Needle-like structures discovered on positively charged lightning branches

Abstract

The lightning flashes used in this work were mapped using data from the LO-FAR (LOw Frequency ARray) radio telescope. Due to its effective lightning protection system, LOFAR is able to continue to operate during thunderstorm activity[1]. The Dutch LOFAR stations consist of 38 (24 core + 14 remote) stations spread over 3200 km 2 in the northern Netherlands. The largest baseline between core stations is about 3 km, the largest baseline between remote stations is about 100 km. From each station we use 6 dual-polarized low band dipole antennas (LBA), sampled at 200 MHz, to observe the 30-80 MHz band. The raw time series data were saved to the transient buffer boards, which continuously buffer the last 5 s of data from a maximum of 48 dual-polarized antennas per station. The resulting relative timing accuracy is better than 1 ns. See [2] for more details on LOFAR. When a lightning flash occurs within the area enclosed by the Dutch LOFAR stations, as observed by www.lightningmaps.org, the transient buffer boards are stopped and the data is read to disk. The method we used to map each lightning flash has three major steps. In the first step we fitted plane-waves to the time of pulses received by individual LOFAR stations. Note that the LOFAR stations are less than 100 m in diameter and the lighting is many kilometers from the closest LOFAR station, so that a plane-wave approximation is very good for individual LOFAR stations. These plane-waves were used to identify non-functional antennas, and the intersection of their arrival directions gave a rough first estimate of the flash location, accurate to a few kilometers. Since each station has its own clock and cable delays, in the second step we found the clock offsets between the different LOFAR stations by simultaneously fitting the locations of multiple events and station clock offsets to the measured times of radio pulses, with a Levenberg-Marquardt minimizer. In order to achieve the highest precision, we chose to fit locations of 5 events that created pulses that were strong but not saturating, had a simple structure, and did not change shape significantly across different stations. After fitting, the root-mean-square difference between the modeled and measured arrival times of the radio pulses was around 1 ns. The resulting station clock offsets are consistent with LOFAR station clock calibrations, which are known

Needles and Lightning Leader Dynamics Imaged with 100–200 MHz Broadband VHF Interferometry

“…Combing all clues from the above analysis, we find that a mechanism described by Hare et al () in their supplementary material regarding line charge density redistribution of a leader channel with a two‐layer charge structure is more consistent with our observations than the leader‐disconnection mechanism. As shown in Figure , a positive leader is propagating leftward due to the ambient electric field.…”

“…A nearby bidirectional leader initiated approximately 500 m from the preexisting positive leader that suppressed nearby needles, induced new flickers, created a disconnection on the main positive channel, and resulted in the needle extinguishment behind the disconnection. All these rarely observed yet perhaps common lightning processes and their interactions were analyzed and shown fundamentally consistent with a mechanism described by Hare et al () regarding a two‐layer leader charge structure, providing a new picture of needle and leader dynamics as summarized in Figure . Note that all results are derived from only one lightning flash, more observations are needed to validate these findings.…”

“…Needles from positive leader channels were first reported by Hare et al (2019) and were inferred to originate from the positive leader and propagate outward. Here we definitively image the positive leader path from sources of six recoil leaders traversing the positive channel and confirm that the needles originate from the conducting positive leader channel.…”

The Plasma Nature of Lightning Channels and the Resulting Nonlinear Resistance

“…It has been suggested that the negative differential resistance behavior of lightning channels plays an important role in the mechanism of current cutoff, which in its turn makes some flashes transfer charge to ground by a series of (discrete) return strokes, while others by a single stroke followed by a long continuing current (Krehbiel et al, 1979;Hare et al, 2019;Heckman, 1992;Mazur et al, 1995). Recent review articles argue that the role of negative differential resistance in the channel cutoff remains to be quantified (Mazur & Ruhnke, 2014;Williams, 2006;Williams & Heckman, 2012;Williams & Montanyà, 2019).…”

“…The behavior of the steady-state resistance of arc channels has been used to discuss the phenomenology of lightning channels (Hare et al, 2019;Heckman, 1992;Krehbiel et al, 1979;Mazur & Ruhnke, 2014;Williams, 2006;Williams & Heckman, 2012;Williams & Montanyà, 2019). Steady-state plasma arcs exhibit the so-called negative differential resistance, that is, the resistance decreases with increasing electrical current.…”