Recoil Leaders Research Articles

Bipolar cloud‐to‐ground lightning flash observations

AbstractBipolar lightning is usually defined as a lightning flash where the current waveform exhibits a polarity reversal. There are very few reported cases of cloud‐to‐ground (CG) bipolar flashes using only one channel in the literature. Reports on this type of bipolar flashes are not common due to the fact that in order to confirm that currents of both polarities follow the same channel to the ground, one necessarily needs video records. This study presents five clear observations of single‐channel bipolar CG flashes. High‐speed video and electric field measurement observations are used and analyzed. Based on the video images obtained and based on previous observations of positive CG flashes with high‐speed cameras, we suggest that positive leader branches which do not participate in the initial return stroke of a positive cloud‐to‐ground flash later generate recoil leaders whose negative ends, upon reaching the branch point, traverse the return stroke channel path to the ground resulting in a subsequent return stroke of opposite polarity.

Recoil leader formation and development

Abstract The existing interpretation in the lightning literature, based on field measurements, defines recoil leaders as negative leaders. However recoil leaders are floating conductors, and, based on this physical assumption, they should be defined as bipolar and bidirectional leaders. This physics-based assumption has never previously been verified experimentally. Such verification, reported in this paper, has been obtained from observations of branched upward positive leaders from a tall tower using a high-speed video system synchronized with electric and magnetic field change and luminosity measurements on the ground. The analysis of these observations clearly reveals the nature of recoil and dart leaders as bidirectional and bipolar electrodeless discharges that develop from a small region along a path of the decayed channels of a previous positive leader, or a positively charged return stroke of negative CG flashes.

The initial stage processes of rocket‐and‐wire triggered lightning as observed by VHF interferometry

We examine VHF interferometric images, channel‐base currents, and broadband electric field waveforms of the initial stage (IS) in two rocket‐and‐wire triggered lightning flashes. Two types of negative leaders, termed “long‐duration” and “short‐duration” leaders, were imaged by the VHF interferometers during the IS of the two flashes. There were three leaders that had relatively long durations of more than a few milliseconds. These three leaders were not accompanied by a significant change of channel‐base current during their early stages of development, indicating that they corresponded to intracloud (IC) discharges that were not connected to the grounded triggered‐lightning channel. Two of these three leaders eventually connected to the triggered‐lightning channel and initiated initial continuous current (ICC) pulses. The third long‐duration leader apparently developed from the vicinity of an isolated negative charge region toward an upper‐level positive charge region and toward a branch of the grounded channel; it served to bridge the positive charge region and the triggered‐lightning channel, resulting in the opposite polarity portion of the bipolar ICC. The short‐duration negative leaders had durations of some hundreds of microseconds. These negative leaders apparently recoiled along the conductive channels created by branches of the upward positive leader (UPL); they initiated ICC pulses when the grounded channel was sufficiently conductive. It follows that ICC pulses can be initiated either by recoil leaders or via interception of separate in‐cloud leaders by a grounded current‐carrying channel.

High-speed video of lightning and x-ray pulses during the 2009–2010 observation campaigns in northeastern Spain

This paper summarizes results obtained by the 2009–2010 field campaigns conducted in northeastern Spain using a high speed camera, VHF interferometer, VLF lightning location system and scintillation detectors for energetic radiation. The main purpose was to obtain new insight into the lightning processes which emit x/γ radiation. For just one (very close) lightning strike, 99 x-ray pulses were emitted within 0.86ms and had an energy higher that 10keV. This burst was associated with the final stages of a downward negative leader. The high-speed camera data also allowed to identify various previously unreported features in different types of cloud-to-ground lightning flashes. At video frame rates of 10,000fps, some branches of negative leaders of cloud-to-ground flashes were illuminated for the duration of only one frame. During positive cloud-to-ground flashes, the high speed videos showed an intense activity of recoil leaders. In the case of intra-cloud lightning, the VHF interferometer network did not present a clear relation between detections and visible leader. Instead, bursts of VHF detections corresponded visually to a long-lasting bright channel under the cloud base.

Physical processes during development of upward leaders from tall structures

The objective of our study was to identify and interpret the various processes during development of upward positive leaders from tall structures. We provide a physical interpretation for the varying luminosity of the leader channel during its initial ascent and the pulsing luminosity during much later stages of the established leader. Our analysis confirms that pulsed luminosity is a result of recoil leaders intercepting the current-carrying channel connected to ground. This interception produces an M-component. Applying the bidirectional leader concept and a simplified electrostatic approximation, we introduce physical models of the dart leader-return stroke sequence and the M-component.

Positive leader characteristics from high‐speed video observations

Based on analyses of high‐speed video recordings of cloud‐to‐ground lightning in Brazil and the USA, the characteristics of positive cloud‐to‐ground (+CG) leaders are presented. The high frame rates permitted the average, 2‐dimensional speeds of development along the paths of the channels to be resolved with good accuracy. The values range from 0.3 to 6.0 × 105 ms−1 with a mean of 2.7 × 105 ms−1. Contrary to what is usually assumed, downward +CG leader speeds are similar to downward −CG leader speeds. Our observations also show that the speeds tend to increase by a factor of 1.1 to 6.5 as they approach the ground. The presence of short duration, recoil leaders (RLs) during the development of positive leaders reveal a highly branched structure that is not usually recorded when using conventional photographic and video cameras. The existence of the RLs may help to explain observations of UHF‐VHF radiation during the development of +CG flashes.