A Study of Lightning Initiation Based on VHF Radiation

Abstract

ABSTRACT We analyzed the characteristics of the VHF (63 MHz) radiation and correlated wideband electric field records for 113 cloud-to-ground (CG) flashes and 33 intracloud (IC) flashes in five different storms in South Florida during the summer of 1981. Each of the stepped leaders in the CG flashes was preceded by active VHF radiation with an average duration of 34 msec. In 28% of the CG flashes, preliminary variations in the electric field records were observed which are similar to the B,I,L pattern reported by Clarence and Malan (1957). In these cases, the VHF radiation started only a few milliseconds prior to the preliminary electric field change. The VHF pulse repetition rate increased from the beginning of the preliminary breakdown to a few milliseconds prior to the return stroke at which time it decreased. There was a variation in the VHF pulse rate at about the time of stepped leader initiation determined by the first steady electric field change. The VHF radiation started at the same time as the electric field change in all the IC flashes. The uniqueness of the combined patterns in the VHF and electric field records suggests that a system can be designed to discriminate between CG and IC flashes tens of milliseconds prior to the occurrence of a potential return stroke. For 25 of the flashes displayed with l-µsec resolution, the ground-based electric field and the 2 MHz envelope of VHF radiation were correlated with the electromagnetic field with a bandwidth of 20-MHz recorded at an altitude of about 4 km in a WC-130 aircraft. These records showed that the VHF radiation observed at the ground correlates much better with the wider bandwidth field than the lower frequency electric field.