Observations of stepping mechanisms in a rocket-and-wire triggered lightning flash

We present 10 high‐speed video images that depict the bottom 150 m of a downward‐negative, dart‐stepped leader in a rocket‐and‐wire triggered flash, recorded at 240 kiloframes per second (4.17 μs frame integration time), along with correlated measurements of the X‐ray emission at 50 m, electric field derivative (dE/dt) at 80 m, and the rocket‐launch‐tower current beneath the leader. We observed discrete segments of secondary channel that exhibited luminosity above that of the surrounding corona streamers and were distinctly separate and beneath the downward‐extending leader channel. These segments appear similar to the space stems or space leaders that have been imaged in long negative laboratory sparks. Multiple simultaneous pulses in X‐ray emission, dE/dt, and launch tower current were recorded during the time that the leader steps were imaged. The leader extended at an average downward speed between 2.7 × 106 and 3.4 × 106 m s−1.