A subsequent-stroke stepped leader repeatedly colliding with the remnants of the preceding stroke at different altitudes

In a three-stroke negative cloud-to-ground flash, the leader of Stroke 2, while forming a new, heavily branched path to ground, briefly collided with the defunct (non-luminous) channel/branches of Stroke 1 at heights of about 2.0 km, 1.6 km, again 2.0 km, and 90 m above ground level. Each of the first three (higher-altitude) collisions was associated with luminosity waves originating from the collision point, including an upward reflection-type wave along the colliding leader channel and one or more transmitted waves along the residual branches of the preceding stroke. Based on the estimated extension speeds and RF field signatures, we attributed the transmitted luminosity waves to dart-stepped leaders developing in relatively short segments of the decayed channel. As a result of the last collision, at a height of 90 m above ground level, a branch of the Stroke 2 leader entered the lower part of the residual (non-luminous) Stroke 1 channel and connected to the ground, producing a return stroke. Additionally, the heavily-branched Stroke 2 leader created a ground termination about 950 m away from its ground termination common with Stroke 1. We observed four X-ray pulses with peaks ranging from 130 to 750 keV during a 3.5-ms or so interval around the time of the higher-altitude collisions during the leader stage of Stroke 2, but no detectable X-ray pulses during the leader stages of Strokes 1 and 3. The probability of random occurrence of 4 X-ray pulses within a 3.5-ms time interval at our site is 0.00099.