Upward Positive Leader Research Articles

Attachment processes of negative flashes with multiple return strokes to a tall tower observed at close distances

Two downward negative flashes (F1612 & F1613) with multiple return strokes struck to the 356-m Shenzhen Tower were analyzed. The high-speed camera captured upward positive connecting leaders (UCL) for the first strokes of both flashes and two subsequent strokes of F1612. The observations also show that none of the downward negative leader (DNL) completely propagated to the tower tip before the occurrence of the return stroke (RS), so that we inferred that the UCL initiated from the tower tip might exist for each RS of the two flashes. The possible height of the UCL and DNL connecting point above the tower tip for each RS was estimated, which showed a positive correlation to the speed of corresponding DNL. The striking distances estimated for 8 subsequent RSs in F1612 were in a range from 14.6 m to 85.4 m and that for 3 subsequent RSs in F1613 were from 24.2 m to 66.2 m. There was a weak positive linear relationship between the DNL speed and striking distance for subsequent RSs in the two flashes. The DNL speed showed a positive correlation to the peak current of corresponding RS.

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.

FDTD Simulation of a Small‐Scale Charged Airplane Model in an Ambient Electric Field between Two Flat Electrodes

When an airplane flies in an electric field under a thundercloud, electric fields at the edges and projected portions of the airplane are enhanced and leaders emanate from there. When a positive leader emanating upward from the airplane connects with a downward negative leader from the bottom of an ordinary thundercloud and a negative leader emanating downward from the airplane connects with an upward positive leader from the ground, a large lightning current flows along the channel bridged between the thundercloud and the ground through the airplane. Recently, a method to reduce the risk of lightning strikes to airplanes has been proposed. It controls the charge on the surface of an airplane to suppress the electric field at edges and projected portions of the airplane. In this paper, an airplane under a thundercloud is represented with a vertical conducting bar or a horizontal conducting bar with a small projected portion, which is placed between impulse‐high‐voltage‐applied two flat electrodes, and it is analyzed using the finite‐difference time‐domain (FDTD) method. Corona and leader discharges emanating from edges and projected portions of an airplane model are considered with their engineering representations: 40‐μS/m and 0.02 S/m conducting regions for corona and leader discharges, respectively. The airplane model is not pre‐charged or pre‐charged negatively. It follows from the FDTD‐computed results that pre‐charging an airplane model with a relevant amount of negative charge can avoid discharges from and to the airplane model. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Low Frequency Magnetic Field Observations of Natural Positive Leaders Featuring Stepwise Propagation

AbstractWe examine the low‐frequency (LF) magnetic pulses recorded for the positive leader in two natural lightning flashes as recorded on the high‐speed video camera. The captured positive leader in both cases propagated in a stepwise manner at least over several milliseconds according to the concurrent sequence of LF magnetic pulses. By comparing with previous studies on positive leaders under different situations, the average inter‐pulse intervals (11.7 and 20.3 μs, respectively) obtained for our natural cases are shorter than that of upward positive leaders in rocket‐triggered (typically >25 μs) and tower‐initiated lightning (62 μs in one case). Hence, the stepwise propagation of positive leaders in natural lightning is likely more frequent than that in object‐induced lightning, reflecting a possible influence of ambient electric field and other meteorological factors on the stepwise feature of positive leader. The stepwise extension might be common for positive leaders developing in various scenarios.

Polarity Asymmetries in Rocket‐Triggered Lightning

AbstractThe dissonant development of positive and negative lightning leaders is a central question in atmospheric electricity. It is also the likely root cause of other reported asymmetries between positive and negative lightning flashes, including the ones regarding: stroke multiplicity, recoil activity, leader velocities, and emission of energetic radiation. In an effort to contrast lightning leaders of different polarities, we highlight the staggering differences between two rocket‐triggered lightning flashes. The flash beginning with upward positive leaders exhibits an initial continuous current stage followed by multiple sequences of dart leaders and return strokes. On the other, in its opposite‐polarity counterpart, the upward development of negative leaders is by itself the entire flash. As a result, the flash with negative leaders is faster, briefer, transfers less charge to the ground, has lower currents, and smaller spatial extent. We conclude by presenting a discussion on the three fundamental leader propagation modes.

Observation of an upward lightning flash with 21 upward positive leaders initiated from different wind turbines in wind farm

AbstractAs the wind turbine height increases, the probability of multiple upward leaders flashes within the wind farm increases. In this study, an upward lightning flash containing 21 upward positive leaders (UPLs) initiated from different wind turbines in Zhangbei Wind Farm was analysed. The average speed of the UPLs was 1.2 × 105 m/s, which mostly decreased to zero before reaching the cloud height. Only four UPLs developed to cloud height, with prolonged periods of bright light on three of them and with a minimum duration of 126 ms. A total of nine return strokes occurred on the other two dissipated leader channels, and the numbers of strokes on the two channels was six and three, respectively. The average interstroke interval for the return strokes was 10.5 ms. Most of these nine return strokes had peak currents smaller than 10 kA. The high‐speed video frames confirmed that competition does exist between the UPLs, and the range of shielding effects increases as the height of the ULs increases. The effect of fan rotation on the initiation of upward leaders and the shielding effect between the wind turbines and upward leaders are discussed.

Ionization Activity Detected During Dark Periods in Long Air Positive Sparks

AbstractThe dark period in long air gap discharges has been assumed in the literature as the time between consecutive streamer current pulses when ionization and luminosity are absent. These dark periods are also present in natural lightning, in processes such as the inception and propagation of upward positive leaders, the development of needles, as well as transient luminous events in the upper atmosphere. Only recently, faint luminosity has been observed during dark periods, challenging this assumption. This paper aims to study any possible electrical activity during dark periods by means of experiments supported by computer simulation. Therefore, an experimental platform, including low‐current measurements, Schlieren and standard photography as well as ultraviolet (UV)‐photon detection was used to observe the electrical‐optical‐thermal characteristics of the dark period. A complementary numerical model was used to estimate the streamer space charge spatial distribution and its drift during dark periods. It is found that faint UV and visible light during the dark period is emitted exactly at the location of the low‐density streamer stem channel. This process is accompanied by the generation of an electronic current in the order of hundred microamps to milliamps. The simulation results show that this ionization activity occurs due to strong reduced electric fields in the residual stem channel above 112 Td, which is mainly determined by a combination of applied voltage, space charge distribution, and localized heating. Thus, the presented results show that there is indeed ionization activity during dark periods in long air gaps, which maintains a continuous glow‐like discharge.

Observation and Modeling of Dart Leader Development in an Altitude‐Triggered Lightning Flash

AbstractThe dart leader development process of an altitude‐triggered lightning flash with eight return strokes was analyzed. The analysis, based on the high‐speed camera and electric field data, revealed three different processes of dart leaders: (a) normal dart leader process; (b) dart leader process with chasing leader; and (c) dart leader process with bidirectional leader. These dart leaders are very different in terms of the average 2‐D speed and the electric field change. Moreover, in the bidirectional leader process, the 2‐D speed of the upward positive leader is greater than that of the downward negative leader. A piecewise‐speed source‐charge model was used to simulate the electric field change of the normal dart leader. The superposition of downward positive leader was used to simulate the fading and decaying process of the leader tail, which appeared in the chasing leader and bidirectional leader. The bidirectional leader model was used to simulate the bidirectional leader process, and the final simulation results are in good agreement with the measured electric field.

Upward Bipolar Lightning Flashes Originated From the Connection of Recoil Leaders With Intracloud Lightning

AbstractThe present work shows high‐speed videos of two upward flashes that started with positive upward leaders and, instead of being followed by negative subsequent return strokes, they were followed by positive subsequent return strokes. In both cases, after the positive leaders developed, recoil leaders (RL) appeared in their decayed branches as would be usual in negative upward lightning flashes. However, in these flashes the negative end of a recoil leader connected to a positive leader of an intracloud (IC) flash nearby. The connection initiated a downward positive leader that re‐ionized the decayed channel of the upward flash all the way to the tower giving origin to a positive subsequent return stroke. This work shows that RL do play an important role in the occurrence of bipolar upward flashes and their interaction with IC flashes can provide explanations for all types of bipolar upward flashes initiated by upward positive leaders.

First Measurements of Low‐Medium Frequency Magnetic Radiation for an Altitude‐Triggered Lightning Flash

AbstractAn altitude‐triggered lightning flash was conducted on the Field Experiment Base on Lightning Sciences, China Meteorological Administration, in the summer of 2021. The low‐medium frequency magnetic field (B‐field) of altitude‐triggered lightning was measured for the first time with a sensitive magnetic sensor. The B‐field waveforms of precursors and sustained upward positive leader (UPL) show the obvious damped‐oscillatory waveform, which is caused by propagation losses and reflections of lightning current at both extremities of steel wire. The propagation of UPL during the inception of attempted downward negative leader (DNL) is revealed by B‐field signals with microsecond resolution, showing that the bidirectional leaders develop in an uncoordinated manner. The sustained DNL occurs about 5.5 ms after the inception of sustained UPL in our observations, and the measurements indicate that the stepping of DNL was much more frequent than that of UPL during the stage of sustained propagation.

The Physical Experimental Modelling of the Formation Processes of Upward Discharges from Grounded Objects in the Artificial Thunderstorm Cell’s Electric Field

The results of the physical modelling of the formation processes of upward discharges from grounded objects in the artificial thunderstorm cell’s electric field are presented. We established the considerable influence of the electrode tip’s radius on the pulse streamer corona stem’s parameters and, subsequently, on the probability of the transformation of the impulse streamer corona first flash’s stem into a first stage of upward leader. We determined the diapason of the optimal tip radii for a lightning rod or lightning conductor, which allows for the most probable formation of the first impulse streamer corona, with the parameters providing the best conditions for the upward leader’s start, the purpose of which is the lowering of the probability of lightning striking the object under protection. A considerable difference between the electrical characteristics of the first impulse corona flash with and without the streamer–leader transition was established. It was shown that the amplitude of the streamer corona flash current impulse is considerable, but not the main defining factor of the streamer–leader transition. It is established that the charge value of the streamer corona first flash is not a threshold requirement for the formation of the upward leader from a ground object, but only defines the probability of the successful upward leader formation. Based on the analysis of the experimental data received, we suggest that there is a dependency between the probability of upward positive leader formation from the grounded objects and the charge value of the first pulse streamer corona flash for the rod (centered) and rope (elongated) lightning conductors and objects in the electric field of the thundercloud and downward lightning leader. The obtained results can be used for mathematical modelling of the formation processes of upward discharges from grounded objects in the artificial thunderstorm’s electric field, as in a natural thunderstorm situation.

A numerical analysis model for inception and propagation of positive upward leader from UHV DC transmission lines

As the upward leader from the transmission line plays a vital role in the interception of lightning downward leader, the proper description of upward leader characteristics is beneficial for improving the evaluation accuracy of lightning performance for UHV DC transmission lines. Based on the leader discharge theory, a novel numerical analysis model to simulate the inception and propagation process of positive upward leader from the transmission line was proposed. The model-based calculation results show good agreement with the data of laboratory long spark discharge, natural lightning observations and operation experience of transmission lines, which verifies the validity of this model. Moreover, the influence of operating voltage, protection angle and tower height on the upward leader characteristics for the typical UHV DC transmission line was analyzed. The results show that the lightning interception ability of upward leader from the positive polar conductor is enhanced with the increase of voltage level, protection angle, and tower height, which theoretically illustrates that installing the positive polar conductor closer to hillsides, adopting the negative protection angle and reducing the tower height are effective lightning protection measures for UHV DC transmission lines.

Two successive bidirectional leaders propagated in triggered lightning channel

A stepped leader propagated along the previous return-stroke channel in triggered lightning. After the stepped leader decayed, the first bidirectional leader went through the process of initiation, propagation and dissipation. Then the second bidirectional leader initiated at the termination of the decayed first bidirectional leader and propagated toward the ground, generating the fourth return-stroke. The observations were synchronously performed through a high-speed camera and electromagnetic field measurements. The first bidirectional leader was characterized by similar average upward and downward velocities of 0.76 × 106 m/s and 0.67 × 106 m/s. The velocity of the upward positive leader of the first bidirectional leader was noticeably fluctuated, ranging 0.39 × 106–1.78 × 106 m/s. The second bidirectional leader was characterized by a sustainable propagating upward end with an average velocity of 1.82 × 106 m/s. The velocity fluctuation trend of the upward end depends on the neutralization amount of the residual negative charge and the positive charge in UPL.

Observation of five types of leaders contained in a negative triggered lightning

A negative triggered lightning involving five types of leaders was recorded by high-speed camera using frame rate of 20,000 fps and fast antennas at different distances. Five types of leaders contained one upward positive stepped leader, one upward positive dart leader, ten downward negative dart leader, one bidirectional leader and three downward negative dart-stepped leaders were propagated successively in the same channel. The upward positive dart leader occurred after initial continuous current pulse with average 2-D speed of 1.40 × 106 m/s and started second continuous current process. The bidirectional leader was transformed from decaying unidirectional leader and showed the unique electric field changes. Faster return strokes are found to be induced by downward leaders propagating evenly and deposit more positive charge in the following residual channel. The positive charge can inhibit the potential initiation of an upward positive leader and boost the propagation of the next downward negative leader.

Close Observation of the Evolution Process during Initial Stage of Triggered Lightning Based on Continuous Interferometer

The discharge signal in the initial stage of lightning is weak. The revelation of the discharge mechanism at this stage depends especially on close observation. In this study, a continuous interferometer (CINTF) was used to observe the initial stage of the upward positive leader (UPL) of the triggered lightning in Conghua District, Guangzhou City, Guangdong Province. The positioning error of CINTF for a close-range radiation source was analyzed, and the positioning error calibration method of CINTF for a specific close-range radiation source was studied, which improved the observation accuracy of elevation angle at the initial stage of the UPL of the triggered lightning. With the rise of the rocket, the positioning error in altitude during the initial stage of the UPL increased obviously. Under the layout condition of the Guangzhou field experiment site for lightning research, when the positioning results of the elevation angle of the initial stage of the UPL were 40°, 50°, and 60°, respectively, the calibrated altitude positioning error could be reduced by about 11 m, 14 m, and 20 m, respectively. On the basis of the calibrated observation results, the evolution characteristics of the initial stage of the UPL were studied, and its discharge mechanism was revealed. The precursor current pulse (PCP) was generated by a weak upward positive breakdown and a subsequent strong downward negative breakdown near the rising rocket tip, which was in the form of a single pulse. The precursor current pulse cluster (PCP cluster) and initial precursor current pulse cluster (IPCP) were both signs of self-sustaining development of the UPL. After the PCP cluster, self-sustaining development stopped immediately. The self-sustaining development after IPCP could be short-term or continuous.

In‐Cloud Discharge of Positive Cloud‐To‐Ground Lightning and Its Influence on the Initiation of Tower‐Initiated Upward Lightning

AbstractWe conducted a comprehensive observation of tower‐initiated upward lightning (UL) during the summer season from 2012 to 2020, using high‐speed video and the Beijing Broadband Lightning Network (BLNET). A total of 25 UL flashes were observed, and 21 of them were triggered by nearby positive cloud‐to‐ground (CG) lightning. It is interesting to note that eight (38%) of the parent +CG lightning flashes have more than one return stroke (RS). The distance between the UL and the grounding point of its parent +CG lightning was 4–38 km, where nine (43%) of the UL were triggered less than 2 ms after the positive RS, and 11 (52%) of them were triggered after a long time delay of 5–187 ms. Based on high‐speed video and mapping results, the short time delay was associated with the reactivation of the decayed established leader above the tower. The long time delay was associated with the negative leader extending to the region above the tower during the long continuing current. The positive E‐field change during RS or continuing current period favored the inception of an upward leader. Although both preliminary breakdown (PB) and RS in a +CG flash produced positive E‐field changes, an upward positive leader induced by PB aborted its sustained propagation after +RS.

Electromagnetic Characteristics of Upward Leader Initiated From the Canton Tower: A Comparison With Rocket‐Triggered Lightning

AbstractBy using the synchronous observation data obtained at the Tall‐Object Lightning Observatory in Guangzhou (TOLOG) of 2019, the electromagnetic characteristics of upward positive leader (UPL) ascending from the 600‐m high Canton Tower are examined, and are compared with the magnetic field (B‐field) radiation of UPL in rocket‐triggered lightning. Before the inception of sustained UPL, small electric field (E‐field) pulses are superposed on the fast E‐field changes. The timescale of B‐field pulses corresponding to the E‐field pulses ranges from 5 to 9 μs, and the inter‐pulse interval is about 30 μs, which are both similar to the B‐field pulses associated with the precursors of rocket‐triggered lightning. Measurements show that the precursor‐like stage is likely common for upward lightning initiated from the Canton Tower. Moreover, the UPL channel of tower‐initiated upward lightning extends significantly in the first several ms with the initial average two‐dimensional (2‐D) velocity of 8.77 × 105 m/s (3‐D velocity of 11.0 × 105 m/s), which is one order of magnitude faster than the UPL initiated from the wire tip of rocket‐triggered lightning, indicating that initiation of UPL from Canton Tower benefits from a substantial E‐field enhancement of nearby lightning discharges. However, this favorable condition is rapidly consumed during the UPL development, causing the average 2‐D velocity of UPL to decrease rapidly and maintain at about 0.5 × 105 m/s. It is noted that the variation in the 2‐D speed of sustained UPL differs from tower‐initiated upward lightning reported in the literature, which is possibly associated with the physical and geometric properties of the tower.

On the Transition From Precursors to the Initial Upward Positive Leader in Negative Rocket‐Triggered Lightning

AbstractPrecursory current pulses (precursors) and initial upward leader pulses are examined to characterize the charge transfer of precursors and the sustained upward leader during the early stage of rocket‐triggered lightning. According to the analysis of six negative triggered flashes, it is found that both precursors and the initial upward positive leader (iUPL) pulse are led by a small deflection that appears at about 25 μs prior to the major pulses. The characteristics of precursors and iUPL pulses are very similar except that the precursors generally transfer less electric charge (35.07 μC on average) than iUPL pulses do (64.73 μC on average). Some marginal differences in the parameters (e.g., rise time, duration) may be attributed to the modification of the current pulses caused by the different impedances of the leader channel and the steel wire. The ascending rocket causes the enhancement of electric field (E‐field) in the close vicinity of the wire tip, and therefore the breakdown of the iUPL is initiated by a stronger E‐field than that of the precursors. The step lengths of the precursors (defined as the extended length of the wire tip lifted by the rocket between two adjacent isolated precursors) are similar to that of iUPL pulses. Our analyses indicate that the transition from precursors to the iUPL occurs when the E‐field around the wire tip is strong enough, by the accumulation of more positive charge, to launch a meter‐scale breakdown and the ensuing charge transfer to ground.

A Distinct Class of High Peak‐Current Lightning Pulses Over Mountainous Terrain in Thunderstorms

AbstractCloud‐to‐ground strokes, narrow bipolar events, and energetic in‐cloud pulses are known classes of high peak‐current lightning processes that occur in thunderstorms. Here, we report one more distinct class of high peak‐current events observed exclusively over mountainous terrain, usually above 2,000 m altitude, in the continental Unites States. These events, which we call mountain‐top energetic pulses (MEPs), are bipolar pulses with negative radiated field polarities. MEPs are generated between the high mountain tops and compact overhead thunderclouds. Evidence supports the hypothesis that MEPs are produced by terrain‐initiated upward positive leaders propagating in high electric fields due to the proximity of the low negative charge regions of the thunderstorms. This scenario further suggests the possibility that MEPs are associated with downward terrestrial gamma‐ray flashes, and their high peak currents imply that they may produce elves.

Differences Between Flashes With and Without Return Strokes in Rocket‐Triggered Lightning

AbstractBased on simultaneous observation of currents and electric field changes, the differences have been identified between flashes with and without return strokes in rocket‐triggered lightning. The triggered lightning events without return stroke have an obvious no‐current period up to some milliseconds before the current flow is reestablished, then a relatively large amplitude (1.2 kA or so) and fast risetime (0.7 μs or less) pulse similar to those of the return stroke current are reestablished between the upward positive leader and the ground. The triggered lightning events with return strokes have no no‐current period in the initial current variation, and the current pulse of wire destruction and plasma channel reestablishment process is similar to the M‐component pulse, featuring a small amplitude (0.17 kA or so) and slow risetime (57 μs or more). The stronger initial stage is usually followed by return strokes.