Luminosity characteristics of dart leaders and return strokes in natural lightning

Streak-camera photographs were obtained in daylight for 23 subsequent strokes in five Florida negative cloud-to-ground flashes. Out of the 23 return-stroke streaked images, only 11 were accompanied by leader streaked images, while all 23 leaders were identified in corresponding electric field records. Thus, 12 subsequent leaders (one of which created a new channel to ground) failed to produce luminosity above the daylight background level. The brightest three dart-leader/return-stroke sequences from two flashes have been examined for relative light intensity as a function of time and height. Dart-leader light waveforms appear as sharp pulses with 20-to-80 % risetimes of about 0.5-1 mu s and widths of 2-6 mu s followed by a more or less constant light level (plateau). The plateau continues until it is overridden by the return-stroke light waveform, suggesting that a steady leader current flows through any channel section behind the downward moving leader tip before the return-stroke front has passed that channel section. Return-stroke light pulses near ground have 20-to-80 % risetimes of about 1-2 mu s and amplitudes a factor of 2 to 3 greater than those of the dart-leader light pulses. As opposed to the return-stroke light pulses that suffer appreciable degradation during the upward propagation of the return-stroke front, the dart-leader light pulses preserve their shape, and the pulse amplitude is either more or less constant or increases as the leader approaches ground. The average electric field intensity across the dart-leader front, whose length is inferred from measured light-pulse risetimes and propagation speed to be of the order of 10 m, should be at least an order of magnitude greater than the average electric field intensity across the return-stroke front, whose length is inferred to be of the order of 100 m.