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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]

• 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]

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]

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]