Channel Luminosity Research Articles

Evolution of the Leader Discharge in Bi‐Directional Propagation System in Altitude‐Triggered Lightning

AbstractOn 18 June 2023, comprehensive observations were conducted to an altitude‐triggered lightning flash. Upward positive leader (UPL) and downward negative leader (DNL) in a bi‐directional development system were detected simultaneously by a high‐speed camera, together with the coordinated measurements of magnetic field and very‐high‐frequency (VHF) emissions. High‐speed images reveal, for the first time, the enhancement of the UPL's propagation speed by DNL in the bi‐directional leader system. Concretely, the upward positive leader initially originates from a suspended wire tip and propagates at a two dimensional (2D) speed of 5.32 × 104 m/s, and after about 6.3 ms, its propagation speed was enhanced to 1.12 × 105 m/s when the stepped DNL started advancing at an average speed of 1.44 × 105 m/s. Additionally, based on the evolution of channel luminosity and the variations of magnetic radiation, it is found that there is a consistency in luminosity variation between the ascending channel and the descending channel in the bi‐directional leader system, and the amplitude of the magnetic field increases when the negative discharges start at the bottom wire end with intensive VHF emissions. Those facts indicate that the DNL has an effect, may be a positive one, on the UPL's development in the early stage of altitude‐triggered lightning.

Comparison of Lightning Channel Luminosity Versus Time Profiles in the Infrared and Visible Ranges

AbstractInfrared (IR) luminosity of lightning channel in the 3–5 μm range usually persisted throughout the entire interstroke interval, which is in contrast to the simultaneously recorded visible (0.4–0.8 μm) luminosity that always decayed to an undetectable level prior to a subsequent return stroke pulse. A longer visible luminosity period at the end of flash tended to be associated with a longer IR afterglow period following the decay of visible luminosity (and by inference current) to an undetectable level. At the end of flash, the IR luminosity persisted up to about 1 s, and the median IR afterglow duration was a factor of 10 longer than the median visible luminosity duration. The IR luminosity often exhibited a hump when the visible luminosity was monotonically decaying or undetectable, with the corresponding channel temperature being likely around 3400 K.

Characteristics of negative breakdowns in extinguished channels of a positive cloud-to-ground flash

The characteristics of a natural positive cloud-to-ground lightning flash which had only one return stroke are investigated in detail using synchronous detections of lightning very-high-frequency (VHF) interferometer, fast antenna, high-speed video and S-band Doppler weather radar. The flash initiated at 5.8 km above the ground and at the edge of an enhancing convective cell, where a positive bi-level charge structure was involved. The initial negative leader developed upward, forming multiple branches, while the positive leader was recognized 22.2 ms later. The breakdown with different polarities exhibited distinct properties. Three significant negative breakdowns initiating in the previously established but extinguished negative channels were well detected by the VHF interferometer, with a duration of 0.30 ms, 0.06 ms and 0.20 ms, and propagation speed of 1.9 × 106 m/s, 2.2 × 107 m/s and 1.9 × 107 m/s, respectively. The first one occurred 0.3 ms before the return stroke, its occurrence promoted the development of the downward positive leader that was getting close to the ground, leading to an obvious acceleration and an enhancement of the channel luminosity. The second and the third negative breakdowns occurred during the continuing current after the return stroke, acting as the causation of the positive M-components. Symmetric V-shaped and hooked pulses on the fast electric field change waveform and obvious increased luminosity of the grounded channel were well detected. The occurrence of negative breakdowns in the pre-conditioned channels served for producing a backward surged current wave that promote the discharge process through the grounding/grounded channel.

Laboratory generated symmetrical-waveshape lightning current versus arc channel luminosity

Identifying a relation between current and channel luminosity is beneficial for producing an approximation to channel current via channel optical signal. In this paper, damped sinusoidal oscillation currents with more or less symmetrical waveshapes are injected into a tungsten copper hemispherical air gap and into a graphite rod air gap. Repeatability and reliability in channel luminosity measurement are compared between two air gap channels. Parametric correlation analyses, involving 12 sequences of lightning currents with a current peak from 13.0 kA to 34.4 kA, a current 10–90% risetime from 9.7 μs to 20.2 μs, and a current half-peak width from 21.6 μs to 46.4 μs, show the luminosity signatures also exhibit more or less symmetrical pulses, although they lag behind the corresponding current pulses. In initial primary pulses of these damped sinusoidal oscillations, the current peak is approximately direct proportional to the luminosity peak. Direct proportional relations are also observed between current 10–90% risetime and luminosity 10–90% risetime, between current half-peak width and luminosity half-peak width, as well as between current charge transfer and luminosity-time integration. These features are similar to those found in classical M-component current pulses associating with the M-component mode of charge transfer. Applicability of the results is further discussed to estimate the channel current from the channel luminosity.

Experimental investigation on correlation between fast-front lightning current and the resultant arc channel luminosity

Temporal variation of the luminosity of lightning channel can provide a proxy for the temporal variation of current flowing through it. In this study, seven groups of lightning currents with fast-front waveforms, spanning a 10–90% risetime range from 2.1 to 9.5 μs, a half-peak width range from 35.5 to 147.8 μs, and an amplitude range from 3 to 30 kA, are injected into a 15-mm air gap between a pair of graphite rod electrodes. An analysis is given of waveform associations and of waveform parameter correlations between current pulses and luminosity pulses emitted by the current-carrying channel. The luminosity signature is found to be generally divided into three stages: initial rising stage, initial fast decay stage, and later slow decay stage. During the later slow decay stage, the luminosity follows faithfully with the current (i.e., linearly correlated with current), whereas no such a feature is found in the previous two stages. The current peak is roughly, linearly correlated with the luminosity peak. A roughly linear relationship is found between the current risetime and the luminosity risetime, but no direct correlations are observed between their half-peak widths. In regard of optical signature, it is concluded that the laboratory-generated lightning currents, which are featured with relatively short front times and relatively slow decay after peak, are of similar nature as the return-stroke current in naturally-occurred lightning arc.

Relationship Between Brightness and Current of the Propagating Positive Leaders in Laboratory High Voltage Atmospheric Discharges

The discharge current of propagating lightning leaders is critical to understand lightning physics and to design lightning protection systems but almost impossible to be measured directly with present-day technology. In this paper, we have investigated the relationship between luminosity and discharge current of propagating positive leaders in laboratory high voltage atmospheric discharges. The continuously propagating positive leader channels were recorded by high-speed video frames. The brightness distribution of the propagating positive leader channel, which was evaluated by the gray value across the central line, obeyed a normal distribution. The mean grayscale pixel value of the propagating positive leader channel central lines obtained in high-speed video frames was compared with the average discharge current measured in the exposure duration of corresponding frames. There is a statistically significant linearity correlation between the discharge current and the channel luminosity of the propagating positive leader. The result further suggests a potential that the discharge current of propagating positive lightning leaders may be obtained based on the optical information observed in the time domain.

The origin of ball and bead lightning from an expanded lightning channel

It is shown how ball and bead lightning can originate from a narrow lightning channel which has been expanded by an ionizing wave. The resulting plasma structure development is followed for 10 s, and it is shown that the electric field developed can act as an electrostatic trap for negative burning particles. Previously it has been shown that a secondary lightning stroke current travelling down the expanded channel is necessary to explain the strength of the channel luminosity. This same secondary current can constrict the channel irregularly, via the magnetic pinch effect, to produce elongated segments of the channel; these become ball and bead lightning. All the plasma balls/beads will be “phantom plasma” structures unless some negatively-charged burning particles (fuel such as soil, copper wire or smoke) are trapped in the electric field distribution providing illumination.

Influence of slit alignment and slit width on the luminosity measurement of arc discharge channel

The luminosity of lightning channel is closely related to the lightning current. In the present paper, a lightning channel luminosity detection system (LCLDS), comprising of an avalanche photodiode module, a neutral density filter, an extension tube and an adjustable horizontal slit, is assembled. By using the impulse current generator, lightning impulse currents with different amplitudes and also different waveforms were produced in a 15-mm open-air gap between two graphite-rod electrodes. The luminosity and the current of discharge channel were measured synchronously. The experiments looking into the influencing factors of luminosity measurement, including the alignment of the silt and the slit width, were carried out. Obtained results show that the slit opening center of LCLDS should be aligned to the center of the discharge gap. Otherwise, in the case of improper alignment, the measured luminosity waveforms associated with the same injected current might be quite different in the amplitudes as well as in the waveforms. Moreover, the peak luminosity increases linearly with increasing slit width, whereas the luminosity waveforms are practically unaffected, implying that, in the practical application, the slit width can be adjusted to alter the luminosity signal output according to the actual need. Obtained results here can establish the foundation to develop the correlation between the current and the corresponding luminosity in the future.

Triggered lightning sky waves, return stroke modeling, and ionosphere effective height

AbstractGround waves and sky waves measured 209 km and 250 km south of six triggered lightning flashes containing 30 strokes that occurred in the half‐hour before sunset on 27 August 2015 are presented and analyzed. We use a cross‐correlation technique to find the ionospheric effective reflection height and compare our results to previous techniques that calculate effective height based on the time delay between ground wave and sky wave time domain features. From the first flash to the last flash there is, on average, a 1.6 km increase in effective ionospheric height, whereas no change in effective ionospheric height can be discerned along the individual strokes of a given flash. We show to what extent the triggered lightning radiation source can be described (using channel‐base current, channel geometry, and channel luminosity versus time and height) and speculate that a well‐characterized source could allow a more accurate determination of the electromagnetic fields radiated toward the ionosphere than has been done to date. We show that both channel geometry and the change in return stroke current amplitude and waveshape with channel height (inferred from measured channel luminosity versus height and time) determine the waveshape of the ground wave (and presumably the upward propagating wave that results in the sky wave) and that the waveshape of the ground wave does not appear to be related to the current versus time waveform measured at the channel base other than a roughly linear relationship between the two peak values.

High‐speed video observation of stepwise propagation of a natural upward positive leader

AbstractUsing a high‐speed video camera operated at 150,000 frames per second, we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in Beijing. The stepwise development of the upward positive leader was clearly revealed during its self‐sustained propagation toward the cloud base, providing the first optical evidence for the stepping processes of a positive leader. The leader developed with definitive intermittent pauses and re‐establishment with abrupt jump of the leader top. Obvious brush‐like corona zone emitting outward from the leader top was identified in the frame of stepping, and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping, indicating that a current wave was generated at the leader top and subsequently traveled down the channel. The channel luminosity decreased during the leader pause stage, with the corona zone dimmed correspondingly. The positive leader experienced an average interstep interval of 61.7 µs (ranging between 30 µs and 120 µs) and an average 2‐D speed of 8.1 × 104 m/s. The transient speed of the step jump was estimated to be larger than 7.3 × 105 m/s with an average step length of 4.9 m. The need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics.

Initiation and propagation of cloud-to-ground lightning observed with a high-speed video camera.

Complete evolution of a lightning discharge, from its initiation at an altitude of about 4 km to its ground attachment, was optically observed for the first time at the Lightning Observatory in Gainesville, Florida. The discharge developed during the late stage of a cloud flash and was initiated in a decayed branch of the latter. The initial channel section was intermittently illuminated for over 100 ms, until a bidirectionally extending channel (leader) was formed. During the bidirectional leader extension, the negative end exhibited optical and radio-frequency electromagnetic features expected for negative cloud-to-ground strokes developing in virgin air, while the positive end most of the time appeared to be inactive or showed intermittent channel luminosity enhancements. The development of positive end involved an abrupt creation of a 1-km long, relatively straight branch with a streamer corona burst at its far end. This 1-km jump appeared to occur in virgin air at a remarkably high effective speed of the order of 106 m/s. The positive end of the bidirectional leader connected to another bidirectional leader to form a larger bidirectional leader, whose negative end attached to the ground and produced a 36-kA return stroke.

Ground‐level observation of a terrestrial gamma ray flash initiated by a triggered lightning

AbstractWe report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude‐triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a ground‐level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.

Attachment process in subsequent strokes and residual channel luminosity between strokes of natural lightning

AbstractWe examined high‐speed video and electric field records of 23 subsequent strokes following the previous stroke channel in three natural negative lightning flashes, which were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Five strokes exhibited faintly luminous formations (FLFs) occurring in a single, pre‐return‐stroke frame and ranging from 130 to 908 m in length between the lower end of downward leader and the prospective strike point. The FLFs were inferred to be not streamers (as in first strokes) but manifestations of an increase in conduction current in the defunct channel between the leader and ground in response to the increasing electric field produced by the descending leader. Further, in eight strokes, we observed residual channel luminosity persisting over many frames (for 4.7 to 18 ms), through the pre‐return‐stroke frame. The residual luminosity was apparently associated with a stronger channel heating and a larger channel radius (and hence a lower temperature decay rate), both associated with the relatively long preceding continuing current. Presence of either FLF or residual channel luminosity did not appear to significantly influence the mode and velocity of propagation of the descending leader.

Lightning M-components with peak currents of kilo amperes and their mechanism

Characteristics of six M-components with peak currents of kilo amperes are analyzed by using the data from the artificially triggering lightning experiment in Shandong 2009, including the current at the base of the discharge channel, the surface electric field change at 30 m from the channel, and the high-speed camera records. The M-component is associated with the abrupt enhancement of channel luminosity with a duration of less than 1ms. Both the current and electric field are characteristic of relatively symmetric waveforms with a rise time of several tens of microseconds. The simultaneous records show that the electric field starts to change and reaches its peak earlier than the channel base current. Because of the obvious continuing current flowing in the channel, the conductivity of the discharge channel prior to the M-component is better than that prior to the leader-return strokes. The channel condition before the occurrence of the M-component is crucial to current amplitude and rise time. According to the waveforms of current and electric field recorded simultaneously, it is inferred that the M-component evolves from up to down and the evolvement continues after contacting the ground, and then an upward reflected process is induced. Further analysis shows that the interaction between these two processes is likely to be changed with altitude.

Lightning leader characteristics in the Thunderstorm Research International Program (TRIP)

We have used high speed streaking photographic techniques to time‐resolve the luminous compnents of cloud‐to‐ground lightning flashes. All recordings were made during our participation in the Thunderstorm Research International Program (TRIP), conducted at the Kennedy Space Center, Florida, during the summers of 1977 and 1978, and at the Langmuir Laboratory, near Socorro, New Mexico, during the summer of 1979. Twenty one dart leaders, four dart‐stepped leaders and three stepped leaders were recorded, the majority under daylight conditions. The mean two‐dimensional propagation speed of the dart leaders, evaluated over channel lengths less than or equal to 0.8 km above ground, is 11×106 m/s, with a range of 2.9 to 23×106 m/s. Several of the dart leaders reveal a decrease in propagation speed as ground is approached. However, four of the dart leaders in two separate flashes show an increase in speed near the ground, an observation not previously reported in the literature. In two multistroke flashes, we examine the variation of dart leader propagation speed along the channel and find very similar behavior for different strokes in the same flash. The speed variations that we observe may be predominantly caused by geometrical variations of the channel. The dart leader propagation speeds reported in this study are compared with the earlier works of Schonland, McEachron, and Kitagawa and Brook. Agreement among the studies is good, with a common range of observed dart leader propagation speed of 2 to 23×106 m/s. The major discrepancy among these studies is the observation, by Schonland, of a distribution of dart leader propagation speeds positively skewed toward the lower limit of reported values. Eleven of the dart leaders are analyzed at upper and lower levels along the visible channel to give 22 dart ‘lengths’. They range from 7 to 75 m with a mean of 34 m. For these 22 determinations, we calculate a correlation coefficient of 0.85 between the dart length and the dart leader propagation speed. The correlation of greater dart length with higher propagation speed is consistent with the slower decay of channel luminosity due to the greater initial input of energy to the channel by the faster and, presumably, more energetic dart leader. Four dart‐stepped leaders are examined in detail with regard to variation of propagation speed, step length, stepping interval, and luminous intensity during propagation between the cloud base and ground. Significant differences in the tendencies of these parameters are found within these four leaders. For example, one dart‐stepped leader recording shows a decreasing propagation speed and an increasing step interval near ground, whereas another shows the opposite behavior. In the best event recorded, several of the individual steps reveal a photographic film density structure, with the lower portion of the step exhibiting a distinct, bright tip that fans out into a symmetrically diffuse image in the upper portion of the step. Our analysis indicates that this spread in the upper portion of the step image is not the result of streaking photography distortion but, rather, represents the luminous structure of the step. We estimate that the step image is recorded in less than 1 µs. Consequently, with a measured step length of ∼20 m, the luminous pulse must propagate along the step at a speed of at least 2×107 m/s. The mean propagation speed for three stepped leaders is found to be 1.1×106 m/s. All three stepped leaders are very faint, and were recorded only in the last 100–200 m above ground. Two stepped leaders and one dart‐stepped leader do not propagate completely to ground before initiation of the return stroke. Apparently, these leaders are met by an upward propagating discharge at heights above ground of 20, 30, and 20 m, respectively. Other stepped and dart‐stepped leader cases are indeterminate because an obstacle or the horizon prevent the recording of the leaders near the ground. Connecting discharges are not observed for any of the dart leader events with a resolution of 10 m at 5 km, implying that upward discharges initiated by the approach of dart leader do not occur or are substantially less than a few tens of meters in length. Dart leaders, apparently, propagate completely to ground.