Broadband Radio Interferometer Observations of Initial Breakdown Pulses

The initial negative leader development of a lightning flash is typically accompanied by initial breakdown pulses (IBPs), which are manifest in electric-field measurements as bipolar waveforms many tens of microseconds long, superimposed by shorter ~1-μs spikes. The underlying IBP mechanism is not fully understood. IBPs often occur deep within thunderclouds, and so cannot be resolved optically. In this study, we investigate the first few milliseconds of several close (<10 km range) intra-cloud (IC) and cloud-to-ground (CG) flashes from multiple Florida thunderstorms using broadband HF/VHF interferometer (INTF) data, and fast and slow electric field measurements. Many instances of IBPs occur during the flashes, with similar characteristics for both IC and CG flashes. The observations reveal that characteristic IBPs are associated with apparent step-wise vertical elongation of the flash, with breakdown appearing to propagate hundreds of meters at speeds on the order of 10⁷ m/s. During the IBPs, the VHF power is also relatively large, and the VHF sources appear to partially propagate through regions of previous activity, suggesting that recurrent breakdown within a volume is common and perhaps required to propagate the negative leader forward. During periods between IBPs, the VHF sources are weaker in power and propagate at a slower speed on the order of 10⁵ m/s, more typical of negative leader propagation.

The INTF was developed by New Mexico Tech and was deployed at Kennedy Space Center (KSC) in 2016 and 2017. It employed three 100-m baselines in 2016, with 14-88 MHz bandwidth, 16-bit resolution, and 180 MSps sample rate. The recording of a fast antenna with 100-μs time constant was synchronously-digitized with the INTF data. Three slow antennas were also operated at KSC with a 10-second time constant at 50 kSps sample rate.