Detection of in‐cloud lightning with VLF/LF and VHF networks for studies of the initial discharge phase

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Abstract
Lightning initiation and the associated in‐cloud parts of lightning flashes have been studied by comparing thunderstorm data from two independent networks, LINET and SAFIR‐type systems, operating in the VLF/LF and VHF regime, respectively. The two networks respond to radiation pulses with different length scales; an event detected by VLF/LF must be hundreds of meters long. In all 12 storms studied, up to half of the first in‐cloud events detected with the VHF networks were found to be closely time‐correlated with the first VLF/LF signal. Range‐normalized VLF/LF signal amplitudes of the time‐coincident events (TCEs) are comparable to amplitudes of weak cloud‐to‐ground strokes. Without measured preparatory VHF emission activity, initial breakdown in TCEs seems to start directly with a strong discharge step producing signatures in VLF/LF records. The TCE data are consistent with lightning initiation via a runaway breakdown mechanism that extended over hundreds of meters.