The mechanisms of stepped and dart leaders in cloud-to-ground lightning strokes

Abstract

Recent observations indicate the rather complete neutralization of volume space charges by induced positive corona streamers. New photographic observations of high‐tension power lines in rain and snow storms reveal strong and heavily branched corona streamers from such hydrometeors. A study of uniform field streamer breakdown in midgap in clean gases, just published, has provided additional information essential to the delineation of the processes active in the forging of the channel of the cloud‐to‐ground stroke. On the basis of these findings, consistent sequential qualitative analysis is presented resolving the following moot questions: 1. How is the negative charge stored on raindrops in cubic kilometers of cloud volume drained and channeled to yield a pilot leader channel some meters in diameter? 2. How is this hydrometeor‐supported discharge able to leave the cloud base and forge into clean, initially un‐ionized air to give a stepped leader? 3. What causes stepping? 4. How is the channel several kilometers in length maintained conducting for the tens of milliseconds needed for it to reach ground? 5. How is it that the stepped leader of some meters diameter leads to a return stroke channel with the most intense luminosity millimeters in diameter and the larger luminous conducting envelope no more than 4 cm in diameter? 6. Why is the pilot leader channel zigzagged? 7. What starts the dart leader stroke and how is the channel maintained conducting for the tenths of seconds between successive strokes?

Laboratory investigation of positive streamer discharges from simulated ice hydrometeors

“…These effects are likely to result in a significant enhancement of the electric field near the ice crystal extremities that, in certain cases, may be sufficient to support corona discharges including positive streamers. Loeb (1966) first suggested that hydrometeor-triggered positive streamer systems could effectively 'funnel' space charge, concentrating it at the origin of the positive streamer system such that the resultant local electric-field enhancements might support the formation of a negative stepped leader. The idea of local electric-field enhancement by positive streamer systems was further explored by Griffiths and Phelps (1976) by means of a model investigation, leading to the conclusion that a small number (usually less than ten) of successive intensifying positive streamer systems, each initiated by individual positive streamers and developing in the same volume as previous streamer systems, could lead to a significant electric-field enhancement at their collective origin.…”

Lightning path simulation based on the stepped leader: Electrical conductivity effects

“…With reference to the best known category, the cloud-to-ground lightning, the main stages are essentially the preliminary breakdown, the stepped leader, the connecting leader and the return stroke, however subsequent stages may also occur (Volland, 1984;Uman, 1987). This study deals with the macroscopic development of the stepped leader channel (Uman, 1987;Golde, 1977;Chalmers, 1967;Loeb, 1966Loeb, , 1968Schonland and Collens, 1934;Raizer, 1991), which establishes the atmospheric ionized path to complete the lightning.…”

Numerical simulation of positive streamer development in thundercloud field enhanced near raindrops

“…The first stage of lightning initiation was analyzed in [3][4][5][6][7][8][9][10][11][12][13]. Obviously, to initiate lightning, the field strength at least locally must be above the E br /P g , but the E/P g magnitudes measured in thunderclouds do not exceed 3-4 kV/cm/atm [14][15][16].…”