Low Frequency Magnetic Field Observations of Natural Positive Leaders Featuring Stepwise Propagation.

We examine the low‐frequency (LF) magnetic pulses recorded for the positive leader in two natural lightning flashes as recorded on the high‐speed video camera. The captured positive leader in both cases propagated in a stepwise manner at least over several milliseconds according to the concurrent sequence of LF magnetic pulses. By comparing with previous studies on positive leaders under different situations, the average inter‐pulse intervals (11.7 and 20.3 μs, respectively) obtained for our natural cases are shorter than that of upward positive leaders in rocket‐triggered (typically >25 μs) and tower‐initiated lightning (62 μs in one case). Hence, the stepwise propagation of positive leaders in natural lightning is likely more frequent than that in object‐induced lightning, reflecting a possible influence of ambient electric field and other meteorological factors on the stepwise feature of positive leader. The stepwise extension might be common for positive leaders developing in various scenarios. Plain Language Summary: Many observations have been made with respect to upward positive leaders in rocket‐triggered lightning and tower‐initiated lightning. From the previous observations, it is obvious that the upward positive leader sometimes propagates in a stepwise manner. When it comes to natural lightning, many reliable conjectures have been put forward about the stepwise propagation of positive leaders, but there is still a lack of solid observations. Whether the positive leader in natural lightning also propagates in a stepwise manner remains an open question. In this paper, we analyze the propagation of positive leaders in two natural lightning flashes using high‐speed imaging and concurrent measurements of the associated radio frequency magnetic fields. It aims to understand the variability in the propagation manner of positive leaders in natural lightning and revealing the differences and similarities of leader propagation between natural and object‐initiated lightning. From the low‐frequency magnetic field measurements, the positive leaders in natural lightning could also propagate in a stepwise manner. Our analysis shows that the inter‐pulse intervals of positive leaders in observed natural lightning are shorter than that previously observed in tower‐initiated and rocket‐triggered lightning. Key Points: First coordinated observations of stepwise propagation for the positive leader of natural lightning flashesThe average inter‐pulse interval of positive leader stepping in two cases is estimated to be around 12 and 20 μs, respectivelyThe stepping of positive leaders in natural lightning appears to be more frequent than that of the object‐initiated upward positive leaders [ABSTRACT FROM AUTHOR]