Abstract: The negative dart leader-return-stroke process, accompanied by broadband electromagnetic radiation and high-energy emissions, represents a key focus in lightning physics. During the summer of 2023, collaborative observations of rocket-triggered lightning are conducted by State Key Laboratory of Chemistry for NBC Hazards Protection and Chinese Academy of Meteorological Sciences, leveraging the platform of China Meteorological Administration Field Experiment Base for Lightning Science (CMA_FEBLS). Specifically, a negative lightning event is triggered by an upward positive leader when the rocket-tethered wire ascends to an altitude of 223 m above ground level on 18 July 2023. Using self-developed detection equipment, collaborative observations and measurements are performed on the discharge process of this rocket-triggered lightning event. This effort results in the acquisition of high-temporal-resolution, multi-parameter complementary datasets, integrating multiple observations, including high-speed video footage, low-to-mid-frequency magnetic field data, electric field data, very high frequency (VHF) signals, and high-energy radiation records captured during the dart leader-return-stroke discharge phase. Observations from two dart leader-return-stroke processes (denoted as RS1 and RS2) of the aforementioned negative rocket-triggered lightning event are analyzed. Results demonstrate strong temporal synchronization among lightning current, electric field, magnetic field, VHF radiation, and X-ray pulse measurements. Notably, intense VHF signals and high-energy X-ray signals are generated during the stage of the strokes when the downward leader propagated through the vaporized steel wire channel and downward to the ground, ultimately initialing a return stroke. Although the charge transfer amounts of two processes show no significant difference (approximately -2.61 C for RS1 and -2.14 C for RS2), their dart leader propagation speeds differed substantially. The propagation speed of RS1 leader reached 10⁷ m·s^-1, which corresponded to intense VHF radiation and continuous X-ray pulses. In contrast, RS2 leader propagates at a speed of 10⁶ m·s^-1, an order of magnitude lower than two-dimensional propagation speed of RS1. Consequently, the VHF radiation and X-ray pulses observed during RS2 are comparatively weaker, and X-ray pulses exhibit a discrete pattern.
Furthermore, the temporal evolution of X-ray pulse emissions is found to be consistent with the process by which the leader descended along the steel wire channel, grounded, and triggered the return stroke. This temporal correspondence confirms a direct correlation between X-ray emissions and the initiation of return strokes, thereby providing critical observational evidence for elucidating the strong-field radiation mechanism of X-rays during the lightning return stroke phase.