Unique UV images of lightning leading to new insights into its propagation and attachment

Development of hot lightning channels is facilitated by essentially cold streamers emanating from their extremities. Negatively-charged channels extend intermittently, with each step culminating in a strong streamer burst. Thus, streamers play a crucial role in lightning, yet they are poorly understood, because they emit primarily ultraviolet (UV) light, which usually goes undetected. Here, we present unique UV images of both positive and negative streamers leading to new insights into the lightning propagation and attachment. A downward-extending negative channel induced a ~20-m long positively-charged hot channel developing from a roof-top vent pipe and having ~70-m long positive streamers emanating from its tip. The positive streamers extended toward the negative streamers of a descending-channel branch, as normally happens during the lightning attachment process. However, the expected attachment to the vent pipe failed, and ground connection occurred ~200 m away from it. This failure to connect to the vent pipe was likely due to the self-choking effect of the large negative space charge that was suddenly introduced by the step-related streamer burst ahead of the lower end of the descending channel. The observed abrupt change of the propagation path to ground has important implications for the validity of the traditional lightning protection concepts.