Observations of the Initiation of Lightning Flashes

The mechanism that initiates lightning flashes is still unknown. Recent observations of the initial events in lightning flashes near Langmuir Laboratory in New Mexico may help us to understand the lightning initiation mechanism. The observations were obtained from a close storm on August 5 2013, whose complete sequence of lightning was observed with with a broadband VHF interferometer (INTF), the Langmuir Laboratory Lightning Mapping Array (LMA), and fast and slow antennas. Detailed analysis of the observations show that flashes for which the initial VHF radiation sources was stronger than about 5 dBW (4 watts) were initiated by a previously unidentified form of fast positive breakdown. No activity was detected prior to the positive breakdown, either by the sensitive INTF or fast electric measurements. This and other features of the observations indicate that the breakdown occurs in virgin air and is produced by dielectric streamer processes in localized regions of strong electric fields. We observed both normal intracloud and cloud-to-ground discharges to be initiated by such breakdown. The results are fundamentally consistent with those obtained from modelling studies by Liu et al. (Phys. Rev. Lett.109, 025002, 2012), in which positive sprite streamers were shown to be initiated by purely dielectric breakdown, without the need of an initiating event such as a cosmic ray or energetic electron avalanches. We speculate that all lightning flashes are initiated by the fast positive events.