Propagation Mechanism of Branched Downward Positive Leader Resulting in a Negative Cloud‐To‐Ground Flash

Our basic knowledge of downward positive lightning leaders is incomplete due to their rarity and limited ability of VHF mapping systems to image positive streamers. Here, using high‐speed optical records and wideband electric field and magnetic field derivative signatures, we examine in detail the development of a descending positive leader, which extended intermittently via alternating branching at altitudes of 4.2 to 1.9 km and involved luminosity transients separated by millisecond‐scale quiet intervals. We show that the transients (a) are mostly initiated in previously created but already decayed branches, at a distance of the order of 100 m above the branch lower extremity, (b) extend bidirectionally with negative charge moving up, (c) establish a temporary (1 ms or so) steady‐current connection to the negative part of the overall bidirectional leader tree, and (d) exhibit brightening accompanied by new breakdowns at the positive leader end. One of the transients unexpectedly resulted in a negative cloud‐to‐ground discharge. Both positive and negative ends of the transients extended at speeds of 106–107 m/s, while the overall positive leader extension speed was as low as 103–104 m/s. Wideband electric field signatures of the transients were similar to K‐changes, with their millisecond‐ and microsecond‐scale features being associated with the steady current and new breakdowns, respectively. For transients with both ends visible in our optical records, charge transfers and average currents were estimated to be typically a few hundreds of millicoulombs and some hundreds of amperes, respectively. Plain Language Summary Our knowledge of downward positive lightning is very limited. Positive lightning constitutes only about 5% of the global lightning activity, but it causes the most severe damage to various objects and systems, as well as most transient luminous events in the mesosphere. The leader processes in positive lightning remain a mystery and are widely debated in the atmospheric electricity community. In this paper, we present two new findings on propagation mechanisms of branched downward positive leader: (a) its ability to produce an opposite‐polarity (negative) lightning discharge to ground and (b) its unusual mode of propagation involving bidirectional transients, which temporarily reactivate individual leader branches and facilitate their alternating extension. Key Points Branched positive leader extending at an average speed of 103–104 m/s res