On the Step Type of Continuous Propagating Positive Leader in Laboratory‐Scale Long Spark Discharges.

Positive leader steps are currently not well understood. Many recent findings challenge the previous understanding of the positive leader steps occurring in continuous leader development. In this work, we investigate the stepping propagation of positive leaders in laboratory spark discharge by using a high‐speed video camera and a synchronized electrical parameter measurement system. Two types of steps were distinguished in the positive leader continuous development process based on the current pulse features. One type of step exhibits a bell‐shaped current pulse, while the other type of step exhibits a steep current rise. The former type vanishes when the rising rate of the ambient electric field increases, whereas the latter type, which could be led by a separate luminous structure, does not. Plain Language Summary: Lightning is one of the most impressive, commonly experienced geophysical phenomena, driven by the propagation of positively and negatively charged, thermally‐ionized plasma channels, known as positive and negative leaders. Leaders propagate in a continuous or stepwise manner. Knowledge of leader steps is critical to understanding the radio‐frequency electromagnetic radiation and even the high energy physical phenomenon. Based on natural or triggering lightning observations, laboratory‐scale discharge experiments, and numerical simulations, some important insights (like the spontaneous emergence of space stems and the bi‐directional development of the space leaders) about negative leader steps have been given by researchers. Positive leader steps are more mysterious. We investigate positive leader steps by performing laboratory spark discharge experiments. Using a high‐speed video camera and a synchronized electrical parameter measurement system, two types of steps were distinguished in the positive leader continuous development process based on the current pulse features. One type of step exhibits a bell‐shaped current, while the other type of step exhibits a steep current rise. The former type vanishes when the rising rate of the ambient electric field increases, whereas the latter type, which could be led by a floating luminous formation, does not. Key Points: Positive leader steps superimposed on the continuous leader propagation are classified into two types based on the current featuresThe steep‐rise type steps are accompanied by the abrupt leader channel elongation, while the bell type steps maybe notThe bell type steps will disappear with the rising rate increase of the ambient electric field, while the steep‐rise type steps do not [ABSTRACT FROM AUTHOR]