Initial Breakdown Pulses Accompanied by VHF Pulses During Negative Cloud-to-Groud Lightning Flashes

This study compares waveforms recorded by "broadband" very low frequency/low-frequency/ medium-frequency (VLF/LF/MF) electric field change sensors (bandwidth -0-2.5 MHz) and very high frequency (VHF) sensors (bandwidth 186-192 MHz) during the initiation of 20 negative cloud-to-ground lightning flashes. In the first 2 ms of each flash, initial breakdown (IB) pulses are detected with the VLF/LF/ MF sensors. Comparison shows that all classical IB pulses are accompanied by VHF pulses, where classical IB pulses are defined herein as bipolar with duration >10 mu s and amplitude >25% of the largest IB pulse amplitude in the flash. There are on average 47% of IB pulses (of all amplitudes and durations) that are accompanied by VHF pulses within +1 mu s. There are also many VHF pulses with no associated IB pulses. These observations indicate that the initial in-cloud lightning channel extension process (es) occurs very fast and at multiple length scales, since substantial electromagnetic radiation is emitted in the VLF/LF/MF and VHF bands. Plain Language Summary There is a lack of understanding of how a lightning flash initiates, as this process usually takes place deep inside thunderclouds. Electromagnetic pulses emitted during lightning initiation, which can be measured from a safe distance, help us to understand lightning better. We use arrays of low-frequency (similar to 0-2.5 MHz) and very high frequency (186-192 MHz) receivers and compare their recordings registered during initiation of 20 cloud-to-ground lightning flashes. We found that the larger pulses detected during lightning initiation in low-frequency records were systematically accompanied by pulses detected in the very high frequency records. This observation indicates that the initial lightning extension process occurs very fast and at multiple length scales and that emitted electromagnetic radiation covers a very large range of frequencies.