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

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 1kA. 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 2.358kA, 0.627ms,0.078ms, 0.165ms, 0.417C, 2.172ms, and 579A, 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 10(7)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 300s 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. Plain Language Summary As a typical charge transfer process during lightning discharge, most of M components are often associated with current pulse having a peak amplitude of 100-200A and a charge transfer of 0.1 to 0.2 C. However, some large M components transfer as much charge as a return stroke and their peak currents may be up to several kiloamperes, which often cause severe damage. Although there was some research on large M components, the research mainly focused on charge transfer and current characteristics. Up to now, there is no widely accepted conclusion on the initiation of large M components. To better understand the formation cause of large M components, we thoroughly analyze the current characteristics of M components and the related discharge processes based on channel-base current data, high-speed video observations, and the radiation source locations of flash-continuous interferometer observations acquired from triggered lightning flashes. The study reveals that large M events are often caused by fast positive streamers followed by a possible recoil event or by a dart leader to ground if an active channel exists.