CORRELATED HIGH-SPEED VIDEO AND RADIO INTERFEROMETRIC OBSERVATIONS OFA CLOUD-TO-GROUND LIGHTNING FLASH

A six-stroke cloud-to-ground lightning flash has been studied using ob servations from a high-speed video camera (1000 frames s(-1)) and a VH F radio interferometer (1-mu s time resolution), as well as additional electric, magnetic, and optical measurements. The flash produced stro kes along two channels to ground and a long (550 ms) continuing curren t. The video observations provided time-resolved pictures of stepped a nd dart leaders, short and long continuing currents! and M components during the continuing currents,and complemented and confirmed the inte rferometer observations of flash structure and development. The M comp onents were initiated by fast negative streamers inside the cloud whic h 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 continuin g 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 e vent occurrences indicated that the conducting channels of the continu ing current both expanded and contracted with time. An M-type event wa s 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-step ped leaders (such as occurred during the third stroke) are similarly e xplained. Because the stranded charge is observed to be greatest for i nitial strokes, new channels to ground of stepped and dart-stepped lea ders are expected to follow initial strokes, as is usually observed. T he video and electric field observations indicate that all return stro kes have at least a short continuing current, of the order of one or a few milliseconds. The results also reinforce the well-known observati on that long continuing currents tend to follow relatively weak return strokes.