Correlated high-speed video and radio interferometric observations of a cloud-to-ground lightning flash

A six-stroke cloud-to-ground lightning flash has been studied using observations from a high-speed video camera (1000 frames s(-1)) and a VHF radio interferometer (1-mu s time resolution), as well as additional electric, magnetic, and optical measurements. The flash produced strokes along two channels to ground and a long (550 ms) continuing current. The video observations provided time-resolved pictures of stepped and dart leaders, short and long continuing currents! and M components during the continuing currents,and complemented and confirmed the interferometer observations of flash structure and development. The M components were initiated by fast negative streamers inside the cloud which propagated into the conducting channel of the continuing current and subsequently brightened the channel to ground. We call this sequence an M event. Dart leaders and the fast streamers of M and K events were found to be significantly brighter than stepped leaders and continuing currents to ground. A number of streamers did not initiate M events that were identical to in-cloud K streamers. Analysis of the M and K event occurrences indicated that the conducting channels of the continuing current both expanded and contracted with time. An M-type event was also: observed during a dart leader. It is proposed that the channel multiplicity within the flash resulted from cutoff of the channel to ground while charge continued to flow down the channel from the stroke source region, stranding negative charge along the channel. Dart-stepped leaders (such as occurred during the third stroke) are similarly explained. Because the stranded charge is observed to be greatest for initial strokes, new channels to ground of stepped and dart-stepped leaders are expected to follow initial strokes, as is usually observed. The video and electric field observations indicate that all return strokes have at least a short continuing current, of the order of one or a few milliseconds. The results also reinforce the well-known observation that long continuing currents tend to follow relatively weak return strokes.