Dwyer et al. [2003, 2004a] recently reported measurements of bursts of x‐rays, with energies up to ∼250 keV, originating from dart leaders and possibly the return strokes of rocket‐triggered lightning. In this paper, these x‐ray observations are compared with the relativistic runaway electron avalanche (RREA) model. It is found that for dart leaders the standard RREA model is inconsistent with the observed spectrum and flux of the x‐ray emission. This result implies that the cold runaway electron model may be applicable to dart leaders. In this model, runaway electrons are directly accelerated out of the bulk electron population, produced by the ionization of the neutral atoms. However, this mechanism requires an electric field an order‐of‐magnitude larger than the conventional breakdown field. Because the flux of runaway electrons is very sensitive to the electric field strength, x‐ray observations of lightning provide a novel tool for determining the fields associated with the leaders.
