This work presents the first simultaneous X-ray measurement and high-speed video observation of the propagation of a lightning leader producing X-rays. As a result, the three-dimensional leader distance from the X-ray measurement and, for the first time, the conditions of the preexisting channel during the leader propagation were observed. Although four leaders in this seven-stroke flash followed the same path to ground, X-rays were only observed during the leader before the return stroke with the highest peak current. The fact that the other three leaders following the same path to ground did not produce detectable X-rays confirms the hypothesis that leader line charge density is an important factor that determines X-ray production. The fact that X-rays was recorded only when the leader tip was at a certain portion of the lightning channel confirms that the orientation of the leader plays an important role in the detection of X-rays.
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