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• Recent progress in studying the breakthrough phase of the attachment process is reviewed. • During the BTP, hot channels of both the DL and UCL extend toward each other inside the CSZ. • During the BTP, the current rises from the UCL level to tens of kA (for negative first strokes). • Initial conductivity of common streamer zone is expected to be of the order of 10−5 S/m. Recent progress in studying the breakthrough phase (BTP) of the attachment process in lightning and long sparks is reviewed. The main focus is on the new insights gained from recent observations for 3 types of electric discharges: natural lightning (Nag et al., 2012; Tran and Rakov, 2017a,c), rocket-and-wire triggered lightning (Howard et al., 2010; Hill et al., 2016), and long sparks (Kostinskiy et al., 2016). The BTP (also known as the final jump) starts when the poorly-conducting streamer zones, developing ahead of the hot channels of negative downward leader (DL) and positive upward connecting leader (UCL), come in contact and a common streamer zone (CSZ) is formed. The beginning of BTP (establishment of CSZ) is usually marked by an abrupt current rise, a burst of dE/dt pulses (referred to as leader burst or LB), and hard X-ray emission. During the BTP, hot channels of both DL and UCL extend toward each other inside the CSZ, resulting in its shrinking, until the high-impedance CSZ is eliminated and low-impedance connection of DL to the grounded object is established. The process of bridging of CSZ by hot leader channels is accompanied by the formation of slow front (SF) in the channel current and in electric and magnetic field waveforms at both close and far distances from the channel. Attempted or relatively weak hot-channel connections producing current surges and associated field pulses superimposed on the SF (SF pulses) can occur. The SF lasts some microseconds and ends at the onset of the submicrosecond-scale fast transition (FT), which signifies the end of BTP. During the BTP, the current rises from the UCL level of the order of tens to hundreds of amperes to about 50% of the overall (SF + FT) current peak, which is of the order of tens of kiloamperes (for negative first strokes). This two orders of magnitude current rise during the BTP occurs before the collision of hot leader channels inside the CSZ; that is, before the onset of return stroke proper. [ABSTRACT FROM AUTHOR]

Simultaneous electric field and high-speed video camera records of two flashes, labeled 1593 and 1594, were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Flash 1593 was an upward negative flash whose upward positive leader initiated from a 257-m high tower, located 8.8 km from LOG. It contained six leader/return-stroke sequences, all of which developed along the upward leader path and terminated on the 257-m tower. Flash 1594 was a three-stroke downward bipolar flash whose first stroke was positive and terminated on a 60-m tower. The two subsequent strokes were negative and terminated on the same 257-m tower as the six strokes of flash 1593. All strokes terminated on the 257-m tower exhibited very similar narrow bipolar electric field waveforms with damped oscillatory tails. Characteristics of those electric field waveforms are examined and compared to similar observations found in the literature. [ABSTRACT FROM AUTHOR]

A terrestrial gamma-ray flash (TGF) observed at ground level is presented. It was recorded at the Lightning Observatory in Gainesville, Florida, on June 13, 2014. Ground-based observations of TGFs are very rare. To date, only two positively identified ones are found in the literature. Our TGF was associated with a single-stroke negative cloud-to-ground discharge. It had a duration of 16 μ s and consisted of 6 pulses, two of which exceeded the upper measurement limit of 5.7 MeV. The pulses apparently corresponded to individual photons, which is a characteristic feature of TGFs. The TGF began 191 μ s after the return-stroke electric field peak. The stepped leader duration was as short as 3.9 ms. There was essentially no energetic radiation detected during the leader process. The NLDN-reported return-stroke peak current was as high as 224 kA. The characteristics and occurrence context of the LOG-recorded TGF are compared to those of the two similar events found in the literature. In all three cases there was evidence of a channel carrying appreciable current to ground at the time of TGF, and the associated (preceding or concurrent) cloud-to-ground discharge processes were unusually intense. [ABSTRACT FROM AUTHOR]

Abstract: Return-stroke peak current is one of the most important measures of lightning intensity needed in different areas of atmospheric electricity research. It can be estimated from the corresponding electric or magnetic radiation field peak. Electric fields of 89 strokes in lightning flashes triggered using the rocket-and-wire technique at Camp Blanding (CB), Florida, were recorded at the Lightning Observatory in Gainesville, about 45km from the lightning channel. Lightning return-stroke peak currents were estimated from the measured electric field peaks using the empirical formula of Rakov et al. (1992) and the field-to-current conversion equation based on the transmission line model (Uman and McLain, 1969). These estimates, along with peak currents reported by the U.S. National Lightning Detection Network (NLDN), were compared with the ground-truth data, currents directly measured at the lightning channel base. The empirical formula, based on data for 28 triggered-lightning strokes acquired at the Kennedy Space Center (KSC), tends to overestimate peak currents, whereas the NLDN-reported peak currents are on average underestimates. The field-to-current conversion equation based on the transmission line model gives the best match with directly measured peak currents for return-stroke speeds between c/2 and 2c/3 (1.5 and 2×108 m/s, respectively). Possible reasons for the discrepancy in the peak current estimates from the empirical formula and the ground-truth data include an error in the field calibration factor, difference in the typical return-stroke speeds at CB and at the KSC (considered here to be the most likely reason), and limited sample sizes, particularly for the KSC data. A new empirical formula, I =−0.66–0.028rE, based on data for 89 strokes in lightning flashes triggered at CB, is derived. [Copyright &y& Elsevier]

Abstract: We examine initial rising portions of electric field waveforms of negative first return strokes in natural cloud-to-ground lightning recorded simultaneously at near and far distances from the lightning channel. The near and far field-measuring stations are located at Camp Blanding and in Gainesville, Florida, respectively, separated by a distance of about 45km. A total of five return strokes had been recorded in 2007–2008, four of which were analyzed in detail (one was not suitable for analysis due to saturation of electric field waveform at the far station). Field waveform characteristics, including overall zero-to-peak and 10-to-90% risetimes, duration of slow front, fast transition 10-to-90% risetime, and magnitude of slow front relative to the peak, were found to be similar to those reported from other studies, in which the field propagation path was over ground (as opposed to sea water). It is shown, via modeling, that the slow front in electric field waveforms at far distances is primarily due to the radiation field component, while at near distances it is composed of comparable contributions from all three components of electric field. For both measured and model-predicted waveforms, the durations of the slow front appear to be similar at near and far distances from the lightning channel. [Copyright &y& Elsevier]

Abstract: Triggered-lightning properties, including dart-leader charge density, return-stroke propagation speed, dart-leader electric potential, dart-leader propagation speed, and dart-leader current, inferred from return-stroke current and very close electric field measurements, are presented. Although most of the estimates are based on relatively crude models, they are all generally in good agreement with independent measurements and/or theoretical considerations found in the literature. The results are likely to be applicable to subsequent strokes in natural lightning. [Copyright &y& Elsevier]

• An upward flash was triggered from a 257-m tower in florida by a +CG 45 km away. • Six return strokes of the upward flash were all initiated by bidirectional leaders. • All six return-stroke field waveforms at 8.8 km exhibited early zero-crossings. • Two scenarios giving rise to the narrow field waveforms are discussed. We examined in detail the morphology and evolution of an upward negative flash initiated from a 257-m tower in Florida. High-speed video camera images and wideband electric field records, as well as ENTLN data, were used. The upward negative flash was induced (triggered) by a single-stroke 50-kA +CG that occurred about 45 km from the tower. The 257-m tower flash contained 6 leader/return stroke sequences and 1 attempted leader that almost terminated on the tower. All the leaders exhibited bidirectional extension. Electric field waveforms produced by the return strokes (measured at a distance of 8.8 km) were bipolar and abnormally narrow (exhibited earlier zero crossings). In order to examine the origin of the observed earlier zero crossings, we used two return-stroke models of transmission line type (MTLL and MTLE) to see what model input parameters are responsible for this feature. Within the limits of those models, the observed earlier zero crossings could be explained only by a narrow input current waveform or/and its fast amplitude decay with height. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• We measured the electric field waveforms produced by both RTL and NL at comparable far distances using same instrumentation, where the radiation field is dominant. Compared to previous work done by Mallick & Rakov (2014), we increase the sample size of RTL data and compare with NL data. • The majority of the parameters show comparable values. The distributions for 10%-to-90% risetime and HPW show less scatter for RTL than NL. The plateau type slow front occurs in RTL strokes more often than in NL strokes. The initial electric field peak and the various time parameters of field waveforms are very low. • For subsequent strokes in NL and field propagation over land, HPW values are typically of the order of 10 μs. • For 184 subsequent strokes in NL and 139 strokes in RTL, we found that the field zero-crossing time for RTL is slightly smaller than for NL. • The smaller electric field pulse widths that we measured for RTL compared to NL could be due to: 1. A faster current decay with height in RTL return strokes. 2. The bottom part of the channel for RTL is straighter than for NL. Using wideband electric field records obtained at the Lightning Observatory in Gainesville (LOG), Florida, we examined in detail the characteristics of far electric field waveforms produced by two categories of lightning return strokes. The first category includes return strokes of any order in lightning flashes triggered by a rocket extending a grounded wire toward the overhead thundercloud, which we label RTL (rocket-triggered lightning) strokes. The second category includes return strokes of order 2 and higher (that is, only subsequent strokes) in natural cloud-to-ground lightning flashes, which we label NL (natural lightning) strokes. Properties of RTL and NL strokes are expected to be similar. A total of 139 RTL strokes from years 2013 to 2016 and 184 NL strokes from years 2013 to 2015 were examined in this study. All strokes transported negative charge to ground and occurred at similar distances from LOG. Similarities and dissimilarities of waveform parameters for the two categories of strokes and their possible explanations are discussed in this paper. This is the first detailed comparison of electric field waveforms of RTL and NL strokes recorded at approximately the same distance (some tens of kilometers) and with the same instrumentation. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we compare five engineering return stroke models, extended to include the presence of an elevated strike object, namely the Bruce–Golde (BG) model, the transmission line (TL) model, the traveling current source (TCS) model, and the two modified TL models (MTLL and MTLE). The model-predicted current profile along the channel and along the strike object, as well as radiated electric and magnetic fields at different distances are presented and discussed. The comparison is made assuming the same ‘undisturbed current’ (that is the idealized current that would be measured at the tower top if the current reflection coefficients at its both extremities were equal to zero) and the same return-stroke speed. Except for the case of very close (50m) electric field, it is found that the computed electromagnetic fields associated with a strike to a 168-m tall tower are less model-dependent than those corresponding to a strike to flat ground. [Copyright &y& Elsevier]

Recent studies of thunderstorm-related enhancements of fluxes of energetic radiation and particles at ground level suggest that removal of mid-level negative charge from the cloud by negative cloud-to-ground (-CG) lightning flashes or normal intracloud (IC) flashes serves to abruptly terminate those enhancements. However, it was not clear if the electron-accelerating electric field responsible for flux enhancements at ground was primarily between the main negative charge region and ground (produced due to the dominant effect of negative cloud charge) or between the mid-level negative and lower positive charge regions inside the thundercloud. Here, we report that these flux enhancements can be also abruptly terminated by inverted intracloud flashes and hybrid lightning flashes (inverted IC followed by negative CG). Based on the analysis of 13 events of these two types, we provide first evidence that the electric field between the mid-level negative and lower positive charge regions in the thundercloud can be responsible for the flux enhancements at ground level. • Lightning discharges can influence the evolution of enhanced fluxes of energetic radiation and particles (TGEs). • TGEs can be abruptly terminated by inverted intracloud flashes (ICs) and by hybrid flashes (inverted IC followed by -CG). • Electron accelerator between main negative and lower positive cloud charge regions is capable of producing TGEs. [ABSTRACT FROM AUTHOR]

• Expressions to estimate the current peak from the far-field peak considering channel tilt are derived. • Expressions to reconstruct the entire current waveform from the field waveform are derived. • The expressions are derived for three most widely used engineering return-stroke models. • This is the first attempt to solve the inverse problem (infer sources from fields) for inclined channels. The inclination of a lightning return-stroke channel significantly affects the waveform of the associated electromagnetic field. In this paper, we have derived mathematical expressions to estimate the peak and waveshape of lightning return-stroke current at the channel base from the waveform of electromagnetic field observed more than some tens of kilometers away from an inclined lightning channel. Three models of the lightning return stroke are considered, which are the transmission-line (TL) model, the modified TL model with linear current decay with height (MTLL), the modified TL model with exponential current decay with height (MTLE). The peak of the lightning return-stroke current at the channel base is estimated from the peak of observed lightning electric or magnetic field using an analytical relation between the peak of channel-base current and that of the radiation field component for an inclined channel. The waveform of the channel-base current is reconstructed from the electric or magnetic field waveform using a relation between the channel-base current and the sum of induction and radiation field components for an inclined channel. It has been shown that the peak and waveshape of current at the base of an inclined channel are sufficiently accurately estimated with the proposed expressions. This work is the first attempt to solve the inverse problem (infer source parameters from electromagnetic fields) for the case of inclined lightning channel. Solving this problem is important for developing methodology of remote measurements of lightning currents. [ABSTRACT FROM AUTHOR]

Although, due to the high variability of key parameters such as the return-stroke speed, it is impossible to determine the lightning current accurately from the remotely measured electric or magnetic field for a given event, we show in this paper that, for an assumed return-stroke model, the statistical estimation (e.g. in terms of mean values and standard deviations) is possible. We show additionally that for the transmission line (TL) model, the equation permitting to infer the mean value of the return-stroke current from the mean value of electric or magnetic field and the mean value of speed has the same functional form as the well-known TL current—far field relationship. This result gives to some extent a theoretical justification to the use of lightning location systems to infer parameters of lightning current statistical distributions from measured fields alone. [Copyright &y& Elsevier]