Subsequent Strokes

Most lightning flashes are composed of multiple strokes. All strokes other than the "first" are referred to as "subsequent" strokes. A subsequent stroke is initiated by a downward leader (mostly dart leader) following the channel of a previous discharge after a typical inter-stroke interval of 35 ms. The dart leader charge is deposited again along the lightning channel and when the dart leader reaches the ground a subsequent return stroke occurs. This dart leader/return stroke sequence can be repeated several times within about one second total duration of a lightning flash. A lightning flash with up to 20 and more strokes can exceed a total duration of more than one second. Flashes with a large number of subsequent strokes typically show a significant "flickering" of the lightning channel also seen with the naked eye.

A typical flash has three to four subsequent strokes. In Austria, about 50 % of the ALDIS detected flashes are multi-stroke flashes. This shows that multi-stroke discharges are common and not exceptional.

The impulsive component of the current in a return stroke is often followed by a "continuing current" which has a magnitude of tens to hundreds of amperes and a duration up to hundreds of milliseconds. Continuing currents usually occur in subsequent strokes. Between 30 % and 50 % of all negative cloud-to-ground flashes contain long continuing currents. The high amount of transferred charge during the continuing current phase is assumed to be the main cause of forest fire initiation and perforation of metal sheets due to lightning.

A possible dependence of lightning parameters on geographical location has been pointed out for many years, in particular the peak current of first strokes, however, no conclusive evidence has been reported in literature until now.

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