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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]

• 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 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]

• 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]