Runaway Electrons and Positive Streamers from Hydrometeors: Cofactors in Lightning Initiation?
Abstract
Knowledge of the physical mechanisms involved in the initiation of lightning remains incomplete. Observations have shown peak thundercloud electric fields to be an order of magnitude weaker than the dielectric strength of air, suggesting that lightning initiation is not a simple case of dielectric breakdown. Both hydrometeor- initiated positive streamers and cosmic ray-seeded runaway electron avalanches have been proposed as lightning initiators due to their ability to occur in electric fields significantly weaker than the dielectric strength of air. The historically favored positive streamer mechanism requires electric fields that appear to be a few times larger than peak observed fields, while the electric field requirements of the more recently favored runaway electron mechanism are comparable. These two hypotheses are often presented as solutions to the lightning initiation problem, but in fact they are at best only partial solutions. The relevant physical result of both mechanisms is similar, consisting of a local intensification of the thundercloud electric field over distances on the order of 10 meters. What remains unexplained is how this local intensification of the thundercloud electric field leads to the appearance of a lightning leader channel. First we present a hypothesis on the mechanism of local electric field intensification, where it is suggested that an intense local electric field may be generated in a thundercloud via a strong runaway breakdown event followed immediately by a series of hydrometeor-initiated positive streamer systems. Next we present a hypothesis of lightning leader formation, where it is suggested that the locally intensified electric field may support a process analogous to the precursors of `stepping' in a negative stepped leader.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMAE21A0992P
- Keywords:
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- 3304 Atmospheric electricity;
- 3324 Lightning