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

International audienceLightning 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