Dart-stepped-leader step formation in triggered lightning

Abstract

Dart-stepped-leader step formation in triggered lightning is documented with high-speed video recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) and linear streak film with a temporal resolution of about 1 µs. Locally luminous points and segments of channel both separate and below the main descending leader tip were recorded on the high-speed video. Bidirectional leaders were imaged initiating at the locally luminous points below the main channel tip, points that remain stationary during the interstep process. The average speed of five bidirectional leaders was 8.4 × 105 m/s upward and 4.8 × 105 m/s downward, assuming 1.5 µs between successive images. The main dart-stepped-leader channel tip moved downward between steps. Leader steps extended below the bottom of the previous bidirectional leader. Processes that can be seen between steps on high-speed video are generally below the noise threshold of the streak film, which shows primarily the newly formed steps.