Narrow bipolar events (NBEs) are impulsive and powerful intracloud discharges. Recent observations indicate that some NBEs exhibit a slanted orientation rather than strictly vertical. However, the inclination of NBEs has not been considered in previous transmission line models, leading to uncertainty when evaluating their characteristics based on electromagnetic fields. This paper investigates the propagation effects of slanted NBEs using a newly developed slanted rebounding-wave model. It is found that the calculated results using the proposed model match well with measurements for both vertical and slanted NBE cases. The inclination of the NBEs significantly affects the electromagnetic fields at close distances, while the effects weaken as the observation distance increases, where the fields are dominated by the radiation component. The slanted rebounding-wave model improves the agreement with respect to a purely vertical channel and can be extended to any discharge geometry at arbitrary observation distances.

One of the outstanding questions in lightning research is how dart leaders (also called recoil leaders or K-leaders) initiate and develop during a lightning flash. Dart leaders travel quickly (106-107 m/s) along previously ionized channels and occur intermittently in the later stage of a flash. We have recently reported some insights into dart leader initiation and development based on our BIMAP-3D observations. In this presentation we will expand on that work by combining observations and modeling to try to understand the observed dart leader behaviors. BIMAP-3D consists of two broadband interferometric mapping and polarization (BIMAP) systems that are separated by 11.5km at Los Alamos National Laboratory. Each station maps the lightning VHF sources in a 2D space, and the combination of the 2-station measurements provides a detailed 3D source map. A fast antenna is also included at each station for electric field change measurements. Our previously reported observations suggest dart leaders commonly exhibit an initial acceleration, followed by a more gradual deceleration to a stop. We also modeled the dart leader electric field change with a simple configuration of two point-charges, finding that the modeled tip charge increased in magnitude during the initial acceleration in some simple cases. We now employ a more sophisticated model to better understand the distribution of charge along the dart leader channel, and the background electric field in which the dart leader develops.Presented at the AGU 2023 Fall Meeting

A very close high-speed video observation of lightning attachment to a building revealed novel details regarding the leader streamer zone dynamics. Upward leaders propagate in a steady and unbranched manner, displaying a uniformly luminous corona brush. The exception being the upward connecting leader (UCL) just before connection, when its streamer zone increases in size and develops a more filamentary pattern. Downward negative leaders have 3-m long multiple streamers emanating from each negative leader tip. In some occasions, plasma formations known as space stems are seen to form in the location previously occupied by negative streamers. Space stems have luminosities comparable to the main leader channel, but are detached from it by 4 m. Some space stems display streamers of their own, including cases where streamers are emanating from both ends. The space stem formation hampered the propagation of the negative leader that was closest to the UCL.

On June 2nd and 3rd, 2019, 65 sprites were captured with a Phantom V2010 camera recording at 100,000 frames per second from Langmuir Laboratory (LL) in New Mexico. An extra sensitive slow-antenna known as LEFA, located 25 km east of LL, measured E-fields simultaneous with the video observations. Data from the Earth Networks Total Lightning Network (ENTLN) was used to locate their parent flash. By correlating all these datasets, we found the largest fraction of sprites with current signatures (45%) observed to date. These measured sprites have strong electromagnetic signatures comparable in magnitude to the largest current moments previously reported in the peer-reviewed literature, with range-normalized electric field changes of half the amplitude of their parent flashes, and current moments of up to 2742 kA km, as derived from a new computationally-efficient technique introduced here. Comparison to high-speed optical recordings shows also that optically-large sprites tend to have larger electrical currents.

The physical mechanism of Narrow Bipolar Events (NBEs) has been studied for decades but it still holds many mysteries. Recent observations indicate that the fast breakdown discharges that produce NBEs sometimes contain a secondary fast breakdown that propagates back in the opposite direction but this has not been fully addressed so far in electromagnetic models. In this study, we investigate fast breakdown using different approaches that employ a Modified Transmission Line with Exponential decay (MTLE) model and propose a new model, named “rebounding MTLE model”, which reproduces the secondary fast breakdown current in NBEs. The model provides new insights into the physics of the fast breakdown mechanism.