Fast Breakdowns Observed in the Initial Leaders of Two Energetic Compact Strokes

Abstract We have observed 1 fast positive breakdown (FPB) and 20 fast negative breakdowns (FNBs) for the first time in the initial leader of two energetic compact strokes (ECSs) using a broadband interferometer. It was found that the FPB propagated upward over a distance of 30 m with a speed of , while the FNBs propagated downward over a distance ranging from 25 to 173 m with a speed ranging from to . The geometry of the fast breakdowns indicates that each ECS leader channel consists of multiple branches. More than half of the FNBs are found to propagate nearly vertically and straightly with their progression direction angles relative to the vertical lines being less than 10°. In addition, for each of the fast breakdowns, which include the FPB and FNBs, we have identified a corresponding LF electric field change pulse.