On the Nature of Lightning Dart Leaders as Observed with Three Dimensional Broadband Interferometry

Defender: Daniel P. Jensen

Date: April 22, 2024 @ 10am

Speare 113

Abstract:

Dart leaders are a type of electrical discharge that propagates quickly (106-107 m/s) and re-ionizes decayed paths created earlier in a lightning flash. Dart leaders have been observed for nearly 100 years, but many open questions have remained about how they are initiated and how they develop. Recent improvements in technology and instrumentation have dramatically improved our ability to observe and analyze dart leaders, especially the development of three dimensional broadband interferometers (3DINTF). With 3DINTF data we observe that dart leaders typically exhibit an initial acceleration followed by a gradual deceleration, and that passing branch junctions in the flash structure often causes rapid variations in dart leader speed. Combining 3DINTF and electric field change observations with an electrostatic model of the conductive leader channel we find that changes in dart leader speed can be explained by changes in the electric field at the leader tip. The modeled tip electric field is too low to allow propagation into virgin air, which explains why dart leaders are typically confined to previous discharge channels. 3DINTF observations also provide insights into how dart leaders are initiated. Other groups previously identified a process known as “VHF twinkling”, where flickering VHF sources appear somewhat randomly along parts of a positive leader channel. We observed that the region emitting VHF twinkling expands both towards the leader tip and back to the flash origin in between dart leaders. The twinkling region expanding towards the origin then appears to initiate a dart leader if it does not die out or get interrupted by a dart leader on another channel.