Abstract: Needle discharges, characterized by fine-scale filamentary plasma structures within lightning channels, represent a key manifestation of localized electric field intensification and intricate channel dynamics. Although previous case studies have reported their occurrence, systematic comparisons of their behavior during different lightning discharge phrases remain rare. Elucidating these discharges is important as they offer insights into both the microphysical charge redistribution processes along lightning channels and the transient electric field during discharge events.
This investigation utilizes high-speed video observations with sub-microsecond temporal resolution, synchronized with electric field measurements obtained at Guangzhou Tall-object Lightning Observation Station. This dataset facilitates detailed examination of needle discharge characteristics, including their initiation, extension, flickering behavior, and propagation during both leader stage and return-stroke stage of a positive cloud-to-ground flash. Results reveal clear contrasts between two stages. During leader stage, needle discharges initiate 164-40.5 ms preceding the return-stroke onset, with average initial lengths of approximately 140 m. In contrast, return-stroke stage needles predominantly developed within 21.3 ms post-onset, with significantly reduced initial lengths averaging 76.3 m. Flickering activity show striking differences: Each needle flickers 2.67 times on average in the leader stage, whereas the value increases to 7.20 in return-stroke stage, with shorter intervals (8.36 ms versus 1.43 ms) and maximum extension up to 1560 m. Propagation dynamics further highlight the stage dependence, with both initial extension speed and flickering speed significantly higher during return-stroke stage (1.15×10⁶ m·s^-1 and 1.53×10⁶ m·s^-1) compared to leader stage (0.48×10⁶ m·s^-1 and 0.56×10⁶ m·s^-1).
Overall, results reveal that needle discharges exhibit distinct multi-stage evolutionary patterns controlled by distinct mechanisms. During the leader stage, longer yet less frequently reactivated needles form preferentially, whereas return-stroke stage facilitates repeated high-speed reactivation and rapid spatial extension driven by intense current injection. These results enrich understanding of fine-scale discharge physics and provide observational constraints for multi-scale models of lightning dynamics.