Griffiths and Phelps Lightning Initiation Model, Revisited

Tilles J.N., Liu N., Stanley M.A., Krehbiel P.R., Rison W., Stock M.G., Dwyer J.R., Brown R., Wilson J.

Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown. Recent discoveries, such as sprites, jets, terrestrial gamma ray flashes, and fast positive breakdown, highlight the diversity of complex phenomena that thunderstorms can produce, and point to the possibility for electrical breakdown/discharge mechanisms beyond dielectric breakdown theory based mainly on laboratory experiments. Here we present one such confounding discovery, termed fast negative breakdown, that does not fit with our current understanding of dielectric breakdown. Our adaptation of radio astronomy imaging techniques to study extremely transient lightning-associated events confirms that electrical breakdown in thunderstorms can begin with oppositely-directed fast breakdown of negative polarity, similar and in addition to fast positive breakdown expected from conventional dielectric theory and recent observations. The discovery of fast negative breakdown calls for an addendum to the physical description of electrical discharges in air. Recent studies have shown that lightning is initiated by a newly-recognized discharge process called fast positive breakdown. Here, the authors present observational evidence of fast breakdown but of negative polarity, seemingly contrary to current understanding of discharge physics.

Abbasi R.U., Abu‐Zayyad T., Allen M., Barcikowski E., Belz J.W., Bergman D.R., Blake S.A., Byrne M., Cady R., Cheon B.G., Chiba J., Chikawa M., Fujii T., Fukushima M., Furlich G., et. al.

Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$\mu$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.

Silva C.L., Merrill R.A., Pasko V.P.

The initial stage of in-cloud lightning development is characterized by a series of initial breakdown pulses (IBPs), observed as abrupt electric field changes at a remote sensor. Recent studies have attributed this process to the stepwise elongation of a negative lightning leader channel and used transmission line (TL) models to interpret the observed electromagnetic emission. In these models a current pulse is injected at the base of the channel, propagates along it, and the current parameters are adjusted to fit the measured IBPs. In this paper we explore the limitations of TL models by comparing four of its variants: the classic TL (with no attenuation) and three modified transmission line models that have different current attenuation characteristics, i.e., following linear, exponential, and Gaussian functions. For a compact channel (less than a few hundred meters long) all four models tend to produce similar electromagnetic signatures, and nearly identical waveforms can be obtained by simply varying the channel length. It is impossible to simultaneously identify, with confidence, both channel length and the speed of current wave propagation at the source by matching a far-field waveform. The field-to-current conversion factor for in-cloud sources is not a constant and can vary by as much as 2 orders of magnitude depending on channel length and current pulse risetime. This conclusion contrasts with the assumption used by the National Lightning Detection Network, that the field-to-current conversion can be performed by a multiplicative constant, as it is done for return strokes in cloud-to-ground lightning.

Rison W., Krehbiel P.R., Stock M.G., Edens H.E., Shao X., Thomas R.J., Stanley M.A., Zhang Y.

A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown. How lightning is initiated inside storms has been a long-standing and fundamental question. Here, the authors report observations of a previously unrecognized type of discharge, called fast positive breakdown, that is found to initiate many and potentially all lightning discharges in storms.

Milikh G.M., Likhanskii A.V., Shneider M.N., Raina A., George A.

Formation of the streamer zone of a leader is an outstanding problem in the physics of electric discharges which is relevant to laboratory leaders, as well as to the leaders formed by lightning. Despite substantial progress in the theoretical understanding of this complicated phenomenon, significant puzzles, such as the low propagation velocity of a leader compared to the fast streamers, remain. The objective of this paper is to present 2-D plasma simulations of the formation and propagation of the streamer zone of a leader. In these simulations we will generate a group of streamers that propagate in a discharge gap while interacting with each other. It is shown that interaction between the streamers significantly reduces their propagation velocity. This explains why the leader, which consists of many streamers, is much slower than a single streamer formed in the same discharge gap. It is shown that the mean velocity suppression of the group of streamers is determined by the inter-streamer distance. The critical value of the packing factor of the streamers at which the interactions between them can be neglected, and thus the discussed process can be treated as caused by a single streamer, is obtained.

Lyu F., Cummer S.A., McTague L.

We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightning events with very high peak currents: the well-known narrow bipolar events, and a previously unreported type that we call energetic in-cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicate a likely connection between the two, which further suggests the possibility of downward directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.

da Silva C.L., Pasko V.P.

To date the true nature of initial breakdown pulses (IBPs) and narrow bipolar events (NBEs) in lightning discharges remains a mystery. Recent experimental evidence has correlated IBPs to the initial development of lightning leaders inside the thundercloud. NBE wideband waveforms resemble classic IBPs in both amplitude and duration. Most NBEs are quite peculiar in the sense that very frequently they occur in isolation from other lightning processes. The remaining fraction, 16% of positive polarity NBEs, according to Wu et al. (2014), happens as the first event in an otherwise regular intracloud lightning discharge. These authors point out that the initiator type of NBEs has no difference with other NBEs that did not start lightning, except for the fact that they occur deeper inside the thunderstorm (i.e., at lower altitudes). In this paper, we propose a new physical mechanism to explain the source of both IBPs and NBEs. We propose that IBPs and NBEs are the electromagnetic transients associated with the sudden (i.e., stepwise) elongation of the initial negative leader extremity in the thunderstorm electric field. To demonstrate our hypothesis a novel computational/numerical model of the bidirectional lightning leader tree is developed, consisting of a generalization of electrostatic and transmission line approximations found in the literature. Finally, we show how the IBP and NBE waveform characteristics directly reflect the properties of the bidirectional lightning leader (such as step length, for example) and amplitude of the thunderstorm electric field.

Luque A., Ebert U.

We introduce the generic structure of a growth model for branched discharge trees that consistently combines a finite channel conductivity with the physical law of charge conservation. It is applicable, e.g., to streamer coronas near tip or wire electrodes and ahead of lightning leaders, to leaders themselves and to the complex breakdown structures of sprite discharges high above thunderclouds. Then we implement and solve the simplest model for positive streamers in ambient air with self-consistent charge transport. We demonstrate that charge conservation contradicts the common assumption of dielectric breakdown models that the electric fields inside all streamers are equal to the so-called stability field and we even find cases of local field inversion. We also discuss the charge distribution inside discharge trees, which provides a natural explanation for the observed reconnections of streamers in laboratory experiments and in sprites. Our simulations show the structure of an overall "streamer of streamers" that we name collective streamer front, and predict effective streamer branching angles, the charge structure within streamer trees, and streamer reconnection.

da Silva C.L., Pasko V.P.

[1] In this paper we present modeling studies of air heating by electrical discharges in a wide range of pressures. The developed model is capable of quantifying the different contributions for heating of air at the particle level and rigorously accounts for the vibration-dissociation-vibration coupling. The model is validated by calculating the breakdown times of short air gaps and comparing to available experimental data. Detailed discussion on the role of electron detachment in the development of the thermal-ionizational instability that triggers the spark development in short air gaps is presented. The dynamics of fast heating by quenching of excited electronic states is discussed and the scaling of its main channels with ambient air density is quantified. The developed model is employed to study the streamer-to-leader transition process and to obtain its scaling with ambient air density. Streamer-to-leader transition is the name given to a sequence of events occurring in a thin plasma channel through which a relatively strong current is forced through, culminating in heating of ambient gas and increase of the electrical conductivity of the channel. This process occurs during the inception of leaders (from sharp metallic structures, from hydrometeors inside the thundercloud, or in virgin air) and during their propagation (at the leader head or during the growth of a space leader). The development of a thermal-ionizational instability that culminates in the leader formation and propagation is characterized by a change in air ionization mechanism from electron impact to associative ionization and by contraction of the plasma channel. The introduced methodology for estimation of leader speeds shows that the propagation of a leader is limited by the air heating of every newly formed leader section. It is demonstrated that the streamer-to-leader transition time has an inverse-squared dependence on the ambient air density at near-ground pressures, in agreement with similarity laws for Joule heating in a streamer channel. Model results indicate that a deviation from this similarity scaling occurs at very low air densities, where the rate of electronic power deposition is balanced by the channel expansion, and air heating from quenching of excited electronic states is very inefficient. These findings place a limit on the maximum altitude at which a hot and highly conducting lightning leader channel can be formed in the Earth’s atmosphere, result which is important for understating of the gigantic jet (GJ) discharges between thundercloud tops and the lower ionosphere. Simulations of leader speeds at GJ altitudes demonstrate that initial speeds of GJs are consistent with the leader propagation mechanism. The simulation of a GJ, escaping upward from a thundercloud top, shows that the lengthening of the leader streamer zone, in a medium of exponentially decreasing air density, determines the existence of an altitude at which the streamer zones of GJs become so long that they dynamically extend (jump) all the way to the ionosphere.

Marshall T., Stolzenburg M., Karunarathne S., Cummer S., Lu G., Betz H., Briggs M., Connaughton V., Xiong S.

[1] The initial breakdown stage of 10 intracloud lightning flashes that may have produced terrestrial gamma ray flashes (TGFs) is studied with wideband E-change, multiband B-change, and VHF lightning mapping data; these flashes fit published criteria known to be associated with TGFs. The (x, y, z, t) locations of fast initial breakdown pulses (IBPs) were determined with E-change data using a time-of-arrival (TOA) technique. Each IBP includes one or more fast-rising subpulses. Previous research has shown that a typical intracloud flash initiates just above the main negative cloud charge (MNCC), then an initial negative leader propagates upward in 1–20 ms to the bottom of the upper positive cloud charge (UPCC), thereby establishing a conducting path between the MNCC and UPCC. TOA locations indicate that IBPs are directly related to the initial negative leader. The IBPs primarily occur in short (

Stenbaek-Nielsen H.C., Kanmae T., McHarg M.G., Haaland R.

Sprites are optical emissions in the mesosphere mainly at altitudes 50–90 km. They are caused by the sudden re-distribution of charge due to lightning in the troposphere which can produce electric fields in the mesosphere in excess of the local breakdown field. The resulting optical displays can be spectacular and this has led to research into the physics and chemistry involved. Imaging at faster than 5,000 frames per second has revealed streamer discharges to be an important and very dynamic part of sprites, and this paper will review high-speed observations of sprite streamers. Streamers are initiated in the 65–85 km altitude range and observed to propagate both down and up at velocities normally in the 106–5 × 107 m/s range. Sprite streamer heads are small, typically less than a few hundreds of meters, but very bright and appear in images much like stars with signals up to that expected of a magnitude −6 star. Many details of streamer formation have been modeled and successfully compared with observations. Streamers frequently split into multiple sub-streamers. The splitting is very fast. To resolve details will require framing rates higher than the maximum 32,000 fps used so far. Sprite streamers are similar to streamers observed in the laboratory and, although many features appear to obey simple scaling laws, recent work indicates that there are limits to the scaling.

Naidis G.V.

The paper discusses the relation between streamer velocity and diameter that follows from an analytical approach to description of the streamer head structure. It is shown that using measured data for streamer velocity and diameter one can evaluate the electric field in the streamer head. The analytical approach predicts that for positive streamers a minimum diameter exists, inversely proportional to the gas density.

Briels T.M., Kos J., Winands G.J., van Veldhuizen E.M., Ebert U.

Petersen D., Bailey M., Beasley W.H., Hallett J.

[1] A brief review of hypothesized mechanisms of lightning initiation is presented, with the suggestion that these mechanisms provide an incomplete picture of lightning initiation. This is followed by two ideas: (1) a combination of previously hypothesized lightning initiation mechanisms as a means for local intensification of the thundercloud electric field, and (2) a process for the formation of a hot lightning leader channel that is analogous to the space leader phase of the laboratory negative stepped leader. Thundercloud electric field observations have consistently yielded peak values that are an order of magnitude weaker than the dielectric strength of air. Various mechanisms have been proposed to explain how lightning can initiate in such weak electric fields, including hydrometeor-initiated positive streamers and cosmic ray-initiated runaway breakdown. The historically favored positive streamer mechanisms are problematic due to requiring electric fields two to three times larger than peak observed fields. The recently favored runaway breakdown mechanisms appear capable of developing in conditions comparable to peak observed fields although it is not clear how these diffuse discharges can lead to creation of a lightning leader. This paper proposes a solution whereby runaway breakdown and hydrometeor-initiated positive streamer systems serve to locally intensify the electric field. Following this local field intensification, it is hypothesized that formation of the initial lightning leader channel is analogous to the formation of a space leader in a laboratory negative stepped leader.

Watson S.S., Marshall T.C.

[1] Narrow bipolar pulses (NBPs) are a class of high-altitude, high-energy discharges that occur during some thunderstorms. We use a modified transmission line model (called MTLEI) with a current that increases exponentially along the propagation channel to test mechanisms that might produce NBPs. Model outputs were compared to measured E data from a single NBP collected at near and far field locations. We were unable to fit the measured data using the fast current propagation speeds appropriate for a runaway breakdown/extensive air shower mechanism. Instead, by using currents that travel relatively slowly (6 × 107 m/s), the MTLEI model fit the data reasonably well. This result is compatible with a mechanism that uses runaway breakdown to produce charge carriers along with a moving electric field to drive the main NBP current. Using this model for the measured NBP, we estimate a charge moment of 0.6 C·km.

Sekehravani E.A., Dodge S., Barmada S., Brignone M., Formisano A., Mestriner D., Nicora M., Procopio R.

Yang Q., Wang D., Yang J., Liu H., Wu T., Takagi N.

AbstractUsing a broadband (from 1 to 250 MHz) interferometer with high temporal‐spatial resolution, we have observed the lightning initiation process of a winter cloud‐to‐ground lightning in Japan with great details. We found that the lightning initiation involved with multiple fast breakdowns behaving like a series of back (downward) and forth (upward) consecutive reflections in a constrained space near the main negative charge region of the lightning. We also found that some initial negative fast breakdowns could propagate with an unusually spread manner. We suggest that fast breakdowns with either reflecting features or spread manners could efficiently utilize the electrostatic energy in a local region with strong electric field for driving subsequent streamers, and therefore may widely exist in lightning initiation processes.

Pantuso J.G., da Silva C.L.

AbstractPositive lightning leaders are a ubiquitous, yet poorly understood, component of lightning flashes. Upward lightning started by positive leaders may be formed when nearby storm activity induces electrical charges in a tall structure, such as communications towers or wind turbines. Alternatively, upward lightning can be triggered with the rocket‐and‐wire technique. In this paper, we introduce a new self‐consistent model for this important discharge mode, one which solves Maxwell's equations under the quasi‐electrostatic approximation. The model also includes a realistic treatment of the nonlinear plasma conductivity within the leader channel. This new computational tool explains the origin of the positive leader speed, of 10s of km/s, as well as why it displays a steady behavior over time. The model also explains the temporal evolution of current to ground measured during the early stages of rocket‐triggered lightning, where the current exhibits a series of small‐amplitude pulses, which disappear over time. The article also outlines straightforward criteria for leader inception, which may have practical applications for lightning protection.

Pu Y., Cummer S.A.

AbstractUsing a 30–250 MHz VHF interferometer, we observed a previously unreported mode of initial lightning development inside thunderclouds. This mode is defined by continuous VHF radiation spanning several km within the first few milliseconds of lightning initiation. Following flash initiation through fast positive breakdown at high altitudes above 9 km, the VHF radiation front of upward negative streamers ascended continuously at a speed of ∼1.0 × 106 m/s, forming a continuous initial breakdown burst (CIBB) about 2 km in length. For the two CIBBs analyzed, the long and narrow CIBB channel was traversed by dart leaders that occurred later in the flash, indicating that the CIBB channel belongs to what becomes the main conducting leader channel. In contrast to classic initial breakdown pulses (IBPs) with sub‐pulses superimposed on the rising edge, CIBBs produced a series of discrete, narrow LF pulses (<10 μs) with an average time interval of 0.20 and 0.14 ms, respectively. We speculate that a CIBB is a continuously developing negative streamer system in the high electric field region at high altitudes, with connections of internal plasma channels producing LF pulses. These results have implications for physical conditions conducive to the formation of a long and continuous negative streamer system.

Li D., Luque A., Rachidi F., Rubinstein M., Neubert T., Zhu Y., Chanrion O., da Silva C., Krehbiel P.R.

AbstractNarrow bipolar events (NBEs) are impulsive and powerful intracloud discharges. Recent observations indicate that some NBEs exhibit a slanted orientation rather than strictly vertical. This paper investigates the effect of the slanted NBEs using a newly developed rebounding‐wave model. The modeling results are validated against the full‐wave Finite‐ Difference Time‐Domain method and compared with measurements for both vertical and slanted NBE cases. It is found that the inclination of the NBEs affects both the waveforms and amplitudes of the electrostatic, induction and radiation components of the electric fields at close distances (≤10 km). However, it primarily influences the amplitudes of the fields for distances beyond 50 km, where the radiation component dominates, resulting in changes of ≥30% when the slant angle exceeds 30°. The slanted rebounding‐wave model improves the agreement with respect to a purely vertical channel and can be extended to any discharge geometry at arbitrary observation distances.

Senay S., Krehbiel P.R., da Silva C.L., Edens H.E., Bennecke D., Stanley M.A.

AbstractMulti‐resolution analysis methods can reveal the underlying physical dynamics of nonstationary signals, such as those from lightning. In this paper we demonstrate the application of two multi‐resolution analysis methods: Ensemble Empirical Mode Decomposition (EEMD) and Variational Mode Decomposition (VMD) in a comparative way in the analysis of electric field change waveforms from lightning. EEMD and VMD decompose signals into a set of Intrinsic Mode Functions (IMFs). The IMFs can be combined using distance and divergence metrics to obtain noise reduction or to obtain new waveforms that isolate the physical processes of interest while removing irrelevant components of the original signal. We apply the EEMD and VMD methods to the observations of three close Narrow Bipolar Events (NBEs) that were reported by Rison et al. (2016, https://doi.org/10.1038/ncomms10721). The ΔE observations reveal the occurrence of complex oscillatory processes after the main NBE sferic. We show that both EEMD and VMD are able to isolate the oscillations from the main NBE, with VMD being more effective of the two methods since it requires the least user supervision. The oscillations are found to begin at the end of the NBEs' downward fast positive breakdown, and appear to be produced by a half‐wavelength standing wave within a weakly‐conducting resonant ionization cavity left behind in the wake of the streamer‐based NBE event. Additional analysis shows that one of the NBEs was likely initiated by an energetic cosmic ray shower, and also corrects a misinterpretation in the literature that fast breakdown is an artifact of NBE‐like events in interferometer observations.

Wemhoner J., Wermer L., da Silva C.L., Barnett P., Radosevich C., Patel S., Edens H.

Calibrated measurements of lightning optical emissions are critical for both quantifying the impacts of lightning in our atmosphere and devising detection instruments with sufficient dynamic range capable of yielding close to 100% detection efficiency. However, to date, there is only a limited number of investigations that have attempted to take such calibrated measurements. In this work, we report the power radiated by lightning in both visible and infrared bands, assuming isotropic emission, and accounting for atmospheric absorption. More precisely, we report peak radiated power and total radiated energy in the combined visible plus near-infrared range (VNIR, 0.34–1.1 μm), around the Hα line (652–667 nm), and for the 2–2.5 μm infrared band. The estimated peak power and total energy radiated by negative cloud-to-ground return strokes in the VNIR range is 130 MW and 20 kJ, respectively. Additionally, we detected peak radiated powers of 12 and 0.19 MW in the Hα and infrared bands, respectively. We cross-reference the optical data set with peak current reported by a lightning detection network. The resulting trend is that optical power emitted around the Hα line scales with peak return stroke current according to a power law with exponent equal to 1.25. This trend, which should be approximately true across the entire visible spectrum, can be attributed to the plasma negative differential resistance of the lightning return stroke channel. We conclude by discussing the challenges in performing calibrated measurements of lightning optical power in different bands and comparing the results with previously-collected data with different experimental setups, observation conditions, and calibration methods.

Bandara S., Marshall T., Stolzenburg M.

Using electric field change data and VHF data of 201 positive Narrow Bipolar Events (+NBEs) detected at close range (< 80 km) at seven sensor sites, three aspects of +NBEs are investigated: the VHF “Development Time” (∆T0), the detailed VHF waveform, and the VHF “Major Trailing Pulses” (MTPs) that follow some +NBEs. The +NBE Development Time study used 137 + NBEs divided into two groups: 72 INBEs (+NBEs that initiate a full lightning flash) and 65 Not-INBEs. These 137 + NBEs had VHF peak powers ranging from 0.1 to 66 kW and ∆T0 values ranging from 20 kW) were all found to have short ∆T0 (< 4 μs), while lower power NBEs (< 7 kW) exhibit the full range of ∆T0 values. The detailed study of VHF waveforms of 201 + NBEs shows that the shape of each +NBE is significantly different at the multiple sensors sites. This finding supports mechanisms in which an NBE is caused by numerous streamers occurring within a few μs. Among 201 + NBEs, 67 + NBEs (with VHF powers of 400–37,500 W) were followed, within 30 μs, by at least one MTP; 39 of these 67 NBEs had a second MTP and 16 of those 39 NBEs had a third MTP. All but one MTP had VHF peak power between 5 W and 270 W; the exceptional case, a single MTP occurring after an NBE with peak VHF power of 6.5 kW, had VHF power of 3.3 kW. MTPs were found to occur from 4.9 to 32 μs after the +NBE or preceding MTP, and they are hypothesized to be evidence of re-radiation in the previously ionized NBE region that is (repeatedly) excited by reflection from an above-cloud ion layer.

Sterpka C., Dwyer J., Liu N., Demers N., Hare B.M., Scholten O., ter Veen S.

Iudin D.I., Sysoev A.A., Rakov V.A.

We review the basic issues of the initiation and development of the lightning discharge, which top the list of the most important and yet unsolved problems of atmospheric electricity. The main challenges of creating theoretical models are due to the fact that the value of the electric strength of atmospheric air exceeds the peak electric fields measured in thunderstorm clouds by approximately an order of magnitude. Among several concepts proposed to explain the process of initiation of a lightning discharge at different times, two ideas stand apart, namely, the hypothesis of the lightning birth caused by the initiation of a positive streamer from the surface of a hydrometeor and the hypothesis of lightning initiation due to the development of a runaway electron breakdown. However, none of these approaches has become universally recognized or dominant due to various difficulties. A fundamentally new mechanism of lightning discharge initiation, which has been proposed recently, is based on the noise-induced kinetic transition occurring in the stochastic field of charged hydrometeors. The proposed approach looks like a sequence of discharge activity transitions from small spatial scales to increasingly larger ones. One of the main features of the proposed hypothesis is that the generation of streamers is determined by the level of small- and mesoscale fluctuations of the electric field in the thunderstorm and is essentially independent of the large-scale field. In this case, the role of the large-scale field is to ensure interaction of the initiated streamers, when they start developing mainly in the direction specified by this field. In the final section of the review, we discuss the fundamental role of the polarity asymmetry in the processes of the initiation and further development of lightning discharges.

Li D., Luque A., Gordillo‐Vázquez F.J., Silva C.D., Krehbiel P.R., Rachidi F., Rubinstein M.

The physical mechanism of Narrow bipolar events (NBEs) has been studied for decades but it still holds many mysteries. Recent observations indicate that the fast breakdown discharges that produce NBEs sometimes contain a secondary fast breakdown that propagates back in the opposite direction but this has not been fully addressed so far in electromagnetic models. In this study, we investigate fast breakdown using different approaches that employ a Modified Transmission Line with Exponential decay (MTLE) model and propose a new model, named "rebounding MTLE model," which reproduces the secondary fast breakdown current in NBEs. The model provides new insights into the physics of the fast breakdown mechanism.

Sterpka C., Dwyer J., Liu N., Hare B.M., Scholten O., Buitink S., Veen S.T., Nelles A.

Here, we present new radio interferometer beamforming observations of lightning initiation using data from the Low-Frequency Array (LOFAR). We show that the first lightning source in the flash increases exponentially in intensity by two orders of magnitude in 15 μs, while propagating 88 m away from the initiation location at a constant speed of 4.8 ± 0.1 × 106 m/s. A second source replaces the first source at the initiation location, and subsequent propagation of the lightning leader follows. We interpret the first source to be a rapidly propagating and intensifying positive streamer discharge that subsequently produces a hot leader channel near the initiation point. How lightning initiates is one of the greatest unsolved problems in the atmospheric sciences, and these results shed light on this longstanding mystery.

Liu F., Lu G., Neubert T., Lei J., Chanrion O., Østgaard N., Li D., Luque A., Gordillo-Vázquez F.J., Reglero V., Lyu W., Zhu B.

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers. We found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with streamer breakdown. The rise times of the emissions for negative NBEs are about 10 μs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~1 ms, consistent with locations deeper in the clouds. For negative NBEs, the emission strength is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause. Strong thunderstorms can reach the lower stratosphere and produce cloud-top blue emissions, affecting the exchange of greenhouse gases between the troposphere and stratosphere. Here, the authors reveal the direct link of blue emissions with the radio signals of one sort of intra-cloud discharges called NBEs.

Attanasio A., da Silva C., Krehbiel P.

Fast breakdown (FB), a breakdown process composed of systems of high-velocity streamers, has been observed to precede lightning leader formation and play a critical role in lightning initiation. Vigorous FB events are responsible for the most powerful natural radio emissions on Earth, known as narrow bipolar events (NBEs). In this paper, an improved version of the Griffiths and Phelps (1976, https://doi.org/10.1029/jc081i021p03671) model of streamer breakdown is used alongside supervised machine learning techniques to probe the required electric fields and potentials inside thunderstorms to produce FB and NBEs. Our results show that the electrostatic conditions needed to produce FB observed in New Mexico at 9 km altitude and FB in Florida at 14 km altitude are about the same, each requiring about 100 MV potential difference to propagate 500 m. Additionally, the model illustrates how electric field enhancement ahead of propagating FB can initiate rebounding FB of the opposite polarity.

Li X., Lu G., Jiang R., Zhang H., Fan Y., Shi T., Qie X., Zhang Y., Ren H., Zhang C., Zhang Y.

Precursory 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.