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 2 3 return-stroke streaked images, only 11 were accompanied by leader st reaked images, while all 23 leaders were identified in corresponding e lectric field records. Thus, 12 subsequent leaders (one of which creat ed a new channel to ground) failed to produce luminosity above the day light background level. The brightest three dart-leader/return-stroke sequences from two flashes have been examined for relative light inten sity as a function of time and height. Dart-leader light waveforms app ear 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 (pl ateau). The plateau continues until it is overridden by the return-str oke light waveform, suggesting that a steady leader current flows thro ugh any channel section behind the downward moving leader tip before t he 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 a nd 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 suffe r appreciable degradation during the upward propagation of the return- stroke front, the dart-leader light pulses preserve their shape, and t he 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-p ulse risetimes and propagation speed to be of the order of 10 m, shoul d be at least an order of magnitude greater than the average electric field intensity across the return-stroke front, whose length is inferr ed to be of the order of 100 m.