Sub-ionospheric Propagation of Preliminary Breakdown Pulses Preceding Negative Cloud-to Ground Lightning Discharges

We present a detailed analysis of broadband electromagnetic measurements of pulse sequences occurring prior to first return strokes of negative cloud-to-ground lightning flashes. Signals generated by lightning discharges were recorded close to the thunderstorm by a magnetic-field receiver and travelled up to 600 km to three distant electric-field receivers. We investigate propagation of the preliminary breakdown pulses (PBPs) in the Earth-ionosphere waveguide. We found that amplitudes of observed pulses are decreasing approximately inverse proportional to the square of the distance. The attenuation of the PBP amplitudes is roughly the same as the attenuation of the corresponding return stroke pulses. The estimated PBP peak currents reach over 100 kA. They may be therefore able to initiate terrestrial gamma ray flashes. The Finite Difference Time Domain simulation shows that there is a significant contribution of the sky wave signals in the PBP waveforms observed beyond 500 km from their source.