Indirectly Measured Ambient Electric Fields for Lightning Initiation in Fast Breakdown Regions

We report a new approach to measure indirectly the ambient thunderstorm electric fields in fast positive breakdown regions. For a given geometry of the discharged fast breakdown region, we show that there is a minimum ambient electric field required to produce a given charge moment change. We apply this approach to the fast breakdown measurements for two events reported by Rison et al. (2016, https://doi.org/10.1038/ncomms10721) and find that the average ambient electric field in the discharged region is at least 1.0 - 1.5 x 10(5) V/m at the 9.5 km initiation altitude of these events. This electric field is close to the runaway electron avalanche electric field and to the critical field for positive streamer propagation. These measurements provide a meaningful starting point for more detailed analyses or simulations of what occurs inside the fast breakdown process that is responsible for the initiation of at least some lightning discharges.