X-ray bursts associated with leader steps in cloud-to-ground lightning

[ 1] X- ray and electric field measurements were made during five nearby negative natural lightning strikes in north central Florida during the summer of 2004. The observed X- ray emission typically was detected similar to 1 ms before the first return stroke, during the stepped- leader phase, and had energies extending up to a few hundred keV. The X rays were produced in discrete, intense bursts emitted in coincidence with the formation of the leader steps, demonstrating unambiguously that the source of lightning X rays is closely related to the stepping process. The X- ray emission from lightning stepped leaders is found to be remarkably similar to that from lightning dart leaders, suggesting that these different types of leaders share a common mechanism. The reported observations have important implications for understanding how runaway breakdown occurs and how lightning leaders propagate.