Bipolar Events Research Articles

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.

On the nature of bipolar flashes that share the same channel to ground

The paper examines the relationship between sequential events in bipolar flashes of two groups sharing the same channel to ground: one initiated by an upward positive leader, the other by a natural flash. Two quite different mechanisms are involved in the occurrence of the bipolar flashes of each group: The bipolar events that start as a natural flash are results of recoil leaders sharing parts of the initial branching structure; the bipolar events that start as upward positive leaders are the result of the interaction between these leaders' initial branching structures and the branches of the intracloud flashes they triggered.

Longitudinal expansion of field line dipolarization

We examine the substorm expansions that started at 1155 UT 10 August 1994 in the midnight sector focusing on the longitudinal (eastward) expansion of field line dipolarization in the auroral zone. Eastward expansion of the dipolarization region was observed in all of the H, D, and Z components. The dipolarization that started at 1155 UT (0027 MLT) from 260° of geomagnetic longitude (CMO) expanded to 351°(PBQ) in about 48 min. The expansion velocity was 0.03–0.04°/s, or 1.9 km/s at 62°N of geomagnetic latitude. The dipolarization region expanding to the east was accompanied by a bipolar event at the leading edge of the expansion in latitudes equatorward of the westward electrojet (WEJ). In the midnight sector at the onset meridian, the Magnetospheric Plasma Analyzer (MAP) on board geosynchronous satellite L9 measured electrons and ions between 10 eV and 40 keV. We conclude from the satellite observations that this dipolarization was characterized by the evolution of temperature anisotropies, an increase of the electron and ion temperatures, and a rapid change in the symmetry axis of the temperature tensor. The field line dipolarization and its longitudinal expansion were interpreted in terms of the slow MHD mode triggered by the current disruption. We propose a new magnetosphere-ionosphere coupling (MI-coupling) mechanism based on the scenario that transmitted westward electric fields from the magnetosphere in association with expanding dipolarization produced electrostatic potential (negative) in the ionosphere through differences in the mobility of collisional ions and collisionless electrons. The field-aligned currents that emerged from the negative potential region are arranged in a concentric pattern around the negative potential region, upward toward the center and downward on the peripheral.

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 > 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.

Locating narrow bipolar events with single-station measurement of low-frequency magnetic fields

We developed a method to locate the narrow bipolar events (NBEs) based on the single-station measurement of low-frequency (LF, 40–500kHz) magnetic fields. The direction finding of a two-axis magnetic sensor provides the azimuth of NBEs relative to the measurement site; the ionospheric reflection pairs in the lightning sferics are used to determine the range and height. We applied this method to determine the three-dimensional (3D) locations of 1475 NBEs with magnetic signals recorded during the SHandong Artificially Triggered Lightning Experiment (SHATLE) in summer of 2013. The NBE detections are evaluated on a storm basis by comparing with radar observations of reflectivity and lightning data from the World Wide Lightning Location Network (WWLLN) for two mesoscale convective systems (MCSs) of different sizes. As revealed by previous studies, NBEs are predominately produced in the convective regions with relatively strong radar echo (with composite reflectivity ≥30dBZ), although not all the convections with high reflectivity and active lightning production are in favor of NBE production. The NBEs located by the single-station magnetic method also exhibit the distinct segregation in altitude for positive and negative NBEs, namely positive NBEs are mainly produced between 7km and 15km, while negative NBEs are predominantly produced above 14km. In summary, the results of comparison generally show that the single-station magnetic method can locate NBEs with good reliability, although the accuracy of 3D location remains to be evaluated with the traditional multi-station method based on the time-of-arrival technique. This method can be applied to track the motion of storm convection within 800km, especially when they move out to ocean beyond the detection range (typically <400km) of meteorological radars, making it possible to study NBEs in oceanic thunderstorms for which the location with multiple ground-based stations is usually not feasible.

Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms.

A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.

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.

Adjunctive Maintenance Lamotrigine for Pediatric Bipolar I Disorder: A Placebo-Controlled, Randomized Withdrawal Study

This study aimed to compare the efficacy of lamotrigine versus placebo in 10- to 17-year-olds with bipolar I disorder (BP-I) who were receiving conventional bipolar disorder treatment. In this randomized withdrawal trial, patients with BP-I of at least moderate severity received lamotrigine during an≤18-week open-label phase. Patients who maintained a stable lamotrigine dose for≥2 weeks and Clinical Global Impression-Bipolar Severity of Illness (CGI-BP[S]) score of≤3 for≥6 consecutive weeks were randomized to double-blind lamotrigine or placebo for≤36 weeks. Of 301 patients enrolled, 298 comprised the open-label intention-to-treat population, with 173 (58%) randomized. Of these patients, 41 (24%) completed the study. In the open-label phase, the mean (SD) baseline CGI-BP(S) rating was 4.4 (0.57), and the mean (standard error [SE]) time to stabilization was 101 (1.6) days. During the randomized phase, mean (SE) time to occurrence of a bipolar event (TOBE) for lamotrigine versus placebo (primary endpoint) was 155 (14.7) versus 50 (3.8), 163 (12.2) versus 120 (12.2), and 136 (15.4) versus 107 (13.8) days for the 3 index mood states (depressed, manic/hypomanic, mixed). The primary stratified log-rank analysis of TOBE was not statistically significant (hazard ratio [HR]=0.63; p= .072); however, the prespecified Cox regression analysis favored lamotrigine (p= .047). In 13-to 17-year-olds, log-rank analysis of TOBE significantly favored lamotrigine (HR= 0.46; p= .015), but not in 10- to 12-year-olds (HR= 0.93; p= .877). Dermatologic events were reported in 4% (open-label phase) and 2%(randomized phase) of patients receiving lamotrigine. Suicidality-related adverse events were reported in 7%(open-label phase) and 7% (randomized phase) of patients receiving lamotrigine. Although the primary analysis failed to detect a benefit of add-on lamotrigine for BP-I in 10- to 17-year-olds, lamotrigine may be effective in a subset of older adolescents. Clinical trial registration information-Lamictal as Add-on Treatment for Bipolar I Disorder in Pediatric Patients; http://clinicaltrials.gov/; NCT00723450.

Insights into high peak current in‐cloud lightning events during thunderstorms

AbstractWe investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in‐cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightning events with very high peak currents: the well‐known narrow bipolar events, and a previously unreported type that we call energetic in‐cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicate a likely connection between the two, which further suggests the possibility of downward directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.

On the field‐to‐current conversion factors for large bipolar lightning discharge events in winter thunderstorms in Japan

AbstractIn this paper we have simulated the far‐field waveform characteristic of large bipolar events (LBEs) occurred in winter thunderstorms in Japan and compared the field‐to‐current conversion factors (FCCFs) of LBEs with that of the lightning cloud‐to‐ground (CG) return stroke (RS) in summer thunderstorm. As for the physical process of LBEs, Wu et al. (2014) considered that LBEs may be very similar to the typical lightning RS (RS‐like process) or caused by an initial continuous current pulse (ICC‐like process) in upward lightning flashes. We assume that the lightning channel length of LBEs ranges from 500 m to 1000 m, and the height of tall object struck by LBEs is from 100 m to 300 m. By using the bouncing wave model, we found that only when the injected current waveform of LBEs is characterized with a symmetric Gaussian pulse, the simulated far‐field waveform of LBEs both for RS‐like process and ICC‐like process is similar to that observed by Wu et al. (2014). For striking tall objects with heights from 100 m and 300 m, the FCCFs of LBEs are positively correlated with its channel length and derivatives of injected current waveform, and the FCCF for RS‐like process is about similar to that for ICC‐like process. However, the FCCFs of LBEs are very different from lightning RS in summer thunderstorm; that is to say, the FCCFs developed for the well‐known lightning RS in summer thunderstorm are not suitable for LBEs.

Physical mechanism of initial breakdown pulses and narrow bipolar events in lightning discharges

AbstractTo date the true nature of initial breakdown pulses (IBPs) and narrow bipolar events (NBEs) in lightning discharges remains a mystery. Recent experimental evidence has correlated IBPs to the initial development of lightning leaders inside the thundercloud. NBE wideband waveforms resemble classic IBPs in both amplitude and duration. Most NBEs are quite peculiar in the sense that very frequently they occur in isolation from other lightning processes. The remaining fraction, 16% of positive polarity NBEs, according to Wu et al. (2014), happens as the first event in an otherwise regular intracloud lightning discharge. These authors point out that the initiator type of NBEs has no difference with other NBEs that did not start lightning, except for the fact that they occur deeper inside the thunderstorm (i.e., at lower altitudes). In this paper, we propose a new physical mechanism to explain the source of both IBPs and NBEs. We propose that IBPs and NBEs are the electromagnetic transients associated with the sudden (i.e., stepwise) elongation of the initial negative leader extremity in the thunderstorm electric field. To demonstrate our hypothesis a novel computational/numerical model of the bidirectional lightning leader tree is developed, consisting of a generalization of electrostatic and transmission line approximations found in the literature. Finally, we show how the IBP and NBE waveform characteristics directly reflect the properties of the bidirectional lightning leader (such as step length, for example) and amplitude of the thunderstorm electric field.

Lightning-initiator type of narrow bipolar events and their subsequent pulse trains

Previous observations show that some narrow bipolar events (NBEs) can initiate intracloud discharges, but the role of NBE as lightning initiation is still unclear. During the summer of 2013, 827 NBEs were detected with a 3-D LF lightning location system in Osaka, Japan. Out of 638 positive NBEs, 103 occurred as the initial events of lightning flashes. These initiator-type NBEs, called “INBEs” in this paper, are always followed by positive pulse trains whose locations show upward propagations probably from the main negative charge region to the upper positive charge region. Most of INBEs develop into intracloud flashes. Only two INBEs develop into positive ground flashes and five INBEs develop into negative ground flashes. Pulse widths and peak amplitudes of electric field change waveforms of INBEs are almost the same as those of normal NBEs. A major difference is that INBEs have much lower discharge heights. Most of INBEs are lower than 10 km while normal NBEs are mainly higher than 10 km. Characteristics of positive pulse trains following INBEs are closely related with discharge heights of INBEs. Higher INBEs are usually followed by weaker, fewer, and less frequent positive pulses with slower upward propagations. As the height increases to above 10 km, NBEs are usually no longer followed by such positive pulses.

Extreme events of stratospheric stationary waves and indications for stratosphere–troposphere coupling: simultaneous analysis in boreal winter

Using monthly stratospheric geopotential height at 20 hPa derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, a planetary wave amplitude index (PWAI) is defined by wave numbers (WN) 1–3 over 55–75°N to indicate the strength of the stratospheric stationary waves. The vertical variability of the stratospheric stationary waves and their possible connection with the stratosphere–troposphere coupling have been investigated in the North Hemisphere winter [December–February (DJF)] for the period of 1950–2010. In terms of the stratospheric PWAI, a pair of bipolar extreme (strong and weak) stratospheric wave events is identified based on the top-ten principle. The comparisons of composite analysis for the bipolar events show that the stratospheric PWAI is an effective indicator for the dynamic coupling between the stratosphere and troposphere in the boreal winter. The results show that the opposite response in the stationary wave activity, atmospheric circulations, precipitation, and surface temperature is not only found in the stratosphere but also in the troposphere and surface. In the stratospheric top-ten extremely strong (strong10) events, the wave amplitude, poleward momentum, and heat fluxes in zonal WN1 tend to increase in the whole atmospheric layer from the stratosphere down to the surface. The polar vortex is enhanced in the stratosphere and reduced in the troposphere. Both the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO) tend to a positive phase with the sea-level pressure (SLP) decreasing in the high latitudes and increasing in the mid–low latitudes. The precipitation tends to increase in the southern Asia and decrease in the southern Europe. The surface temperature becomes warmer in the middle of the Asian–European continent and cooler in southwest Asia and south Europe. In contrast, there is a clear opposing behavior except for a few small areas during the stratospheric top-ten extremely weak (weak10) events.

Large bipolar lightning discharge events in winter thunderstorms in Japan

AbstractUsing a low‐frequency lightning location system comprising nine stations, we have observed and analyzed 374 large and bipolar electric field change waveforms that occurred during the winter of 2012–2013. Since the waveforms are different from those produced by any well‐studied lightning discharge processes, we refer to these source discharge events using a new name: large bipolar events (LBEs). LBEs can be characterized by the following features: (1) All have the same polarity as negative return stroke. (2) All exhibit a single bipolar pulse with a pulse width around 15 µs and similar positive and negative cycles. (3) All are located on the land along the Japan Sea coast, indicating they are probably associated with high grounded objects. (4) Most LBEs produce very large electric field changes that are even larger than that of positive and negative return strokes. (5) Most LBEs are temporally isolated within several milliseconds but are frequently followed by intracloud discharges after tens of milliseconds. (6) Most LBEs produce a single well‐distinguished ionospheric reflection pulse, and the time difference between LBE pulse and the corresponding reflection pulse can be used to calculate ionospheric reflection height. It is speculated that LBE is a type of powerful and transient lightning discharge event produced within a compact region of strong electric field formed when the negative charge layer in thunderclouds is very close to the top of a tall grounded object.

Negative narrow bipolar lightning needs critical cloud height

High within towering thunderclouds, a distinct form of intracloud lightning, known as “narrow bipolar pulse” discharges, can occur. Like other forms of lightning, narrow bipolar events (NBE) can be either negative or positive discharges. These events are known for their high‐powered, short‐distance electrical discharges that produce strong emissions of very high frequency radio waves. Previous research has found that since NBEs take place at relatively high altitudes, it is possible to detect them remotely using satellites. To be able to use the detection of narrow bipolar events to measure cloud behavior or storm dynamics, however, requires a better understanding of the relationship between cloud properties and NBEs.

Joint radio and optical observations of the most radio-powerful intracloud lightning discharges

Abstract. The most radio-powerful intracloud lightning emissions are associated with a phenomenon variously called "narrow bipolar events" or "compact intracloud discharges". This article examines in detail the coincidence and timing relationship between, on the one hand, the most radio-powerful intracloud lightning events and, on the other hand, optical outputs (or lack thereof) of the same discharge process. This is done, first, using coordinated very high frequency (VHF) and optical observations from the FORTE satellite and, second, using coordinated sferic and all-sky optical observations from the Los Alamos Sferic Array. In both cases, it is found that the sought coincidences are exceedingly rare. Moreover, in the handful of coincidences between optical and intense radio emissions that have been identified, the radio emissions differ from their usual behavior, by being accompanied by approximately simultaneous "conventional" lightning radio emissions. It is implied that the most radio-powerful intracloud emission process essentially differs from ordinary incandescent lightning.

Spatial relationship between lightning narrow bipolar events and parent thunderstorms as revealed by phased array radar

AbstractPhased array radar with unprecedented high temporal and spatial resolution is used for the first time to analyze structures of thunderstorms producing lightning narrow bipolar events (NBEs). Locations of NBEs generally correspond well with the deepest convection, but in some thunderstorms extending higher than 15 km, positive NBEs cluster around, rather than right at the center of the core of deep convection. Negative NBEs are generally higher than positive NBEs and are usually produced at the cloud top of the thunderstorm. Positive NBEs, on the other hand, are always located well inside the thundercloud. It seems that negative NBEs can only be produced in very vigorous thunderstorms with cloud tops higher than about 14 km. Numerous thunderstorms with lower height produce few negative NBEs, indicating a height threshold for NBE production. On the basis of these findings, it becomes very convenient and accurate to monitor severe thunderstorms with negative NBEs.