Laboratory Study of Electrical Discharges on Vapor Grown Ice Crystals Subjected to Strong Electric Fields
AuthorPetersen, Danyal A.
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Thundercloud electric-field observations have consistently yielded peak values that are an order of magnitude weaker than the dielectric strength of air at relevant altitudes. Various discharge processes have been proposed to explain how lightning can be initiated in such weak electric fields, including hydrometeor-initiated positive streamers and cosmic ray-initiated runaway breakdown. The historically favored positive streamer discharge process is problematic because it requires electric fields two to three times larger than the largest typically observed. The more recently favored runaway breakdown discharge process appear to be viable in electric fields comparable with those typically observed, but it is not clear how it may lead to creation of a hot lightning leader channel. It has been hypothesized previously by the author that a combination of these two discharge processes offers a more plausible solution, with each process solving a piece of the puzzle. One of the important elements of the positive streamer system discharge process is the generation of initial "seed" positive streamers at the extremities of hydrometeors such as raindrops and ice crystals. The focus of this dissertation is an experimental study designed to investigate the generation of positive streamers and other corona discharges at the extremities of vapor-grown ice crystals. Of primary interest is the determination of the minimum electric field required to generate a positive streamer as a function of ambient air density, ice crystal length, and ice crystal tip geometry. The results of this study show a definite relationship between the minimum electric field required to generate a positive streamer, ambient air density, and ice crystal length. These results are useful insofar as they identify the electric fields required for seed positive streamer production from vapor-grown ice crystals such as are known to exist in the colder regions of thunderclouds. Another interesting result of this study is the finding that the sharpest ice crystal tips can inhibit positive streamer generation while enhancing the production of glow coronas. This result suggests the possibility that regions in thunderclouds characterized by sharp-tipped ice crystal habits may inhibit the initiation of lightning by locally relaxing the thundercloud electric field. The results of this study are a significant improvement over previous studies and offer support and clarification for the hypothesis that hydrometeor-initiated positive streamers may contribute to the initiation of lightning.