The temporal structure of HF and VHF radiations during Florida lightning return strokes

Measurements have been made of the HF and VHF radiation at 3, 30, 139, and 295 MHz produced during lightning return strokes, in correlation with wideband electric field measurements covering from about 300 Hz to 2 MHz. Intense pulses of RF are observed during the first return stroke in a discharge to ground, but with a 10-30 µsec time delay between the beginning of the stroke and the occurrence of significant RF radiation. The delay occurs at all frequencies and is short enough to suggest that the main channel and/or branches are the main initial sources of RF during the first stroke. During a subsequent return stroke, the RF starts an average of 265 µsec prior to the onset of the stroke. At 139 and 295 MHz, the subsequent stroke RF lasts only about 115 µsec; however, at 3 MHz, the radiation frequently persists up to and during the stroke.