VHF radio pictures of lightning flashes to ground

Abstract

We present information obtained from VHF radio pictures of 47 lightning flashes that struck ground 1430 m above mean sea level (msl) to the north of Johannesburg, over a period of 16 years. Radio pictures were obtained using a hyperbolic method, i.e., by taking differences between times at which radio noise from lightning arrived at five widely spaced stations. These data were supplemented by recordings of electric field change. We found that stepped leaders and intracloud streamers emitted pulses and progressed at speeds that averaged 1.6×10 5 ms −1 . Stepped leaders ranged in length from 3 to 13 km. BIL (breakdown‐intermediate‐leader) waveforms of electric field change were caused by stepped leaders whose branched channels followed complicated paths. The electric field changes they recorded in an effective bandwidth of 3.5 kHz could be reproduced faithfully by calculations based on an assumption that the radio sources became charged at the expense of a region near the origin of each leader. Some return strokes radiated trains of noise, called Q noise, whose sources extended at speeds near 10 8 ms −1 . Sources of this Q noise retraced leader channels before extending them at these high speeds. Interstroke processes radiated Q noise whose sources extended at speeds that averaged 8.7×10 7 ms −1 . Most Q sources (98%) were directed vertically, but they pervaded regions that retrogressed at “speeds” that averaged 2.2×10 4 ms −1 in directions away from the starting points of the flashes. This retrogression was directed horizontally, but the vertical Q streamers caused J changes to have positive or negative slopes according to the relative positions of Q sources and the reversal cone. Sixty percent of our sample of ground flashes were endowed with portions that were cloud flashes or were themselves minor components of cloud flashes. A more detailed summary appears at the end of the paper.