Measurement and Comparison of VHF/VLF Radiations of Preliminary Breakdown of Cloud-to-Ground and Intracloud Flashes

The sequence of VLF bipolar radiation pulses and simultaneous VHF activitiesproduced by the preliminary breakdown of both cloud\|to\|ground (CG) flashes and intracloud(IC) flashes are presented. In the case of negative CG flashes, the sequence pulses tend to be of the same polarity as that of the succeeding return stroke, and their separating time average is about 160 μs. The duration of the whole sequence is about 2.2 ms on average. Unlike CG flashes, in the IC flashes from the same thunderstorm the preliminary breakdown is characterized by a positive large bipolar pulses sequence, lasting about 11.5 ms averagely with the pulse spacing time of about 1300 ms. The notable VHF radiation occurs almost at the same time as the onset of the VLF bipolar pulses sequences. The VLF bipolar pulses sequence accompanying by simultaneous VHF activities is the characteristic event during the preliminary breakdown of both CG flashes and IC flashes in Hefei area. During the preliminary breakdown of CG flashes VHF radiation envelope (bandwidth 100 kHz) appears in a quasi\|continuous manner quite different from the intermittent VHF radiation during the initial breakdown of IC flashes. It is interesting to note that although VLF radiation and VHF emission during the initial breakdown are correlated in time, but they do not show an intensity\|correlated tendency. Characteristics of VLF pulses sequences versus return strokes are also presented. The mean ratio between the amplitude of the largest pulse from the sequence and succeeding return stroke amplitude is about 0.43. For 105 cases the mean separation time between the first VLF pulse and the succeeding return strokes is 21 ms, while about 94% cases have their first VLF pulse appear within 40 ms before the onset of the first return stroke.