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

Abstract

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.

Estimating the Electric Fields Driving Lightning Dart Leader Development with BIMAP-3D Observations

“…High speed camera observations of dart leaders initiating outside of clouds indicate that the bright channel initially extends bidirectionally, but the extension in the positive tip direction quickly halts once it reaches the previously observed end of the preconditioned channel (Ding et al, 2024;Mazur, 2016). Unfortunately this extension in the positive direction is not observed in VHF by BIMAP-3D.…”

Estimating the Electric Fields Driving Lightning Dart Leader Development with BIMAP-3D Observations

“…High speed camera observations of dart leaders initiating outside of clouds indicate that the bright channel initially extends bidirectionally, but the extension in the positive tip direction quickly halts once it reaches the previously observed end of the preconditioned channel (Ding et al, 2024;Mazur, 2016). Unfortunately this extension in the positive direction is not observed in VHF by BIMAP-3D.…”