Narrow Bipolar Pulses Research Articles

Predicting the Type of Narrow Bipolar Pulse: A Machine Learning Approach

Narrow Bipolar Pulses (NBP) depicts the electric field (E-Field) changes due to a Compact Intra-cloud Discharge (CID) and are of two types namely Positive NBP (PNBP) and Negative NBP (NNBP). In this study, 437 NBPs were statistically investigated using a dataset collected in Sri Lanka in 2013, 2015, 2016 and 2017. Seven independent variables (Pulse Duration (PD), Rise Time (RT), Slow Front Duration (SFD), Zero Crossing Time (ZCT), Full Width at Half Maximum (FWHM) and Ratio between the Initial and Overshoot Peak Amplitudes (RIOPA)) and one dependent variable (Type of NBP) were analyzed. Two machine learning classification models, the Random Forest (RF) model and the Binary Logistic Regression (BLR) model, were used to predict the dependent variable based on the independent variables. Two models were compared in terms accuracy (ACC), sensitivity (SE), specificity (SP), AUC (Area Under the Curve)-ROC (Receiver Operator Characteristic) curve and kappa statistic. RF model scored the highest in terms of ACC (0.91), specificity (0.94), Kappa statistics (0.82) and AUC (0.98). In conclusion, RF model had the best performance in predicting the type of NBP hence can be used as a suitable automated method to classify the type of NBP.

Combination of narrow bipolar pulses and attempted leaders in Melaka, Malaysia

<span>This paper presents the characteristic of the positive attempted leaders preceded by positive narrow bipolar pulses (NPBPs). Attempted leaders are the preliminary breakdown process with no subsequent event (return stroke). On the other hand, narrow bipolar pulse is the lightning event commonly isolated and produces a strong electromagnetic field (in a short period). Attempted leaders hardly occurred in the tropics, and the preceding of the NBP (the combination) should be considered unique. In this present study, we found four samples in which the arithmetic means of duration of NPBPs pulse was 32.19 µs, with separation between the positive attempted leader was 1.86 ms apart. For the positive attempted leader, the arithmetic means of the whole pulse train, individual pulse and interval pulse of positive attempted leaders were 3.47 ms, 29.66 µs and 486.53 µs, respectively. The pulse train in this study seems to fade out fast compared to the isolated positive attempted leader pulse train. Next, the NPBP's pulse duration in this study shows well in agreement with the type of isolated NPBPs indicates that the association does not affect its pulse duration characteristic already present in the title.</span>

Lightning generated electric field over land and sea at Northern Region of Peninsular Malaysia: Measuring Setup

Lightning is the transfer of significant charge between two charged object, it can appear between cloud-to-cloud, cloud-to-air and cloud-to-ground. All lightning activities are correlated with charge movement and therefore, it can be studied using measurement of electrostatic field (slow field changes), radiation field (fast field changes) and magnetic field affiliated with charge movement. The measuring equipment was a parallel flat plate and vertical whip antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. In principle, both antenna have a similar measuring operation, with the exclusion of the antennas dimension, mode of transient recoder and the association of circuitry characteristic (decay time constant). This measurement setup are commonly used to perform electric field characterization related to various lightning events such as as preliminary breakdown, stepped leader, return stroke, subsequent return stroke, dart leader, dart-stepped leader, narrow bipolar pulses and cloud activity between strokes, such as regular pulse trains, K and M changes and chaotic pulse trains. The measurement systems are located very close to the sea, with respect in obtaining a perspicuous explaination of lightning propagation effects on sea instead on land. Noise-distorted effects in data captured usually introduce in a noisy environment or when the lightning strike far from the measurement station, thus the data gathered from the measurement need to be filter using MATLAB computer simulation before conducting further analysis. A promising wavelet 1-D technique, then, were used in signal denoising process rather using conventional filtration. In this works, the state of art in measuring vertical component of electric field focalized sea area during the thunderstorm days been demonstrate and explained.

Modeling Compact Intracloud Discharge (CID) as a Streamer Burst

Narrow Bipolar Pulses are generated by bursts of electrical activity in the cloud and these are referred to as Compact Intracloud Discharges (CID) or Narrow Bipolar Events in the current literature. These discharges usually occur in isolation without much electrical activity before or after the event, but sometimes they are observed to initiate lightning flashes. In this paper, we have studied the features of CIDs assuming that they consist of streamer bursts without any conducting channels. A typical CID may contain about 109 streamer heads during the time of its maximum growth. A CID consists of a current front of several nanosecond duration that travels forward with the speed of the streamers. The amplitude of this current front increases initially during the streamer growth and decays subsequently as the streamer burst continues to propagate. Depending on the conductivity of the streamer channels, there could be a low-level current flow behind this current front which transports negative charge towards the streamer origin. The features of the current associated with the CID are very different from those of the radiation field that it generates. The duration of the radiation field of a CID is about 10–20 μs, whereas the duration of the propagating current pulse associated with the CID is no more than a few nanoseconds in duration. The peak current of a CID is the result of a multitude of small currents associated with a large number of streamers and, if all the forward moving streamer heads are located on a single horizontal plane, the cumulative current that radiates at its peak value could be about 108 A. On the other hand, the current associated with an individual streamer is no more than a few hundreds of mA. However, if the location of the forward moving streamer heads are spread in a vertical direction, the peak current can be reduced considerably. Moreover, this large current is spread over an area of several tens to several hundreds of square meters. The study shows that the streamer model of the CID could explain the fine structure of the radiation fields present both in the electric field and electric field time derivative.

On the Accuracy of Ray-Theory Methods to Determine the Altitudes of Intracloud Electric Discharges and Ionospheric Reflections: Application to Narrow Bipolar Events.

Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground‐based very low frequency (VLF)/low‐frequency (LF) observations. The simplified ray‐theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high‐frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth‐ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full‐wave Finite‐Difference Time‐Domain (FDTD) approach. The simulated results are compared with ground‐based measurements at different distances in Southern China, and we assess the accuracy of the simplified ray‐theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full‐wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km.

Effects of Propagation of Narrow Bipolar Pulses, Generated by Compact Cloud Discharges, over Finitely Conducting Ground

Propagation effects on the narrow bipolar pulses (NBPs) or the radiation fields generated by compact cloud discharges as they propagate over finitely conducting ground are presented. The results were obtained using a sample of NBPs recorded with high time resolution from close thunderstorms in Sri Lanka. The results show that the peak amplitude and the temporal features such as the full width at half maximum (FWHM), zero-crossing time, and the time derivative of NBPs can be significantly distorted by propagation effects. For this reason, the study of peak amplitudes and temporal features of NBPs and the remote sensing of current parameters of compact cloud discharges should be conducted using NBPs recorded under conditions where the propagation effects are minimal.

Horizontal electric fields of lightning return strokes and narrow bipolar pulses observed in Sri Lanka

Simultaneous measurement of both vertical and horizontal electric field signatures of lightning was carried out in an elevated location in Colombo, Sri Lanka. The experimental setup used in this work was similar to an earlier study carried out by a different group in the late 1980s. To our knowledge, this is the first instance that such a study is conducted in this region. Data were acquired during the active months (April–May) of the southwest monsoon period in 2014. Lightning flashes from the most active thunderstorm was analyzed by selecting 65 Return Strokes (RS), 50 Negative Narrow Bipolar Pulses (NNBP) and 40 Positive Narrow Bipolar Pulses (PNBP). The wave shapes were initially validated against results of a previous study and subsequently via a theoretical method as well. Since the direction and the distance information was not available, rather than the amplitudes, ratios of the peak amplitudes of vertical electric field (Ev) and corresponding horizontal electric field (Eh) were compared. The average ratio for the return stroke was 0.024 ± 0.008. The same for the NNBP was 0.041 ± 0.004. The PNBP had a ratio of 0.031 ± 0.006. The average 10%–90% rise times (Tr) for Ev for RS, NNBP and PNBP was 2.124 ± 1.088 μs, 0.734 ± 0.077 μs and 1.141 ± 0.311 μs respectively. The Tr values for Eh for RS, NNBP and PNBP were 1.865 ± 1.200 μs, 0.538 ± 0.061 μs and 1.086 ± 0.423 μs.

Fluorescence excited in a thunderstorm atmosphere by relativistic runaway electron avalanches

The spectrum and spatiotemporal evolution of the fluorescence of an atmospheric discharge developing in the regime of relativistic runaway electron avalanche (RREA) generation have been calculated without involving the relativistic feedback. The discharges generating narrow bipolar pulses, along with the discharges responsible for terrestrial gamma-ray flashes, are shown to be relatively dark. Nevertheless, the fluorescence excited by a discharge involving RREAs can be recorded with cameras used to record high-altitude optical phenomena. A possible connection between a certain class of optical phenomena observed at the tops of thunderclouds and RREA emission is pointed out.

Characteristics of Narrow Bipolar Pulses observed from lightning in Sri Lanka

A detailed study on electric field characteristics of Narrow Bipolar Pulses (NBP) observed in Sri Lanka is presented here. NBPs analyzed in this work were recorded at a coastal location in the Southern part of Sri Lanka (Matara: 5.95 °N, 8.53 °E), from five highly active consecutive thunderstorm days during the month of May in 2013. The waveforms were recorded with a 10ns resolution within a 100ms time window. Both positive and negative NBPs were observed in this study with the negative type being the most frequent. Parameters presented in this study were the rise time (Tr), zero crossing time (Tz), the duration of slow front (Ts), the full width of half maximum (FWHM), the pulse duration and the ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa). The corresponding average values of negative NBPs for these parameters were found to be 0.58µs, 3.01µs, 0.20µs, 1.38µs, 19.21µs and 0.19 respectively. Similarly, for positive events corresponding values were 1.38µs, 4.66µs, 0.48µs, 1.93µs, 16.42µs and 0.37 respectively. The above values conforms to a much narrower bipolar events when compared to previously reported values which is considered to be caused by the propagation effects of signals captured by the apparatus.

Narrow bipolar pulse locations compared to thunderstorm radar echo structure

AbstractThe locations of 172 positive narrow bipolar pulses (NBPs) found on one day in Florida are superimposed on radar reflectivity data from that day. All 172 NBPs were found within the reflectivity of a thundercloud or at the edge of the reflectivity. The NBPs were classified into three groups: (I) in or above the high‐reflectivity core of the storm, (II) in the convective region but not Group I, or (III) in the anvil region. Groups I, II, and III had, respectively, 79%, 17%, and 4% of the NBPs. Of the 136 NBPs in Group I, 43% occurred within the reflectivity core and 57% occurred above the core. A sequence of 34 positive NBPs during 1 h of one thunderstorm suggests that the majority of NBPs occurred during the rapid growth of two thunderstorm cells. Positive NBPs seem to recur in some storm locations; 67 (39%) of the NBPs were part of a recurrent set. We found 28 cases of NBPs recurring in approximately the same location, including 22 doublets, 3 triplets, 2 quadruplets, and 1 sextuplet. Analyses of one quadruplet and one sextuplet showed that these 10 positive NBPs occurred just above and/or right beside the high‐reflectivity core on the downshear side of the core. Our data lead us to a hypothesis that NBPs occurring between the thunderstorm's upper positive charge and upper negative screening charge are initiated by small‐scale charge regions with positive charge above negative charge, or opposite the orientation of the large‐scale storm charges.

Observations of positive narrow bipolar pulses

AbstractWaveforms of 226 positive narrow bipolar pulses (NBPs) were obtained with five to eight stations of E‐change meters covering an area of 70 × 100 km2. The NBPs had typical average parameters: 10–90% rise time of 2.6 μs, full width at half maximum time of 2.8 μs, zero cross time of 9.9 μs, and range‐normalized amplitude at 100 km of 11.0 V/m. Four main types of positive NBP waveforms were identified: Type A had a simple bipolar waveform with a positive peak and a negative overshoot peak (1% of NBPs), Type B had extra peak(s) superimposed on the overshoot peak (67%), Type C had extra peak(s) on or just after the main positive peak (13%), and Type D had extra peak(s) before the main positive peak (19%). Regardless of type, each NBP waveform maintained its basic shape across a range of 10 to 130 km from its origin. NBP locations, obtained with a time of arrival technique, seemed unrestricted in their horizontal distribution (except for Type C), while NBP altitudes ranged from 7 to 19 km with an average of 13 km. Estimated peak currents were 2–126 kA with an average of 30 kA. Isolation of NBPs from other lightning events was determined for both temporal (660 ms) and spatial (>10 km) quantities; 37% of NBPs were isolated, 38% occurred within 660 ms before a flash, 19% occurred within flashes, and 11% occurred within 660 ms after a flash. The total RMS power radiated by NBPs within 1 kHz–2.5 MHz bandwidth had a range of 5.0 × 106–6.1 × 108 W with an average of 7.8 × 107 W.

Latitude dependence of narrow bipolar pulse emissions

In this paper, we present a comparative study on the occurrence of narrow bipolar pulses (NBPs) and other forms of lightning flashes across various geographical areas ranging from northern regions to the tropics. As the latitude decreased from Uppsala, Sweden (59.8°N) to South Malaysia (1.5°N), the percentage of NBP emissions relative to the total number of lightning flashes increased significantly from 0.13% to 12%. Occurrences of positive NBPs were more common than negative NBPs at all observed latitudes. However, as latitudes decreased, the negative NBP emissions increased significantly from 20% (Uppsala, Sweden) to 45% (South Malaysia). Factors involving mixed-phase region elevations and vertical extents of thundercloud tops are invoked to explain the observed results. These factors are fundamentally latitude dependent. Our results suggest that the NBP emission rate is not a useful measure to monitor thunderstorm severity because regular tropical thunderstorms, where relatively high NBP emissions occur, lack suitable conditions to become severe (i.e., there is modest convective available potential energy and a lack of baroclinity in such regions). Observations of significantly high negative NBP occurrences together with very rare occurrences of positive cloud-to-ground flashes and isolated breakdown pulses in tropical thunderstorms are indicative of a stronger negative screening layer magnitude and weaker lower positive charge region magnitude than those in northern regions.

Electromagnetic fields of a relativistic electron avalanche with special attention to the origin of lightning signatures known as narrow bipolar pulses

In the present study, electromagnetic fields of accelerating charges were utilized to evaluate the electromagnetic fields generated by a relativistic electron avalanche. In the analysis it is assumed that all the electrons in the avalanche are moving with the same speed. In other words, the growth or the decay of the number of electrons takes place only at the head of the avalanche. It is shown that the radiation is emanating only from the head of the avalanche where electrons are being accelerated. It is also shown that an analytical expression for the radiation field of the avalanche at any distance can be written directly in terms of the e-folding length of the avalanche. This model of the avalanche was utilized to test the idea whether the source of the lightning signatures known as narrow bipolar pulses could be relativistic avalanches. The idea was tested by using the simultaneously measured electric fields of narrow bipolar pulses at two distances, one measured far away from the source and the other in the near vicinity. The avalanche parameters were extracted from the distant field and they are used to evaluate the close field. The results show that the source of the NBP can be modeled either as a single or a multiple burst of relativistic avalanches with speed of avalanches in the range of 2–3×108m/s. The multiple avalanche model agrees better with the experimental data in that it can also generate the correct signature of the time derivatives and the HF and VHF radiation bursts of NBP.

Numerical simulations of compact intracloud discharges as the Relativistic Runaway Electron Avalanche‐Extensive Air Shower process

Compact intracloud discharges (CIDs) are sources of the powerful, often isolated radio pulses emitted by thunderstorms. The VLF‐LF radio pulses are called narrow bipolar pulses (NBPs). It is still not clear how CIDs are produced, but two categories of theoretical models that have previously been considered are the Transmission Line (TL) model and the Relativistic Runaway Electron Avalanche‐Extensive Air Showers (RREA‐EAS) model. In this paper, we perform numerical calculations of RREA‐EASs for various electric field configurations inside thunderstorms. The results of these calculations are compared to results from the other models and to the experimental data. Our analysis shows that different theoretical models predict different fundamental characteristics for CIDs. Therefore, many previously published properties of CIDs are highly model dependent. This is because of the fact that measurements of the radiation field usually provide information about the current moment of the source, and different physical models with different discharge currents could have the same current moment. We have also found that although the RREA‐EAS model could explain the current moments of CIDs, the required electric fields in the thundercloud are rather large and may not be realistic. Furthermore, the production of NBPs from RREA‐EAS requires very energetic primary cosmic ray particles, not observed in nature. If such ultrahigh‐energy particles were responsible for NBPs, then they should be far less frequent than is actually observed.

Numerical simulation of narrow bipolar electromagnetic pulses generated by thunderstorm discharges

Using the concept of avalanche relativistic runaway electrons (REs), we perform numerical simulations of compact intracloud discharge (CID) as a generator of powerful natural electromagnetic pulses (EMPs) in the HF-VHF range, called narrow bipolar pulses (NBPs). For several values of the field overvoltage and altitude at which the discharge develops, the numbers of seed electrons initiating the avalanche are evaluated, with which the calculated EMP characteristics are consistent with the measured NBP parameters. We note shortcomings in the hypothesis assuming participation of cosmic ray air showers in avalanche initiation. The discharge capable of generating NBPs produces REs in numbers close to those in the source of terrestrial γ-ray flashes (TGFs), which can be an argument in favor of a unified NBP and TGF source.

Observations of multi-microsecond VHF pulsetrains in energetic intracloud lightning discharges

Abstract. Certain intracloud lightning discharges emit energetic, multi-microsecond pulsetrains of radio noise. Observations of this distinctive form of lightning date from 1980 and have involved both ground-based and satellite-based radio recording systems. The underlying intracloud lightning discharges have been referred to as "Narrow Bipolar Pulses", "Narrow Bipolar Events", and "Compact Intracloud Discharges". An important discriminant for this species of radio emission is that, in the range above ~30 MHz, it consists of several microseconds of intense radio noise. When the intracloud emission is viewed from a satellite, each radio pulsetrain is received both from a direct lightning-to-satellite path, and after some delay, from a path via ground. Thus one recording of the radio emission, if of sufficient length, contains the "view" of the intracloud emission from two different angles. One view is of radiation exiting the emitter into the upper hemisphere, the other for radiation exiting into the lower hemisphere. However, the propagation conditions are similar, except that one path includes a ground reflection, while the other does not. One would normally expect a stereoscopic double view of the "same" emission process to provide two almost congruent time series, one delayed from the other, and also differing due to the different propagation effects along the two signal paths, namely, the ground reflection. We present somewhat unexpected results on this matter, using recordings from the FORTE satellite at a passband 118–141 MHz, with simultaneous data at 26–49 MHz. We find that the 118–141 MHz pulsetrain's detailed time-dependence is completely uncorrelated between the two views of the process. We examine statistics of the 118–141 MHz pulsetrain's integrated power and show that the power emitted into the lower hemisphere, on average, exceeds the power emitted into the upper hemisphere. Finally, we examine statistical measures of the amplitude distribution and show that the 118–141 MHz signal emitted downward is slightly more dominated by discrete, temporally-narrow impulses than is the signal emitted upward.

Unique lightning activities pertinent to tropical and temperate thunderstorms

Observations from two geographical locations show that some lightning events are unique and specific to the respective thunderstorms. From the measurements conducted in a tropical country, Sri Lanka, and a temperate country, Sweden, it is observed that there are some lightning activities that are unique for each location. Narrow bipolar pulses (NBPs) have frequently been observed in the tropical thunderstorms and were not observed in the temperate thunderstorms, whereas isolated breakdown pulses (IBPs) have frequently been observed in the temperate thunderstorms and were not observed in the tropical thunderstorms. It can be speculated that NBPs are the unique features of tropical thunderstorms and that IBPs are the unique features pertinent to the temperate thunderstorms.

Runaway breakdown and electrical discharges in thunderstorms

This review considers the precise role played by runaway breakdown (RB) in the initiation and development of lightning discharges. RB remains a fundamental research topic under intense investigation. The question of how lightning is initiated and subsequently evolves in the thunderstorm environment rests in part on a fundamental understanding of RB and cosmic rays and the potential coupling to thermal runaway (as a seed to RB) and conventional breakdown (as a source of thermal runaways). In this paper, we describe the basic mechanism of RB and the conditions required to initiate an observable avalanche. Feedback processes that fundamentally enhance RB are discussed, as are both conventional breakdown and thermal runaway. Observations that provide clear evidence for the presence of energetic particles in thunderstorms/lightning include γ‐ray and X‐ray flux intensifications over thunderstorms, γ‐ray and X‐ray bursts in conjunction with stepped leaders, terrestrial γ‐ray flashes, and neutron production by lightning. Intense radio impulses termed narrow bipolar pulses (or NBPs) provide indirect evidence for RB particularly when measured in association with cosmic ray showers. Our present understanding of these phenomena and their enduring enigmatic character are touched upon briefly.

Temporal features of different lightning events revealed from wavelet transform

Various lightning events are analysed using wavelet transform. The events include PB pulses, stepped leaders, first return strokes, subsequent strokes associated with negative ground flashes, positive return strokes and narrow bipolar pulses. Time domain electric fields were recorded from tropical and temperate thunderstorms. PB pulses pertinent to the tropical thunderstorms are found to radiate in the average frequency range 51–739kHz. Leaders are found to radiate in the frequency range 87–720kHz. The energy radiated by the negative return strokes is predominant in the range 2.8–40kHz. The energy radiated by the first subsequent strokes predominantly lies in the frequency range 4.5–55kHz. Positive return strokes are found to radiate predominantly in the frequency range 5.5–81kHz. Narrow bipolar pulses are found to predominantly radiate in the frequency range 58–714kHz.

Characteristics of narrow bipolar pulses observed in Malaysia

Narrow bipolar pulses (NBPs) are considered as isolated intracloud events with higher peak amplitude and strong high frequency emission compared to the first return strokes and other intracloud discharges. From 182 NBPs recorded in Malaysia in the tropic, 75 were narrow negative bipolar pulses (NNBPs) while 107 were narrow positive bipolar pulses (NPBPs). The mean duration of NNBPs was 24.6±17.1 μs, while 30.2±12.3 μs was observed for NPBPs. The mean full-width at half-maximum (FWHM) was 2.2±0.7 and 2.4±1.4 μs for NNBPs and NPBPs, respectively. The mean peak amplitude of NPBPs normalized to 100 km was 22.7 V/m, a factor of 1.3 higher than that of NNBPs which is 17.6 V/m. In contrast to the previous studies, it was observed that the electric field change was characterized by a bipolar pulse with a significant amount of fine structures separated by a few tens of nanoseconds intervals, embedded on it.