My research is aimed at terrestrial gamma-ray flashes (TGFs), which are bursts of gamma rays produced in the Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 MeV. They are probably caused by intense electric fields produced above or inside thunderstorms.
According to Marshall et al. (1995, JGR) the electric field inside a cloud before a flash is of about 100 kV/m. However, I'm not sure about what can be consider as "normal" knowing the heterogeneity of charge distribution inside a cloud.
Above cloud as the density decrease the breakdown electric field also decrease (for instance simple calculations based on Pasko, 1996 give a breakdown electrical field of ~4kV/m at 50 km ).
Electric field of a few kV/m have been measured above cloud (10-16 km), Marshall et al. (1996, JGR)
Even though the measured electric field on scales of meters may reach 100 kV/m in regions in which lightning is initiating, on very small local scales, say within a mm of the tip of an ice crystal, significant enhancement should occur. Also, at the tip of a propagating lightning discharge, fields are probably quite high (or the discharge would not sustain itself and propagate).
I am guessing that you are interested in what causes a TGF. If we assume the TGF is the result of a strong runaway breakdown event, then we would need something like 1/10th of the ordinary breakdown field (the so-called "break-even" field) extended over a kilometer or more (runaway avalanches require long distances to develop). Such fields have been observed by balloons many times. However, to get the intense photon beam, one would need a significant population of runaway electrons in order to generate enough high-energy photons. This means either: 1. a tremendous seed electron population was injected into the high field region by a power cosmic ray (but why would the beam go up if the cosmic ray is not?) or 2: a significantly stronger (than 1/10th breakdown) kilometer-scale electric field that can strongly amplify the runaway electron population. Such fields have not been observed. Neither of these scenarios make a lot of sense. As most TGFs are associated with the early stages of a lightning flash, one should look at how an early-stage lightning leader system might influence things. Can this early-stage leader amplify the electric field nearby? Is dart-like breakdown in the channel able to inject massive amounts of seed electrons into an existing runaway-capable ambient electric field?
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