Abstract
This study examines the initiation of two intracloud (IC) and two cloud-to-ground (CG) lightning flashes using electric field change (FA) sensors and VHF (LogRF) sensors located at seven sites near Oxford, Mississippi, USA. For each flash the initiating event caused a pulse in the LogRF data and started an Initial E-Change (IEC) in the FA data.
The initiating LogRF pulses had powers < 1 W and durations of similar to 1 mu s. Numerous LogRF pulses occurred during each IEC; these pulses had durations <= 3 mu s.
Fewer FA pulses occurred during each IEC; these pulses had durations of <= 7 mu s. During each IEC, a few of the LogRF pulses were coincident with a FA pulse, and most such pairs of pulses enhanced the IEC; no IEC enhancing events occurred without such a coincident pair.
Each flash had 1 or 2 IEC enhancing events soon after the initiating event and 1 or 2 enhancing events shortly before the first classic initial breakdown (IB) pulse occurred. The point dipole moments and durations of IECs of the two IC flashes were (-520C m, 620 mu s) and (-770C m, 1790 mu s) and for the two CG flashes were (9C m, 124 mu s) and (36C m, 130 mu s).
We speculate that the LogRF events were positive corona streamers, that enhancing events occurred when a new streamer extended a previous streamer path, and that this process during the flash initiation developed a nascent channel needed for the negative breakdown of the IB pulses.