A relatively slow-moving, nonluminous lightning
streamer, the existence of which has been postulated but not verified, to provide a physical explanation for the observed intermittent mode of advance of a stepped leader
as it initiates a cloud-to-ground lightning discharge
Whereas the stepped leader descends at an average speed of the order of 105
during its downward motion, it advances only about 50 m at a time with a speed greater than average and then pauses for 50–100 μs before resuming its downward movement. The average downward speed has been associated with an invisible streamer
, the pilot streamer, which is postulated to descend at a uniform speed only slightly in excess of the ionizing speed of electrons
in air and lay down a trail of weak residual ionization along which the stepped leader moves very rapidly in a pulsating manner. The idea of a pilot leader
has been supplanted by more modern theory based on studies of sparks over long distances in the laboratory.
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