Ball lightning has not been produced in the laboratory as yet, but its existence as a real natural phenomenon seems to be assured. The evidence concerning its physical nature is reviewed in the light of known information on atmospheric electricity and the lightning discharge. It is concluded that it is unlikely that ball lightning is a plasma phenomenon, as it is frequently assumed to be. More probably it is a region, containing a strongly inhomogeneous distribution of space charge in the form of highly ionized gas, the ionization being primarily in the molecular form, with few free electrons. Occluded material such as dust, water vapor, and combustible gases may play a significant role in its behavior.
Measurements of pressure pulses from triggered lightning strokes show that they are not the result of strong mechanical shock waves of the type postulated by Abramson et al. as the explanation of channel growth in spark breakdown. Physical arguments, which are applicable also to natural lightning strokes, indicate that the rate of thermal heating in the channel is too slow to allow the development of the required strong ionizing shock front.
Two different models of the temperature and state of ionization in the return stroke of a cloud‐ground lightning stroke are in use in the literature. In the model that has been adopted for interpretation of the spectroscopic data on lightning, the mechanism of capture of free electrons to form negative oxygen ions is disregarded, the ionization being assumed to exist in the form of free electrons and heavy positive ions. In this case the thermalization time of the channel is expected to be short, so that the optical and thermal temperatures are effectively equal. In the model that accepts negative ion formation as a fundamental process, the ionization behind the tip of the return stroke is considered to be rapidly interchangeable between free electrons and heavy atomic and molecular oxygen ions. It is expected on this model that there exists a distinct time lag of the thermal temperature behind the optical temperature.
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