Narrow Bipolar Events Research Articles

Propagation Effects on Distant Electromagnetic Fields Radiated by Narrow Bipolar Events

In this article, we studied the propagation effect of finite ground conductivity and earth's curvature on the far vertical electric fields at ground level radiated by narrow bipolar events (NBEs), within several hundreds of kilometers from the discharge channel. Two different NBE current moments with risetimes of 5 and 8 μs are adopted, and two discharge heights of 5 and 15 km are considered, respectively. Simulation results show that the effect of earth's curvature should be considered when the propagation distance is larger than 200 km, and the propagation effect is closely related the NBE discharge current waveform, the discharge height, the ground conductivity, and the observation distance. With the decrease in ground conductivity, the field attenuation obviously increases. Besides, the attenuation is more when the source contains more high frequency component. It is also found that the propagation effect is more obvious when the discharge height is lower, especially for ground conductivity lower than 0.01 S/m. The reduction of NBE's far field peak caused by the propagation effect can reach about 42%, 60%, and 70% at the propagation distance of 100, 200, and 300 km, respectively. Furthermore, the current moment of NBE will be underestimated by about 5-35% from the observed far field without considering the propagation effect, within 300 km from NBE channel. However, the propagation effect on the estimation of NBE charge moment can be approximately ignored.

Blue Optical Observations of Narrow Bipolar Events by ASIM Suggest Corona Streamer Activity in Thunderstorms

AbstractWhile narrow bipolar events (NBEs) could be related with lightning initiation, their intrinsic physics remains in question. Here we report on optical measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station (ISS) of blue flashes associated with NBEs. They are observed in a narrow blue band centered at 337 nm, with no simultaneous activity at 777.4 nm, considered a strong lightning emission line. From radio waves measured from the ground, we find that 7 of 10 single‐pulse blue events can be identified as positive NBEs. The source altitudes estimated from optical and radio signals agree and indicate that the sources of the blue flashes are located between∼8.5 and∼14 km, in a cloud reaching 14–15 km altitude. The observations suggest that single‐pulse blue flashes are from cold ionization waves, so‐called streamers, and that positive NBEs are corona discharges formed by many streamers.

Analysis of a Gigantic Jet in Southern China: Morphology, Meteorology, Storm Evolution, Lightning, and Narrow Bipolar Events

AbstractAt about 22:43:30 BJT (Beijing Time = UTC + 8) on 13 August 2016, two amateur astronomers in Shikengkong, Guangdong province, and Jiahe County, Hunan province, respectively, fortuitously captured a gigantic jet (GJ) event simultaneously, and the GJ exact location could be triangulated. The parent thunderstorm was in a very humid environment (Precipitable Water [PWAT] in excess of 60 mm), featuring high convective available potential energy (CAPE of 2,428 J/kg). The GJ occurred in the region with the coldest cloud top brightness temperature of −64 °C, suggesting the GJ was associated with strong vertical development of the thunderstorm. The vertical cross sections of radar reflectivity also show that the GJ occurred near the thunderstorm strong convection region (overshooting top). The negative cloud‐to‐ground flashes dominated during the thunderstorm evolution. Three positive narrow bipolar events (NBEs) were detected within 30 s before and after the GJ. It indicates that the NBEs were occurred in the upper and middle layers of the thunderstorm (altitude of 11–13 km) with radar reflectivity of 30–35 dBZ.

Kalman Filter and Wavelet Cross-Correlation for VHF Broadband Interferometer Lightning Mapping

Lightning mapping systems based on perpendicular crossed baseline interferometer (ITF) technology have been developed rapidly in recent years. Several processing methods have been proposed to estimate the temporal location and spatial map of lightning strikes. In this paper, a single very high frequency (VHF) ITF is used to simulate and augment the lightning maps. We perform a comparative study of using different processing techniques and procedures to enhance the localization and mapping of lightning VHF radiation. The benchmark environment involves the use of different noise reduction and cross-correlation methods. Moreover, interpolation techniques are introduced to smoothen the correlation peaks for more accurate lightning localization. A positive narrow bipolar event (NBE) lightning discharge is analyzed and the mapping procedure is confirmed using both simulated and measured lightning signals. The results indicate that a good estimation of lightning radiation sources is achieved when using wavelet denoising and cross-correlations in wavelet-domain (CCWD) with a minimal error of 3.46°. The investigations carried out in this study confirm that the ITF mapping system could effectively map the lightning VHF radiation source.

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.

A Retrospective of Findings From the FORTE Satellite Mission

AbstractWe revisit the findings from the FORTE satellite program (1997–2004), which collected optical imaging of lightning as well as optical and radio frequency time series waveforms globally from low‐earth‐orbit. These include surveys of the earth's radio frequency anthropogenic noise environment; earth surface reflectivity at radio frequencies; a scheme for classifying lightning discharge types on the basis of their very high frequency time domain power envelope; insights into the polarization and radiation pattern characteristics of different lightning types, with implications for the underlying discharge processes; and estimates of cloud optical properties based on the statistics of scattered light. Most significantly, FORTE was uniquely suited to capture large samples of data from the rare discharge known as “narrow bipolar events,” enabling detailed examination of their basic characteristics and confirming that they appear to result from fundamentally distinct physical processes compared to other lightning. In particular, despite representing huge charge transfer they evidently produce little‐to‐no light output.

Electric field change and VHF waveforms of Positive Narrow Bipolar Events in Mississippi thunderstorms

Positive Narrow Bipolar Events (+NBEs) in Mississippi thunderstorms were studied using fast antennas (FA, bandwidth 16 Hz - 2.6 MHz) and VHF antennas (Log-RF, bandwidth 186–192 MHz). The waveform characteristics of 201 positive NBEs were determined using both sensors. The +NBEs were classified in two ways: by FA waveform into Types A-D and by +NBE occurrence relative to other lightning events into three groups called Isolated, Not-Isolated, and INBE. (An INBE initiates an intracloud flash.) The FA waveform properties of 188 positive NBEs were mainly in reasonable agreement with previous studies. The VHF waveform properties of +NBEs have not been studied previously. The VHF powers of 201 positive NBE ranged from 0.1–88.4 kW with an average value of 7.8 kW. Types C and D positive NBEs tended to be more energetic (average VHF powers of 9.2 and 13.2 kW) than Types A and B (average powers of 1.9 and 4.0 kW). The INBE group of +NBEs had a larger range of VHF powers (0.2–88.4 kW) than the combined power range of the Not-Isolated and Isolated groups (0.1–26.7 kW). The INBE group also had a larger average peak power (9.9 kW) than the Not-Isolated and Isolated Groups (4.0 and 8.7 kW, respectively). However, 53% of INBEs had peak VHF power < 5 kW, so an INBE does not require a large power to initiate an intracloud flash. For 90% of the 201 positive NBEs the magnitude of the time difference between the FA peak amplitude and the Log-RF peak power was ≤2 μs, but the FA peak amplitude showed almost no correlation with the Log-RF peak power.

Classification of VLF/LF Lightning Signals Using Sensors and Deep Learning Methods.

Lightning waveform plays an important role in lightning observation, location, and lightning disaster investigation. Based on a large amount of lightning waveform data provided by existing real-time very low frequency/low frequency (VLF/LF) lightning waveform acquisition equipment, an automatic and accurate lightning waveform classification method becomes extremely important. With the widespread application of deep learning in image and speech recognition, it becomes possible to use deep learning to classify lightning waveforms. In this study, 50,000 lightning waveform samples were collected. The data was divided into the following categories: positive cloud ground flash, negative cloud ground flash, cloud ground flash with ionosphere reflection signal, positive narrow bipolar event, negative narrow bipolar event, positive pre-breakdown process, negative pre-breakdown process, continuous multi-pulse cloud flash, bipolar pulse, skywave. A multi-layer one-dimensional convolutional neural network (1D-CNN) was designed to automatically extract VLF/LF lightning waveform features and distinguish lightning waveforms. The model achieved an overall accuracy of 99.11% in the lightning dataset and overall accuracy of 97.55% in a thunderstorm process. Considering its excellent performance, this model could be used in lightning sensors to assist in lightning monitoring and positioning.

Understanding the Radio Spectrum of Thunderstorm Narrow Bipolar Events

AbstractThis paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high‐frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9–5) × 105. We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5–10 A‐m and 5–20 μC‐m, respectively, the modeling results indicate that a system of 107–108 streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

Griffiths and Phelps Lightning Initiation Model, Revisited

AbstractIn this paper we reconstruct Griffiths and Phelps' seminal model of streamer systems to test if it can reproduce the key observational features of fast positive breakdown. We first confirm that our implementation is accurate by reproducing the original results. The model describes how a system of positive streamers exhibits an initial exponential charge growth, as a function of position or time, which rapidly transitions into a quadratic steady state. The charge growth is accompanied by substantial electric field enhancement near the onset location, creating favorable conditions for lightning initiation. Due to the relatively low conductivity of streamers (effectively zero in this model), the electric field enhancement is created by the charge deposited in the first few meters of propagation, in the scale length where the charge growth transitions from exponential to quadratic. The quadratic growth of charge, combined with conical system expansion, makes the surface charge density of the moving front constant. The resulting electric field ahead of the streamer system remains nearly constant during its propagation, consistent with the observations of fast breakdown, which reveal a nearly constant propagation velocity, independently of discharge polarity. Minimal changes to the model allow for simulation of narrow bipolar events, reproducing very well their characteristic bipolar electric field change waveform. Despite its simplicity, the Griffiths and Phelps model provides valuable physical insights in the relationship between fast positive breakdown and lightning initiation.

Generation of Seed Electrons by Extensive Air Showers, and the Lightning Inception Problem Including Narrow Bipolar Events

AbstractLightning streamers and leaders need thermal electrons to initiate, but free electrons are extremely rare in the wet air of a thundercloud. Here we analyze the probabilities that high electron densities occur in extensive air showers. We argue that relevant air showers are created by cosmic particles with energies between 1015 and 1017 eV impinging onto our atmosphere. We simulate a large number of air showers and perform a stochastic analysis of their results. We present the available densities of thermal electrons as a function of altitude, time interval, and considered area, while neglecting effects of local electric fields. We find that free electron densities at altitudes between 5 and 13 km can reach values of order 103 cm−3, but only in shower cores with a radius on a centimeter scale. Above 6 km, the availability of extreme free electron densities decreases significantly with increasing altitude. Recent measurements by Rison et al. (2016, https://doi.org/10.1038/ncomms10721) indicate that several streamers must have been triggered simultaneously during discharge inception, and we suggest that an extensive air shower could have been the trigger. Rison's measurements show further that the streamers are laterally separated by several tens of meters, so they must have been triggered by electron densities as low as 1 cm−3. Such low electron densities demand a stochastic approach to streamer initiation near hydrometeors.

Characterizing three types of negative narrow bipolar events in thunderstorms

Data from fast antennas (FAs) with bandwidth of 16 Hz–2.5 MHz and VHF power sensors (Log-RF) with bandwidth of 186–192 MHz are used to examine negative narrow bipolar events, or NNBEs. The main focus is on low-altitude (<8.0 km) NNBEs that initiate negative cloud-to-ground (-CG) flashes; very few low-altitude NNBEs have been studied previously. For comparison, 24 high-altitude (>8.0 km) NNBEs are also examined. The low-altitude NNBEs are found to have two types called NNBE(L) and NNBE(H). NNBE(L)s have a bipolar FA waveform typical of NBEs while NNBE(H)s have a unipolar FA waveform. It is hypothesized that NNBE(H)s may be weak versions of NNBE(L)s in which the second, overshoot part of the bipolar waveform is too weak to detect amid the FA sensor noise. Together the 33 NNBE(L)s and NNBE(H)s occurred at an average altitude of 6.2 km (range 4.6–7.8 km), had average range-normalized (to 100 km) amplitude of 0.4 V/m (range 0.06–1.5 V/m), and had average VHF power of 130 W (range 1–1300 W). These low-altitude NNBE properties are substantially smaller and weaker than the same properties of the high-altitude NNBEs and of positive NBEs that initiate intracloud (IC) flashes; these analyses indicate that -CG flashes are easier to initiate than IC flashes. Visual inspection of the FA and Log-RF data of 868 -CG flashes showed that only 33 flashes (4%) were preceded by either an NNBE(L) or NNBE(H), so 96% of the -CG flashes investigated probably did not begin with an NNBE.

The Foshan Total Lightning Location System in China and Its Initial Operation Results

In the summer of 2013, a three-dimensional (3D)-based Foshan Total Lightning Location System (FTLLS), embedded with differential time of arrival (DTOA) techniques, was installed and started its operation in Foshan, Guangdong Province, China. In this paper, the geographical distribution and set-up information of FTLLS, the estimated locating errors and locating results, as well as its initial operation results are presented. FTLLS consists of nine sub-stations that receive electromagnetic waves associated with lightning discharges and locates VLF/LF (200 Hz–500 kHz) radiation sources in 3D. The remote sub-stations acquired triggered waveforms with a duration of 0.5 ms, a resolution of 12-bits, and a GPS-based sferic time tags of 24 h per day. Cloud-to-ground (CG) lightning events, intra-cloud (IC) lightning events and narrow bipolar events (NBEs) were located by FTLLS. Based on the Monte Carlo simulation, the two-dimensional horizontal location error is basically less than 100 m, and the vertical error (altitude) is less than 200 m when the lightning event occurs within the network. On the other hand, over 14 million lightning strikes were recorded successfully by FTLLS during the period of May to October in 2014, among which IC events, CG events and NBEs accounted for 65%, 34% and 1%, respectively. It is shown that FTLLS is capable of a fine three-dimensional (3D) location, in which the altitude parameters obtained are reasonable and well consistent with observed data in the previous studies. The location results of thunderstorms were additionally verified through simultaneously-observed radar data.

Performance Analysis of Filtered VHF Signals Captured by Lightning Interferometer System

A lightning interferometer is a system that can determine the location of Very High Frequency (VHF) radiation of lightning. Such system has been studied and used for several decades ago. In this project, VHF Interferometer is used to determine the location of VHF radiations emitted by Narrow Bipolar Event (NBE) lightning flash by correlating the signals that received at three antennas. In order to get the better result, cross-correlation algorithm technique is employed to process the signals. This algorithm can determine the time delay between the antennas baselines and then determine the azimuth and elevation angles which are used to locate the location of the NBE flash. This interferometer system also can act as an earlier warning system as it detects the first cloud flash and a forecast can be made before cloud-to-ground flashes occur. Therefore, this interferometer system is very useful for the public citizens in protecting themselves from lightning strikes.

ISUAL‐Observed Blue Luminous Events: The Associated Sferics

AbstractThe blue luminous events (BLEs) recorded by ISUAL (Imager of Sprites and Upper Atmospheric Lightning) radiate unambiguous middle ultraviolet to blue emissions (230–450 nm) but contain dim red emissions (623–754 nm). The BLE appears to be dot‐like on one ISUAL image with an integration time of 29 ms. A few BLEs develop upward into blue jets/starters or type II gigantic jets (GJs). The associated sferics of the BLEs in the extremely low frequency to very low frequency band and in the low‐frequency band exhibit similar patterns to the narrow bipolar events (NBEs) identified in the very low frequency and low‐frequency band. The ISUAL BLEs are conjectured to be the accompanied light emissions of the NBEs. Both upward and downward propagating current obtained from the associated sferics of the BLEs have been found. The source heights of the six BLEs related to negative NBEs are estimated in the range of 16.2–17.8 km. These six events are suggested to occur between the upper positive charge layer and the negative screen charge layer on the top of the normally electrified thunderstorm. The six blue starters, one blue jet, and one type II GJ are inferred to be positive upward discharges from their associated sferics in the extremely low frequency to very low frequency band. Based on the simultaneous radio and optical observations, a NBE is conjectured to be the initiation discharge with rapidly flowing current within the thunderstorm, while a blue jet/starter or a type II GJ is suggested to be the ensuing discharge with slowly varying current propagating upward from the thunderstorm.

Observations of Blue Discharges Associated With Negative Narrow Bipolar Events in Active Deep Convection

AbstractOn 19 August 2012, the Imager of Sprites and Upper Atmospheric Lightning on board the FORMOSAT‐2 satellite captured a sequence of seven blue discharges within 1 min that emanated from a parent thunderstorm over Lake Taihu in East China. The analysis of lightning activity produced in the thunderstorm indicates that at least six of these events occurred in association with negative narrow bipolar events (NBEs) that were concurrent with the blue discharge by less than 1 ms, and negative cloud‐to‐ground occurred within 6 s before each blue discharge, which is in agreement with the modeling presented by Krehbiel et al. (2008). Therefore, the frequent occurrence of negative cloud‐to‐ground could provide the favorable condition for the production of blue discharges, and negative NBEs are probably the initial event of blue discharges. The detection of negative NBEs might provide a convenient approach to detect the occurrence of blue discharges as lightning bolt shooting upward from the top of energetic thunderstorms.

Fast positive breakdown in lightning

AbstractVHF radiation sources produced by positive breakdown during lightning discharges are generally considered to be both weak and slowly propagating. However, as VHF lightning mapping systems have become more sensitive, even this weak radiation can be mapped. In addition to being a faint process, positive breakdown often produces bursts of energetic activity. During the bursts, the VHF emission is extremely bright, and the breakdown propagates at much higher speeds. Here we present VHF interferometric and time‐of‐arrival measurements of such fast positive breakdown events produced during three example flashes. Electric field change measurements show that the fast breakdown process carries positive charge. The extent and velocity of the breakdown is estimated by converting the angular source locations provided by the interferometer into Cartesian coordinates using three‐dimensional lightning mapping observations of the flash as a guide. Fast positive breakdown events are found to extend 100–2400 m into virgin air beyond the tip of the preceding positive leader, at speeds of 0.9–9 ×107 m s−1. The observations expand upon earlier observations of such breakdown and are similar to recently reported results that fast positive breakdown is the cause of high‐power narrow bipolar events.

Initial electric field changes of lightning flashes in two thunderstorms

AbstractThe beginning of all 75 lightning flashes in two small thunderstorms was investigated using an array of electric field change (E‐change) meters and an array of VHF sensors with the goal of determining if an initial E‐change (IEC) preceded the initial breakdown (IB) pulses in each flash. IECs were found at the beginning of all 62 flashes in Storm 1 and all 13 flashes in Storm 2. Hence, it is concluded that an IEC is a fundamental part of most or all lightning initiations and that an IEC is needed prior to the first IB pulse in a flash. IEC durations averaged 0.23 ms for cloud‐to‐ground (CG) flashes (range 0.08–0.54 ms) and averaged 2.7 ms for normal intracloud (IC) flashes (range 0.04–9.8 ms). IEC point dipole moments averaged 26 C m for CG flashes (range 4–86 C m) and averaged −140 C m for normal IC flashes (range −8 to −650 C m). IEC current moments averaged 120 kA m for CG flashes (range 41–410 kA m) and averaged −91 kA m for normal IC flashes (range −2 to −630 kA m). E‐change data support the suggestion that weak narrow bipolar event type events initiate some lightning flashes, but 41 of the 75 flashes had no detectable initiating pulse &gt; 0.04 V m−1 range normalized to 100 km. Two flashes had two IECs; the second IEC of each flash initiated a new lightning channel that propagated in a new direction and at a higher altitude than the original development after the first IEC.

Electrostatic field changes and durations of narrow bipolar events

Using an array of ten sensors, electric field change measurements of 35 positive narrow bipolar events (NBEs) were obtained at close range (≤10 km). At the closest sensor all 35 NBEs had a net electrostatic change (ΔEfast) associated with the main bipolar pulse with amplitudes of 0.4 to 16.3 V/m (not range normalized). At the closest sensor the bipolar pulse of each of the 35 NBEs was followed by a relatively long, slow electrostatic change (ΔEslow) with amplitudes of 0.1 to 43.4 V/m and durations of 0.7 to 33.7 ms. For ΔEfast, estimated 3-D charge moments for 10 NBEs ranged from 0.46 C kmto 1.81 C km with an average and standard deviation of (1.09 ± 0.36)C km. Seven 3-D charge moments were essentially vertically oriented, and the other three moments were tilted at 10°-20° from vertical. The ten 3-D charge moments were overlaid on vertical radar cross sections; six NBEs occurred in weak reflectivity near the upper reflectivity boundary; the other four NBEs occurred near the top of the high-reflectivity core of the thunderclouds. For ΔEslow, we estimated 3-D charge moments for only 3 NBEs; they ranged from 1.11 C km to 2.69 C km with an average of (1.83 ± 0.80)C km. A two-current transmission line model matched the bipolar pulse and the following slow change (ΔEslow) of one NBE reasonably well. The slow change mechanism may be different from the NBE mechanism and similar to the initial E-change before typical lightning flashes.