Stepped Leader Research Articles

On the processes leading to different modes of charge transfer in negative flashes according to fast videos and correlated records of current and electric field

This work discusses the processes involved in different modes of charge transfer in negative lightning flashes, taking as reference video-records obtained with a fast camera and records of current and close electric-field measured at Morro do Cachimbo Station. The data of two downward and one upward negative flashes are explored for considering the features of these processes and for supporting their interpretation, based on the bidirectional and bipolar leader approach. The processes comprise three sequences (stepped leader - first return stroke, dart leader - subsequent return stroke, dart-stepped leader - subsequent return stroke), in addition to the initial continuous current in upward lightning, return strokes following it, continuing current and M-components, the latter observed superimposed on both the continuing current and the tail of return stroke currents. An unreported process of charge transfer, consisting of the reignition of the dissipating leader of an upward lightning from the ground to the cloud is analyzed as well.

CRISIS LEADERSHIP: THE ART OF NAVIGATING ORGANIZATIONAL CHANGE DURING THE COVID-19 PANDEMIC

Organizational transformation is one of the steps leaders can take to survive and adapt to the current situation. Organizations that do not transform in a certain period will be left behind, even collapse, because of the dynamic nature of consumers and markets. Trans- formation is driven by internal factors such as the need for efficiency, revenue enhancement, and increasing employee prosperity. Meanwhile, external factors that trigger transformation include competition with competitors, technological advancement, and demand for adaptation to consumer needs and desires changes. Leaders have many references to adopt for navigating transformation in the normal situation. However, they have few references on navigating transformation in the Covid-19 pandemic situation. The Covid-19 pandemic has implied implementing a remote working system to minimize viral transmission. Such a system poses many challenges to leaders and employees as they have never practiced it. Failure of the leaders to navigate transformation will seriously harm the organization and even lead to collapse. The present study aims to identify some applicable competencies for leaders in navigating organizational transformation during the pandemic. This study used a qualitative method with semi-structured interviews as the data collection technique. The thematic analysis was conducted to analyze responses from interview questions. 14 informants were involved in this study. They consisted of general and senior managers transforming the pandemic situation. The results of this study revealed four major competencies, namely task competencies, human relation competencies, adaptive competencies, and virtual competencies in navigating transformation during the pandemic situation.

Transition in Optical and Radio Features During the Early Development of Negative Intracloud Leader

AbstractWhat happens when the upward in‐cloud (IC) breakdown transitions into horizontally expanding extension? Optical observations of this transition in IC lightning have not previously been reported. We identify the radio and optical signatures of this transition in low‐frequency (LF) magnetic, very high frequency interferometric and space‐borne optical measurements. For initial IC development in stratiform clouds, the ratio of 337/777.4 nm radiance is above unity prior to the transition but is almost always below unity after the transition. In particular, the 337 nm radiance drops significantly while the 777.4 nm radiance remains almost invariant after the transition, suggesting when the dominant illumination process of IC leaders changes from cold streamer discharges to a likely stepped leader. Furthermore, this transition in the LF and optical measurements resembles the reported changes in the cloud‐to‐ground measurements from initial leader to stepped leader, indicating that the initial leader may be physically different from the stepped leader.

New insights into the lightning discharge processes

This review covers selected results of recent observations of lightning discharges performed across the entire electromagnetic spectrum (radiofrequency, optical, and energetic radiation) at the Lightning Observatory in Gainesville, Florida. The most important results include (a) characterization of the preliminary-breakdown, stepped-leader, and return-stroke processes in high-intensity (⩾50 kA) negative lightning discharges, (b) the first high-speed video images of bidirectional leader that made contact with the ground and produced a return stroke, (c) discovery of negative stepped leader branches colliding with the lateral surface of neighboring branches of the same leader, (d) new data on the occurrence context and properties of compact intracloud discharges, and (e) observation of a terrestrial gamma-ray flash that occurred during a bipolar cloud-to-ground lightning discharge. The results serve to improve our understanding of the physics of lightning with important implications for lightning modeling, lightning protection, and high-energy atmospheric physics studies.

High-frequency radio-wave emission by coherent transition radiation of runaway electrons produced by lightning stepped leaders

Lightning can produce multiband radio waves and high-energy radiations. Some of them are associated with the formation of lightning leaders. However, their generation mechanisms are not fully understood yet. Based on the understanding of thermal runaway electrons generated at the leader tip, we propose transition radiation of these runaway electrons as an alternative mechanism for producing very-high-frequency radio signals. Transition radiations are induced when runaway electrons cross the interfaces between lightning coronas and the air. By the use of estimated parameters of electron beams emerging from the leader tips, we calculate their coherent transition radiation and find that the energy spectra and radiation powers are consistent with some detection results from stepped leaders and even narrow bipolar events. Moreover, our model also predicts strong THz radiation during the stepped-leader formation. As a standard diagnosis technique of electron bunches, the proposed coherent transition radiation here may be able to reconstruct the actual properties of electron beams in the leader tips, which remains an open question.

Leading with the (Recently) Successful? Performance Visibility and the Evolution of Risk Taking

It is claimed that by using delimited behavioral interventions, leaders can achieve strategic goals without actively leading. Yet leadership research has paid limited attention to the effectiveness of such behavioral techniques, despite the popularity of influencing individuals through nudges for example. We contribute towards filling this gap with a laboratory experiment. We study a prominent behavioral intervention to facilitate peer learning among followers, by leaders making successful or non-successful followers visible. Critically, our experiment provides a clear benchmark, where success is only due to luck and not due to skill. While prior research has provided mixed evidence whether individuals can discriminate between environmental and individual sources of success, our findings point to clear limitations. Even if learning from successful followers is obviously and transparently ineffective due to dominant environmental factors, the remaining followers do not adjust their choices. Absent a leader stepping in and interrupting the intervention, there is a persistent tendency to imitate lucky projects where nothing substantive can be learned. Our results suggest important take-aways for many rapidly changing business environments, where luck often dominates skill. In these contexts, passive leadership attempting to enlist peer learning can lead to more risk with no gain in expected returns. We discuss the wider implications of our findings for the possibility and limits of behavioral interventions used for passive leadership in general.

Separate luminous structures leading positive leader steps

The physics governing the propagation of lightning leaders and long spark leaders is still not well understood. Positive and negative leaders seem to behave differently. Negative leaders develop in a step manner, guided by the separate luminous structures termed space stems and space leaders. Positive leaders, on the other hand, are generally thought to have no separate luminous structure involved in their propagation. However, a separate luminous structure observed in a positive leader discharge had been reported in recent literature, suggesting that positive leaders may similarly do steps to negative leaders under certain conditions. Here we report the observation of the positive leader step led by a separate luminous structure at high humidity in laboratory lightning-like discharges. We also found the streamer-like common zone connecting the primary leader channel with the separate luminous structure, as well as the bi-directional development of the separate luminous structure. We hope that these findings would contribute to a better understanding of the nature underlying positive long spark leaders and lightning leaders.

On the Step Type of Continuous Propagating Positive Leader in Laboratory‐Scale Long Spark Discharges

AbstractPositive leader steps are currently not well understood. Many recent findings challenge the previous understanding of the positive leader steps occurring in continuous leader development. In this work, we investigate the stepping propagation of positive leaders in laboratory spark discharge by using a high‐speed video camera and a synchronized electrical parameter measurement system. Two types of steps were distinguished in the positive leader continuous development process based on the current pulse features. One type of step exhibits a bell‐shaped current pulse, while the other type of step exhibits a steep current rise. The former type vanishes when the rising rate of the ambient electric field increases, whereas the latter type, which could be led by a separate luminous structure, does not.

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.

Toward a better understanding of negative lightning stepped leaders

Negative lightning leaders observed at very close (within a few kilometers) distances often exhibit heavy branching with many active tips forming a network‐like structure with a descending multi-tip “ionization front” whose transverse dimensions are of the order of hundreds of meters. The presence of such front makes the lightning attachment process more complex than usually assumed. Negative leader branches extend in a step-like manner, with each step necessarily generating a traveling wave moving positive charge from the branch tip up along the channel, like a mini return stroke. The leader stepping process has a number of generally unrecognized consequences. Specifically, a stepped-leader branch tip can collide with the lateral surface of an adjacent branch (usually at an angle of about 90°). Further, the step‐related positive-charge waves propagating from the tip up along the channel can reactivate decayed negative branches at higher altitudes. Overall, a heavily-branched negative stepped leader creates a highly-structured and rapidly-changing electric field pattern inside and in the vicinity of the volume it occupies, which causes complex interactions between the branches.

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.

Application of a Modified Empirical Wavelet Transform Method in VLF/LF Lightning Electric Field Signals

In this paper, to realize a better adaptive method for the lightning electric field signal denoising, we firstly compared the decomposition results of three methods called the EMD (empirical mode decomposition), the CEEMDAN (complete ensemble empirical mode decomposition with adaptive noise), and the EWT (empirical wavelet transform) by artificial signals, respectively, and found that the EWT was better than the other two methods. Then, a MEWT (modified empirical wavelet transform) method based on the EWT was presented for processing the natural lightning signals data. By using our MEWT method, we processed three types of electric field signal data with different frequency bands radiated by the lightning step leader, the cloud pulse and the return stroke, respectively, and the VLF (very low frequency) lightning signals propagating different distances from 500 km to 3500 km, by using the data of the fast electric field change sensors from Nanjing Lightning Location Network (NLLN) in 2018 and the data of the fast electric field change sensors and the VLF electric antennas from the NUIST Wide-range Lightning Location System (NWLLS) in 2021. The results showed that our presented MEWT method could adaptively process different lightning signal data with different frequencies from the step leader, the cloud pulse, and the return stroke; for the lightning VLF signal data from 500 km to 3500 km, the MEWT also achieved a better noise reduction effect. After denoising the signal by using our MEWT, the detection ability of the fast electric field change sensor was improved, and more weak lightning signals could be identified.

Observation of preliminary breakdown and leader propagation characteristics in a negative cloud to ground flash with two strike points

High-speed video and electric field data were used to analyze a preliminary breakdown process and three different types of leader processes in a negative cloud to ground flash with two strike points. The preliminary leader was multi-branched and significantly brighter than the stepped leader, with the speed varying from 1.4 × 106 m/s to 6.4 × 105 m/s in a decreasing trend. Two types of chaotic leaders were proposed and compared. Type 1 chaotic leaders last for approximately 0.3 ms, with speed greater than 107 m/s, and no change in leader type throughout the whole leader process. Type 2 chaotic leaders with a duration of approximately 0.5 ms and speed on the order of 106 m/s transformed into a dart-stepped leader with a continuously decreasing speed in the leader process, and the pulse amplitude of chaotic leaders tended to decrease during the transition. The attenuation of the pulse amplitude in electric field (E-field) and the insufficient amount of charge may be responsible for such a conversion.

The Estimation of Lightning Impulses Superimposed onto Pre-Stressed DC Breakdown Voltages Using the Leader Propagation Method

The impulse breakdown attributes are of rudimentary importance in the insulation coordination in the gas-insulated switchgear (GIS). The critical voltage waveshape for the insulation is the surge over voltages. Usually, such over-voltages will emerge on the electrical equipment while these are burdened under DC or AC working voltages. Therefore, during relatively small-time intervals, the undermined devices are exposed to surge voltages that are superimposed upon the already running DC or AC voltages. From a safety point of view, and for providing reliable power to the consumer, we need to study the breakdown characteristics under such over-voltages, with reference to normal operating voltages (DC or AC). The breakdown mechanism of the GIS under compressed SF6 gas is known to be controlled by a stepped leader propagation method. Although different experimental studies have been conducted by many researchers for different experimental conditions under DC, AC, or impulse high voltages alone, not much research has been performed, in a systematic way, to model and estimate the breakdown voltages of the GIS under such superimposed conditions. This paper presents a systematic model, using a leader propagation technique for the estimation of breakdown voltages for complex voltage conditions, i.e., a lightning impulse (LI) superimposed on pre-stressed DC for different experimental conditions. The estimated values are in good agreement with the measured experimental results.

Microwave radiation associated with stepped leaders of negative cloud-to-ground flashes

In this paper, Very-High Frequency (VHF) and microwave radiation pulses associated with stepped leader pulses (conventional breakdown) are examined. A total of 100 stepped leader pulses (SLPs) with the associated 100 quiet periods (QPs) from ten negative cloud-to-ground (CG) flashes within the reversal distance (<8 km) have been analyzed. The QP is defined as the period between two SLPs when no electrical activity within the fast antenna sensor bandwidth was detected. The measurement system is made up of fast antenna (FA) and slow antenna (SA) sensors, a VHF sensor (60 MHz center frequency with 40 MHz bandwidth), and a microwave sensor (0.97 GHz center frequency with 20 MHz bandwidth). The waveforms were sampled at 2.5 GHz (400 ps). The total durations of the SLPs and QPs were 2.648 ± 1.152 μs and 2.708 ± 1.670 μs, respectively. All microwave and VHF radiation waveforms have been detected as clear individual oscillating pulses. The key finding is that the microwave radiation can be classified into three categories based on the total number of detected individual oscillating pulses during the QP. Two microwave pulses detected during the QP (Category 1) are suggested to be associated with the electron avalanche/corona process at the tip of an existing negative leader and a space stem. The VHF pulses associated with the microwave pulses are suggested to be emitted by propagating streamers. On the other hand, a single microwave pulse detected during the QP (Category 2) is suggested to be emitted by the electron avalanche/corona process of a space stem. The VHF pulses associated with the microwave pulse are suggested to be emitted by propagating streamers. In both categories, 70% of the microwave radiation pulses have been observed to precede VHF radiation pulses with an average lead time of 0.848 ± 1.336 μs. The head-on collision of a downward propagating negative streamer and an upward propagating positive streamer (from space stem) emitted significant microwave radiation. A new negative leader was formed when the head-on collision took place. The microwave radiation pulses emitted by the head-on collision process were observed to precede SLPs with an average lead time of 0.423 ± 0.378 μs. Around 80% of the microwave radiation pulses preceded VHF radiation pulses with average lead time of 0.540 ± 0.596 μs. Therefore, microwave and VHF radiation pulses are suggested to be emitted by different processes of the electrical breakdown mechanism. The microwave radiation is emitted by electron avalanche/corona/head-on collision while the VHF radiation is emitted by propagating streamers.

Stemming the rising tide of anger in the workplace: how to build emotion control among employees before anger spills out

PurposeThe combined impact of the pandemic together with political and social disruption has led to a rising tide of anger that is impacting the workplace. The purpose of this article is to trace the range and depth of anger among employed adults in 2021 and to identify three practical ways for leaders to reduce the risk of anger in their workforce.Design/methodology/approachWeb-based survey of 5,475 employed adults aged 18 and older.FindingsAcross the entire sample, 21% of employees report getting angry more easily or more often lately. While there was no significant difference between men and women, younger workers (younger than 30) were 1.4× more likely (31% for <30 vs 20% for 30+) to report getting angry more easily or more often. Managers were slightly more likely to report increases in anger compared to individual contributors (23% vs 21%).Originality/valueWhile leaders cannot alter the root causes of the rising tide of anger, they can equip the workforce with the emotional and behavioral skills to improve self-management. Three practical steps for leaders are offered: acknowledge, assess and understand the problem by integrating data that helps identify the scope, scale and root cause of the challenges and frustrations the workforce is facing; adopt innovative strategies proven to support mental health; and empower a more resilient workforce by providing the tools and techniques that improve coping skills.

Computation of locations of possible stepped leader attachment points on an aircraft flying under thunderstorm conditions

In case of flight in a thunderstorm region, a lightning strike on the aircraft might occur. For better lightning protection, the locations of points on the aircraft skin where a stepped leader can be attached should be delimited. In this paper, the areas of the skin of an aircraft where a stepped leader might be attached are defined through an approach based on the fact that electrostatics and potential flow theory have the same mathematical background. This is done by computing the distribution of the electrostatic field on the metallic parts of the skin of the aircraft due to the ambient electric field. In any area of the skin, where the electric field resulting from the combination of the ambient electric field and the local electric field can ionize the air, attachment points of a stepped leader might exist. The advantage of this approach is a decreased computational time and computer memory needed, as compared to a full Computational Physics approach. The areas of the skin of a generic airliner delimited through this approach, at least qualitatively, are in agreement with photos taken under real thunderstorm conditions.

Toward a Better Understanding of Negative Lightning Stepped Leaders

Toward a Better Understanding of Negative Lightning Stepped Leaders

Triggered Negative Lightning‐Leaders That Propagated Into Thunderstorm Lower Positive Charge

AbstractWhen the electric field below a thunderstorm or other electrified cloud is around 10 kV/m, it is sometimes possible to initiate (“trigger”) an upward‐propagating lightning‐leader by launching a rocket that uncoils a wire from the ground. The triggered leader propagates upward from the tip of the wire lifted by the rocket. When the channel is hot enough, a flash is visible. Triggering is common when the leader carries positive charge, but not when it carries negative charge. This article is about four flashes consisting of triggered negative leaders that branched into low‐altitude regions of positive cloud charge over Langmuir Laboratory in central New Mexico. Measurements of current and the locations of leader channels are available for three of the four flashes. Some current pulses at the ground for Flash 2 originated at negative leader steps more than 3 km away, which is a greater distance than has been reported from video measurements. Flashes 3 and 4 propagated only into thunderstorm lower positive charge, and the average lightning‐charge densities inside the volumes occupied by these two flashes are remarkably close. Our best estimate of density for Flashes 3 and 4 lies between −4.2 and −1.8 C/km3, which is compatible with the large spread in cloud‐charge densities derived from instruments carried on airplanes or balloons into low positive regions in thunderstorms.

Creating a safe space to speak up.

Sam Foster, Chief Nurse, Oxford University Hospitals, considers why it is so important to create environments where staff can raise concerns, and steps leaders can take to build these.