Characteristics and Discharge Processes of M Events With Large Current in Triggered Lightning

Abstract

Observations have been obtained of M events during triggered lightning flashes with a broadband very high frequency interferometer and measurements of electric field changes, channel-base current, and high-speed video. The current characteristics of 239 M events in 18 triggered lightning flashes are analyzed, and the discharge processes leading to large M currents are investigated. Sixty-eight of the M events (28%) had peak currents exceeding 1 kA. The geometric average values of peak current, duration, 10–90% rise time, half peak width, charge transfer, interval from return stroke to M and background current are 2.358 kA, 0.627 ms,0.078 ms, 0.165 ms, 0.417 C, 2.172 ms, and 579 A, respectively. Compared to other M events, the M events with large peak current occurred closer in time to the preceding return stroke, and their corresponding current changes were more rapid. Three cases associated with the initiation processes of M events reveal that some large M events were initiated by fast positive streamers propagating away at a velocity of about 107 m/s followed by a possible recoil event, or by a dart leader while the channel of continuing current generated by previous leader still existed. It is found that a fast current pulse with 10–90% rise time of less than 300 μs is a necessary, but not sufficient, condition for the occurrence of large M events. The fast current pulse often corresponded to a low/close junction site.