An M-component with a Concurrent Dart Leader Traveling along Different Paths during a Lightning Flash

Abstract:

Using time-correlated high-speed video and electric field change data, we describe a seven-stroke lightning flash in which the fifth return stroke (RS) occurs 0.80 ms after the fourth RS connects to a different ground location 3.3 km away. The fifth RS is 0.34 ms after an M-component starts down the different channel. The fifth stroke involves a dart leader traveling concurrently with, though slower than the M-component. There was no indication of leader advance along this prior channel earlier during the fourth RS. The fourth stroke involves a stepped leader that started from the end of an observed prior dart leader branch which did not previously propagate to ground. The concurrent M-component and dart leader are preceded by an in-cloud event evidenced by a large amplitude, fast electric field change pulse, at 6.1 km estimated altitude, inferred as the connection to the channel for the M-component. The M-component current apparently initiates the dart leader about 40 μs later. A visible channel length of 10,400 m allows for the 2-D propagation speed of the M-component luminosity to be estimated in the range of 1.0 to 1.2 × 10⁸ m s^-1. The concurrent dart leader travels a visible length of 3445 m with 2-D speed of 1.7 × 10⁷ m s^-1, similar to other dart leaders in this flash. Luminosity evolution along the channel through the RS and M-component will also be described. Estimated optical risetimes of three separate M-components in this flash are 80-200 μs at 520 m altitude above ground.