Compact intracloud lightning discharges: 1. Mechanism of electromagnetic radiation and modeling

On the basis of experimental evidence of multiple reflections and modeling, we infer that, from the electromagnetic point of view, the so‐called compact intracloud lightning discharge (CID) is essentially a bouncing‐wave phenomenon. The shortest radiating‐channel length appears to be about 100 m. Reflections, occurring at CID channel ends, influence the magnitude of the overall CID electric field waveform and are responsible for its fine structure, as well as, by inference, for “noisiness” of dE/dt waveforms and for accompanying HF‐VHF bursts. From modeling the CID as a wave traveling on an elevated vertical transmission line and comparing model‐predicted electric fields with measurements, we estimated that the effective current reflection coefficients at channel ends (additionally accounting for current attenuation along the channel) should be in the range of 0 to −0.5, that the wave propagation speed ranges from 0.3 to 3 × 108 m/s, and the channel length is less than 1000 m. In these calculations, we assumed that the current wave had a zero‐to‐peak risetime of 6 μs and a total duration of 30 μs. Influence of current risetime on field waveforms was also examined, and it was found to be typically in the range from about 2 to 8.5 μs.