Electric and/or magnetic fields are generated by stationary charges, uniformly moving charges and accelerating charges. These field components are described in the literature as static fields, velocity fields (or generalized Coulomb field) and radiation fields (or acceleration fields), respectively. In the literature, the electromagnetic fields generated by lightning return strokes are presented using the field components associated with short dipoles, and in this description the one-to-one association of the electromagnetic field terms with the physical process that gives rise to them is lost. In this paper, we have derived expressions for the electromagnetic fields using field equations associated with accelerating (and moving) charges and separated the resulting fields into static, velocity and radiation fields. The results illustrate how the radiation fields emanating from the lightning channel give rise to field terms varying as <mml:semantics>1/r</mml:semantics> and <mml:semantics>1/r2</mml:semantics>, the velocity fields generating field terms varying as <mml:semantics>1/r2</mml:semantics>, and the static fields generating field components varying as <mml:semantics>1/r2</mml:semantics> and <mml:semantics>1/r3</mml:semantics>. These field components depend explicitly on the speed of propagation of the current pulse. However, the total field does not depend explicitly on the speed of propagation of the current pulse. It is shown that these field components can be combined to generate the field components pertinent to the dipole technique. However, in this conversion process the connection of the field components to the physical processes taking place at the source that generate these fields (i.e., static charges, uniformly moving charges and accelerating charges) is lost.

This paper presents the observations and characterization of Cloud-to-Ground (CG) lightning activity in Western Antarctica in a region that covers the Amundsen/Bellingshausen Sea (ABS), the Antarctic Peninsula (AP) and the Weddell Sea (WS). Lightning data have been collected by a lightning detector (Boltek LD-350) and an atmospheric electric field mill (EFM-100) sensors deployed at the Carlini Base on the Antarctic Peninsula (CARL: 62.23 degrees S, 58.63 degrees W). The flash rate and flash multiplicity were analysed for three different seasons within a 1,000 km range, starting at the end of summer (February 2017) and ending in winter (July 2017). Three storm days for each month (within the 1,000 km radius from the LD sensor) with three composite active thunderstorms (labelled as Storm region A, B, and C) for each day have been selected from a collection of storm days between February and July 2017. A total of 355,899 flashes have been recorded with 156,190 Positive CG and 199,709 Negative CG flashes from these 54 thunderstorms. In total, Positive CG flash counts made up around 43.9% of the total detected CG flashes. Most of the Positive CG flashes (> 80%) had only 1 or 2 strokes with a maximum number of 5. For Negative CG flashes, the average multiplicity and the maximum multiple stroke were 1.2 and 16 respectively. Most CG flashes were detected during the summer and fall months. Positive CG flashes were prevalent in Western Antarctic storms even during the winter. The mean, median and range of the ratio of Positive CG to Negative CG flashes were 0.7, 0.718 and 0.217-1.279, respectively.

In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M-component or leader/return-stroke-like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC-carrying channel at relatively short distance from the tower top, resulting in the so-called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M-component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Santis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M-component charge transfer mode (including classical M-component pulses and M-component-type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2-Dimentional Finite-Difference Time Domain method, in which the M-component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Santis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M-component and M-component-type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M-component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M-component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M-component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch-to-channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E-field and 1.9 for the H-field. For a junction height greater than 6 km, the E-field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, ) in which an electric field propagation factor of 1. 8 was inferred for return strokes and mixed-mode pulses, considering that junction points lower than 1 km or so would result in a mixed mode of charge transfer, in which a downward leader/return-stroke-like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 mu s or so) for which the tendency reverses. Finally, model-predicted fields associated with different charge transfer modes, namely, return stroke, mixed-mode, classical M-component, and M-component-type ICC pulse are compared with experimental observations at the Santis Tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed-mode models (1.0 km for the return stroke and 0.8 km for the mixed-mode pulse) are lower than the value of 2.0 km typically used in the literature.

The environmental conditions leading to the bouncing-wave discharge and the subsequent electron beam remain to be investigated in more detailed future studies. The analysis of quasi-static initial electric field changes (IECs) were found at the beginning of all 24 lightning flashes detected within reversal distance (22 Negative Cloud-to-Ground (–CG) and 2 normal Intra-Cloud (IC) flashes) in a tropical storm on June 15th, 2017 close to our station in Malacca, Malaysia (2.314077° N, 102.318282° E). The IECs durations averaged 4.28 ms for –CG flashes (range 1.48 to 9.45 ms) and averaged 11.30 ms for normal ICs flashes (range 7.24 to 15.35 ms). In comparison to Florida storms, the duration of IECs for –CG and IC flashes were 0.18 ms (range 0.08 to 0.33 ms) and 1.53 ms (range 0.18 to 5.70 ms), respectively. Moreover, the magnitudes of E-change for tropical thunderstorm were 0.13 V/m (range 0.03 to 0.44 V/m) for –CG flashes and -0.20 V/m (range -0.13 to -0.27 V/m) for IC flashes. The E-change magnitudes of tropical flashes are significantly larger than Florida flashes.

General characteristics of K-changes, including their duration and probability of occurrence associated with ground flashes in Sri Lanka in the tropics, together with their fine structure, are presented. In 98 ground flashes where the small step changes associated with K-changes are clearly visible, there were about two K-changes per flash on average. The mean K-change time duration observed in this study is 0.38 ms. In 53 of the ground flashes, there were 120 consecutive K-changes. In these cases, the geometric mean of the time interval between K-changes was 12 ms. Analysis of the fine structure of the K-changes reveals the K-changes are always associated with either a chaotic pulse train or a combination of chaotic and regular pulse trains. The results suggest that the small step-like static electric fields identified in the literature as K-changes are the step-like static fields associated with the processes that generate chaotic or a combination of chaotic and regular pulse trains. Thus, at larger distances where the static fields are negligible, K-changes may appear as a chaotic pulse train or a combination of chaotic and regular pulse trains.

In several studies conducted recently, it was shown that equations pertinent to the electric and magnetic fields produced by electrical charges in motion can be used to calculate the electromagnetic fields produced by current pulses propagating along linearly restricted paths. An example includes the case of current pulses propagating along conductors and conducting channels such as lightning. In this paper, it is shown how the technique can be applied to estimate the electromagnetic fields generated by current and charge distributions moving in arbitrary directions in space. The analysis shows that, depending on the way the problem is formulated using the field equations pertinent to accelerating charges, one procedure leads to the generalized dipole equations, which are independent of the velocity of propagation of the current, and the other procedure leads to a set of equations that depend on the velocity. Using the well-tested transmission line model of lightning return strokes as an example, it is shown that both sets of field equations give rise to the same total electromagnetic field.

In this paper, the key finding is that all the examined first classic Initial Breakdown (IB) pulses in tropical flashes within the reversal distance were found to be initiated by a clearly detectable Initial E-field Change or IEC (45 -CG, 32 normal IC, and 3 IC initiated by +NBE). The durations of IECs for both -CG and IC flashes in tropical storms were longer than in Florida storms. On the other hand, for the magnitudes of the E-change, the values were smaller compared to Florida storms with averages of 0.30 V/m compared to 1.65 V/m for -CG flashes, and -0.81 V/m compared to -6.30 V/m for IC flashes. The IEC process of lightning flashes in tropical regions took longer to increase the local electric field in order to produce the first IB pulse because of the smaller magnitude of E-change. On the other hand, in Florida storms, the IEC process took a shorter time to increase the local electric field to produce the first IB pulse because of the larger magnitude of E-change. We found that very high frequency (VHF) pulses for tropical thunderstorms started sometime prior to the onset of the IECs. They started between 12.69 and 251.60 mu s before the initiation of the IEC for two normal IC flashes. The first two VHF pulses were detected alone without narrow IB pulses (fast antenna and slow antenna records) or any pulses from the B-field and dE/dt records. Furthermore, the VHF pulses for three IC flashes initiated by + NBEs were also detected before the onset of the IEC. The IEC started immediately after the detection of the + NBE. It is clear that the IEC is initiated by VHF pulses. It can be suggested that lightning is initiated by Fast Positive Breakdowns or FPBs (which emit strong VHF pulses and large + NBEs) and is followed by several negative breakdowns (weak VHF pulses and/or weak NBE-type pulses) before the IEC started. For the case of normal IC flashes, several weaker VHF pulses (mean values of 41.97 mV and 46.4 mV compared to the amplitudes of the VHF pulses of + NBEs of around 800 mV) were detected before the onset of the IEC. As FPBs can occur with a wide range of VHF strengths and E-change amplitudes, it can be suggested these weak VHF pulses accompanied by narrow IB pulses or weak NBE-type pulses detected before the onset of IEC are actually FPBs followed by negative breakdowns or several attempted FPBs.

We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 µs) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity.

This paper proposes a simple method to take into account non-vertical risers through an equivalent partial inductance. The proposed approach was validated considering several examples and taking as reference full-wave results obtained using a numerical electromagnetics code numerical electromagnetics code (NEC)-4.

This paper presents a seasonal analysis of cloud-to-ground (CG) lightning flash activity in the Western Antarctica using a lightning detector sensor installed at the Carlini Base station. Data obtained from the detection system between February and December 2017 were analyzed. Three common locations and areas of composite active thunderstorms (labelled storm regions A, B, and C) were detected by the sensor within a 1000 km radius from the station. Storm region A was located to the northwest (N/W) of the station and covered the Amundsen/Bellingshausen Sea (ABS), whereas storm region C was located on the southeastern (S/E) side of the station over the Weddell Sea (WS), with distances ranging from 500 to 800 km and bearings of 270 degrees to 360 degrees and 90 degrees to 180, respectively. Storm region B was located around 100 km from the station with the bearings of stroke taken from 0 degrees to 360 degrees. A total of 2,019,923 flashes were detected, of which 43.01% were positive CG and 56.99% were negative CG flashes. The analysis revealed that more than 96% of the CG flashes (both positive CG and negative CG) were produced during the summer and fall seasons as compared with less than 4% during the winter and spring seasons. Most detected lightning strokes (>85%) were located in the central area around the station produced by storm region B and less than 15% were produced by storm region A and storm region C, located in the ocean areas over the Amundsen/Bellingshausen Sea and the Weddell Sea.