High-speed Video Observation of a Lightning Flash Producing X-rays

We report a well-documented case of a negative cloud-to-ground lightning dart leader producing x-rays. Four leaders in this seven-stroke flash followed the same path to ground. X-rays were only observed during the fifth leader before the return stroke with the highest peak current. This work presents the first simultaneous x-ray measurement and high-speed video observation of the propagation of a lightning leader producing x-rays. It was capable of determining the leader distance from the x-ray measurement and the conditions of the pre-existing channel during the leader propagation. The fact that the other three leaders following the same path to ground did not produce detectable x-rays confirms that leader line charge density is likely an important factor that determines x-ray production. The moment of the x-rays detection during the propagation of the fifth leader suggests that the orientation of the leader channel plays an important role in the detection of x-rays.