, with a mean and standard deviation of 33 ± 14 V/m/μs, when normalized to 100 km using an inverse distance dependence. The shapes of first and subsequent stroke fields are similar, but peak subsequent stroke dE/dts are larger than peak first stroke dE/dts in some flashes. The temporal structure of the fast varying fields produced by leader steps near the ground are very similar to return stroke fields. The mean maximum leader dE/dt, at 100 km, is 27 ± 9 V/m/μs. Large amplitude radiation fields produced by cloud discharge processes tend to be bipolar, with either positive or negative initial polarity and usually have several, fast, unipolar pulses superimposed on the initial half cycle. Cloud discharge fields with positive initial polarity usually precede cloud-to-ground flashes and produce a mean maximum dE/dt of 16 ± 8 V/m/μs. The field derivatives for all processes tend to be large when the amplitude of the associated fast field change is large. Estimates of lightning current derivatives, made using range normalized dE/dt measurements, average 155 ± 70 kA/μs, 135 ± 45 kA/μs, and 80 ± 40 kA/μs, for return strokes, leader steps, and cloud discharges, respectively, and a current wavefront velocity of 1 x 10⁸ m/s. These values are about 10 times larger than the maximum dI/dt recorded in strikes to instrumented towers. Lightning field amplitude spectra have been derived by Fourier analyzing dE/dt waveforms, and the spectral amplitudes decrease as 1/f² or faster with increasing frequency in the interval from about 6 to 20 MHz.
Degree ProgramAtmospheric Sciences