Propagation mechanism of downward positive leader launching a negative discharge to ground

Our knowledge of downward positive lightning leaders is incomplete due to their rarity and limited ability of VHF mapping systems to image such leaders. Here, using high-speed optical records and radiofrequency field signatures, we examine in detail the development of a descending positive leader, which resulted in a negative cloud-to-ground discharge. The positive leader extended intermittently, via transients separated by millisecond-scale quiet intervals. Its multiple branches tended to host transients in an alternating manner. We show that the transients (1) are mostly initiated in a previously created but already decayed branch, a few hundreds of meters above its lower extremity, (2) extend bidirectionally (negative charge moving up and positive charge moving down), (3) establish a temporary (about 1 ms) steady-current connection to the outside-of-the-field-of-view negative part of the overall bidirectional leader tree, and (4) exhibit brightening accompanied by additional extension (new breakdown) at the positive end. Both positive and negative ends extended at speeds of 106 to 107 m/s, while the overall positive leader extension speed was as low as 103 to 104 m/s. The wideband field signatures of the transients are similar to K-changes, with their millisecond- and microsecond-scale features being associated with the steady current and new breakdowns, respectively.