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About 200 excellent photographs of cloud-to-ground discharges, taken with a newly designed rotating-film camera, were obtained from very active thunderstorms in 1959 and 1960. The electric-field changes and luminosity variations of the photographed discharges were recorded simultaneously on two oscilloscopes having different time resolution. Fifty per cent of the multiple-stroke flashes, constituting about 90 per cent of cloud-to-ground flashes, are found to involve at least one stroke which is followed by very long continuing luminosity lasting for 40 to 500 msec. This continuing luminosity contains a number of relatively brighter components essentially the same as the M components which are known to follow some strokes having very short time intervals. The analysis of the electric field associated with the continuing luminosity reveals that, during the luminous period, negative charge from the cloud flows to ground continuously. The surges of current (M components) during the continuing luminosity are associated with the small rapid field changes (K changes), and the time interval between M components is statistically the same as that between the K changes generally observed during the interstroke and final period of cloud-to-ground flashes. During the period of continuing luminosity, the lightning channel maintains a level of conductivity which is high enough to support a momentary current increase without involving the leader process. After the channel loses its luminosity, it maintains a lower level of conductivity for 7 to 100 msec, and a subsequent stroke during this period can also follow the same channel. When a longer time elapses (>100 msec), a subsequent stroke, if any, takes a different channel with a new stepped leader.
Kitagawa et al. (Thu,) studied this question.