High resolution imaging of negative leader propagation with LOFAR

The propagation of negative leaders is poorly understood and one of the top questions in lightning research. In the optical, negative leaders are observed to propagate in steps similar to those seen in laboratory experiments, with an average velocity between 10⁵ and 10⁶ m/s. Step formation occurs via a luminous section formed in front of the leader tip, referred to as a “space stem”. This space stem grows bi-directionally and eventually connects with the leader channel, resulting in a surge of current, a luminosity wave traversing back up the channel, and a burst of negative corona streamers emitted from the new tip.

In the VHF, stepping is also observed, but emission associated with space stems has so-far not been identified. Instead, a propagating front of VHF pulse sources is observed, which exhibits a filametary structure at high altitudes.

In this work, we leverage the high tempo-spatial resolution of the LOFAR radio telescope and the high sensitivity and completeness of a new near-field beamforming algorithm (TRI-D) to construct detailed three-dimensional images of negative leader propagation. The spatial resolution of the resulting images is better than 1 m and the time resolution is 100 ns. Studying the distribution of VHF pulse sources, we hope to improve our conceptual understanding of negative leader stepping. Specifically, we want to show whether there is any evidence for space stems and better understand the distribution and interaction of streamers. Here, I present the initial findings of this research.