Fast positive breakdown in lightning

Abstract

VHF radiation sources produced by positive breakdown during lightning discharges are generally considered to be both weak and slowly propagating. However, as VHF lightning mapping systems have become more sensitive, even this weak radiation can be mapped. In addition to being a faint process, positive breakdown often produces bursts of energetic activity. During the bursts, the VHF emission is extremely bright, and the breakdown propagates at much higher speeds. Here we present VHF interferometric and time-of-arrival measurements of such fast positive breakdown events produced during three example flashes. Electric field change measurements show that the fast breakdown process carries positive charge. The extent and velocity of the breakdown is estimated by converting the angular source locations provided by the interferometer into Cartesian coordinates using three-dimensional lightning mapping observations of the flash as a guide. Fast positive breakdown events are found to extend 100-2400 m into virgin air beyond the tip of the preceding positive leader, at speeds of 0.9-9 × 107 m s-1. The observations expand upon earlier observations of such breakdown and are similar to recently reported results that fast positive breakdown is the cause of high-power narrow bipolar events.