The lightning HF radiation at 3 MHz during leader and return stroke processes

Simultaneous recordings of the broadband electric field and HF radiation at 3 MHz were obtained at times before and after the onset of first and subsequent lightning strokes. Data are presented for several hundred negative ground flashes observed in Sri Lanka within a range of 40 km over the land and sea. The stepped leader gave rise to strong 3 MHz radiation, but the peak amplitude of the radiation was less than that of the return stroke. In the return stroke phase, 3 MHz radiation was strongest at the beginning of the first return stroke and gradually decayed completely. The mean duration of the 3 MHz continua of 346 first strokes was 190 μs ( S. D. = 69 μs). In about 99% of the cases 3 MHz radiation in the return stroke phase was accompanied by a burst of multitudinous, fine oscillatory pulses on the broadband electric field. Subsequent strokes, in general, had no 3 MHz radiation in their return stroke phases.