Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud‐to‐ground lightning discharges

Abstract

We analyze 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 traveled up to 600 km to three distant electric field receivers. We found that amplitudes of observed preliminary breakdown pulses, as well as amplitudes of the corresponding return strokes, are attenuated approximately by 2 dB/100 km when propagating in the Earth‐ionosphere waveguide over mountainous terrain. Propagation simulations show that there is a significant contribution of the sky wave signals in the waveforms observed beyond 500 km from their source. The estimated peak currents of the largest preliminary breakdown pulses reach over 60 kA. Such current pulses propagating through in‐cloud lightning leader channels in a strong electric field may be able to initiate terrestrial gamma ray flashes.

Studying Sequences of Initial Breakdown Pulses in Cloud‐to‐Ground Lightning Flashes

“…Tables show the overall ranges of the IBP parameters for the three flash initiations studied. The IBP peak current range of 0.7 to 165 kA is similar to the 3–100 kA range reported by Betz et al (), the 24–154 kA of Karunarathne et al (), the 9–64 kA of Kolmašová et al (), and the 20–189 kA of Kašpar et al (). Our range of current propagation velocities, total charge, and charge moments also approximately agree with the high‐end values but extend lower values than those found by Karunarathne et al ().…”

Initial breakdown and fast leaders in lightning discharges producing long‐lasting disturbances of the lower ionosphere

“…IB radiation was definitively detected up to a distance of 2630 km, and up to 1375 km for early VLF event causative discharges. We note that IB pulses have been detected by broadband (few hundred hertz to 5 MHz) electric field measurements up to distances of ∼600 km [ Kolmašová et al , , ], and radiation from the IB process has been inferred in ELF/VLF (50 Hz to 30 kHz) magnetic field measurements to distances of at least ∼2200 km [ Frey et al , ]. Our observations seem to confirm the inferences of Frey et al [].…”

“…Among all lightning flashes, IB pulses were rarely observed in VLF (with >8 dB SNR) at distances greater than a few hundred kilometers, as will be discussed in the next section. Based on this criterion, we have classified the IB radiation reported in this paper (observed at distances greater than a few hundred kilometers) as “intense.” Recent experimental and modeling studies indicate that peak currents producing IB pulses may be as large as several tens of kiloamps [ Karunarathne et al , ; da Silva and Pasko , ; Kolmašová et al , ], flowing over channels with lengths on the order of a few hundred meters to a kilometer [ Stolzenburg et al , , ; Karunarathne et al , ; da Silva and Pasko , ; Nag and Rakov , ]. Such large currents have been demonstrated to produce initial breakdown observable at a distance of 600 km [ Kolmašová et al , , ].…”

Initial Breakdown Pulses Accompanied by VHF Pulses During Negative Cloud‐to‐Ground Lightning Flashes

“…IB pulses are thought to be generated by currents flowing in the intermittently extending in‐cloud lightning leaders (Stolzenburg et al, ). The IB pulses can be detected hundreds of kilometers from the parent thunderstorm (Kolmašová et al, ; Kotovsky et al, ) and are one indication of lightning initiation. Another indication of lightning initiation is an initial E‐change (IEC) that occurs immediately before the first IB pulse and can be detected only at short distances (typically within 7 km) from the developing discharge (Chapman et al, ; Marshall, Stolzenburg, et al, ; Marshall et al, ).…”