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The performance characteristics of the Earth Networks Total Lightning Network (ENTLN) were evaluated by using as ground-truth natural cloud-to-ground (CG) lightning data acquired at the Lightning Observatory in Gainesville (LOG) and rocket-triggered lightning data obtained at Camp Blanding (CB), Florida, in 2014 and 2015. Two ENTLN processors (data processing algorithms) were evaluated. The old processor (P2014) was put into use in June 2014 and the new one (P2015) has been operational since August 2015. Based on the natural-CG-lightning dataset (219 flashes containing 608 strokes), the flash detection efficiency (DE), flash classification accuracy (CA), stroke DE, and stroke CA for the new processor were found to be 99%, 97%, 96%, and 91%, respectively, and the corresponding values for the old processor were 99%, 91%, 97%, and 68%. The stroke DE and stroke CA for first strokes are higher than those for subsequent strokes. Based on the rocket-triggered lightning dataset (36 CG flashes containing 175 strokes), the flash DE, flash CA, stroke DE, and stroke CA for the new processor were found to be 100%, 97%, 97%, and 86%, respectively, while the corresponding values for the old processor were 100%, 92%, 97%, and 42%. The median values of location error and absolute peak current estimation error were 215 m and 15% for the new processor, and 205 m and 15% for the old processor. For both natural and triggered CG lightning, strokes with higher peak currents were more likely to be both detected and correctly classified by the ENTLN.

A series of vertical cross-sections taken through a small Mesoscale Convective System (MCS) observed over Florida by the dual-polarimetric SMART radar were combined with VHF radiation source locations from a lightning mapping array (LMA) to examine the lightning channel propagation paths relative to the radar-observed ice alignment signatures associated with regions of negative specific differential phase (KDP). Additionally, charge layers inferred from analysis of LMA sources were related to the ice alignment signature. It was found that intracloud flashes initiated near the upper zero-KDP boundary surrounding the negative KDP region. The zero-KDP boundary also delineated the propagation path of the lightning channel with the negative leaders following the upper boundary and positive leaders following the lower boundary. Very few LMA sources were found in the negative KDP region. We conclude that rapid dual-polarimetric radar observations can diagnose strong electric fields and may help identify surrounding regions of charge.

It has been argued in the technical literature, and widely reported in the popular press, that cosmic ray air showers (CRASs) can initiate lightning via a mechanism known as relativistic runaway electron avalanche (RREA), where large numbers of high energy and low energy electrons can, somehow, cause the local atmosphere in a thundercloud to transition to a conducting state. In response to this claim, other researchers have published simulations showing that the electron density produced by RREA is far too small to be able to affect the conductivity in the cloud sufficiently to initiate lightning. In this paper, we compare 74 days of cosmic ray air shower data collected in north central Florida during 2013, 2014, and 2015, the recorded CRASs having primary energies on the order of 1016 eV to 1018 eV and zenith angles less than 38 degrees, with Lightning Mapping Array (LMA) data, and we show that there is no evidence that the detected cosmic ray air showers initiated lightning. Furthermore, we show that the average probability of any of our detected cosmic ray air showers to initiate a lightning flash can be no more than 5 percent. If all lightning flashes were initiated by cosmic ray air showers, then about 1.6 percent of detected CRASs would initiate lightning, therefore we do not have enough data to exclude the possibility that lightning flashes could be initiated by cosmic ray air showers.

We report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude-triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a ground-level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.

The recent discovery of long recovery, early VLF scattering events (LOREs) indicates that the electric field changes from lightning discharges are capable of producing long-lasting disturbances (up to tens of minutes) in the upper mesosphere and lower ionosphere. Comparison of lightning mapping array, broadband (up to 10 MHz) electric field, and VLF (∼300 Hz to 42 kHz) magnetic field measurements shows that the field changes produced by initial breakdown (IB) processes and the following leaders in natural, cloud-to-ground lightning discharges are detectable in VLF magnetic field measurements at long distances. IB radiation has been detected in VLF for lightning discharges occurring up to 2630 km away from the VLF observing station. Radio atmospherics associated with 52 LOREs, 51 regular recovery events, and 3098 flashes detected by National Lightning Detection Network and/or GLD360 were examined for IB radiation occurring up to 15 ms before the return stroke. Our analysis reveals that in contrast to regular recovery early VLF events, LOREs are strongly associated with lightning discharges which exhibit an intense IB process and a fast first leader (typical duration

Time-correlated 1.54 µs high-speed video frames, channel-base current and current derivative (dI/dt), and electric field derivative (dE/dt) measurements are used to analyze the attachment process of triggered lightning dart-stepped leaders. Lengths, speeds, and durations of the upward-connecting positive leaders propagating from the launching structure are measured and calculated. The “leader burst” occurring immediately preceding the dE/dt slow front is demonstrated to be a distinctly different process from the preceding downward dart-stepped leader steps and is associated with the fast increase in channel-base current due to the initial interactions of the downward and upward leader streamer zones. Locations of the leader burst pulses are found to occur within or immediately above the connection region. Pulses superimposed on the dE/dt slow front are shown to occur after the initial connection between the downward and upward leaders and are associated with kiloampere-scale increases in the channel-base current. Subsequent fast-transition pulses are found to produce multiple kiloampere-scale increases in the channel-base current. Observed time delays between dE/dt and dI/dt peaks for slow front and fast-transition pulses confirm the existence of an elevated junction point between the downward and upward leaders. Average downward current wave speeds for fast-transition pulses are found to be a factor of 2 to 2.5 faster than those for slow-front pulses. For 51 dart-stepped leader events, the average total duration of the attachment process, starting with the initial fast current increase and ending with the peak of the final dI/dt fast-transition pulse, is measured to be 1.77 µs.

This study examines coordinated storm and triggered lightning observations made in July–August 2013 at the International Center for Lightning Research and Testing to determine why triggered flashes in Florida typically transition from an upward vertical channel entering the cloud to horizontal structure near the storm's melting level. Data from a balloon-borne electric field meter, a mobile 5 cm wavelength radar, and a small-baseline VHF Lightning Mapping Array acquired during a period in which three flashes were triggered on 1 August confirmed the hypothesis that the transition to horizontal lightning structure just above the melting level occurred in a layer of negative charge. This experiment was the first to provide vertical profiles of the electric field in Florida storms, from which their vertical charge distribution could be inferred. Three dissipating storms observed on different days all had negative charge near the melting level, but a growing mature storm had positive charge there.

We have evaluated the performance characteristics of the Earth Networks Total Lightning Network (ENTLN) using, as ground truth, data for 245 negative return strokes and 138 kiloampere-scale (≥1 kA) superimposed pulses in 57 flashes triggered from June 2009 to August 2012 at Camp Blanding, Florida. The performance characteristics were determined both for the ENTLN processor that had been in service at the time of acquiring triggered-lightning data (June 2009 to August 2012) and for the new ENTLN processor, introduced in November 2012. So, evaluation for the new processor simulates ENTLN output as if the new processor were in service from June 2009 to August 2012. For the same ground-truth data set and the same evaluation methodology, different performance characteristics for those two processors were obtained. For the old processor, flash detection efficiency was 77%, stroke detection efficiency was 49%, percentage of misclassified events was 61%, median location error was 631 m, and median absolute current estimation error was 51%. For the new processor, flash detection efficiency was 89%, stroke detection efficiency was 67%, percentage of misclassified events was 54%, median location error was 760 m, and median absolute current estimation error was 19%.

The time delay between lightning return stroke current and the resultant luminosity was measured for 22 return strokes in eight lightning flashes triggered by the rocket-and-wire technique during the summer of 2014 in Florida. The current-to-luminosity delay measured at the channel base at the 20% amplitude level ranged from 30 to 200 ns with an average of 90 ns and at the 50% amplitude level ranged from 30 to 180 ns with an average of 94 ns. The delays are significantly shorter than that predicted by Liang et al. (2014) from theory. The current-to-luminosity delays increase with increasing current risetime, current risetime varying from 190 ns to 570 ns, but the delay appears not to depend on the peak current value.

We estimated the performance characteristics of the Global Lightning Dataset (GLD360) using rocket-and-wire triggered lightning data acquired at Camp Blanding, Florida, in 2011–2013. The data set consisted of 201 return strokes and 84 kiloampere-scale (≥1 kA) superimposed pulses (initial continuous current pulses and M components) in 43 flashes. All the events transported negative charge to ground. The GLD360 detected 75 strokes and 4 superimposed pulses in 29 flashes. The resultant detection efficiencies were 67% for flashes, 37% for strokes, and 4.8% for superimposed pulses. Out of 75 detected strokes, one (1.3%) was reported with incorrect polarity. The median location error was 2.0 km, and the median absolute current estimation error was 27%. This is the first comprehensive evaluation of GLD360 performance characteristics relative to absolute ground truth, with all previous evaluations being at least in part relative to other locating systems. The results presented in this work may be applicable to regions in and around Florida.

At the end of 120 prereturn stroke intervals in 27 lightning flashes triggered by rocket-and-wire in Florida, residual currents with an arithmetic mean of 5.3 mA (standard derivation 2.8 mA) were recorded. Average time constants of the current decay following return strokes were found to vary between 160 µs and 550 µs, increasing with decreasing current magnitude. These results represent the most sensitive measurements of interstroke lightning current to date, 2 to 3 orders of magnitude more sensitive than previously reported measurements, and contradict the common view found in the literature that there is a no current interval. Possible sources of the residual current are discussed.

We present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground truth, Florida rocket-triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return-stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly measured channel-base currents are available for 290 of them) and 173 kiloampere-scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M components. All these events transported negative charge to the ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return-stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event-time mismatch (the difference in times at which the event is reported to occur by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 µs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a reilluminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.

Dart-stepped-leader step formation in triggered lightning is documented with high-speed video recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) and linear streak film with a temporal resolution of about 1 µs. Locally luminous points and segments of channel both separate and below the main descending leader tip were recorded on the high-speed video. Bidirectional leaders were imaged initiating at the locally luminous points below the main channel tip, points that remain stationary during the interstep process. The average speed of five bidirectional leaders was 8.4 × 105 m/s upward and 4.8 × 105 m/s downward, assuming 1.5 µs between successive images. The main dart-stepped-leader channel tip moved downward between steps. Leader steps extended below the bottom of the previous bidirectional leader. Processes that can be seen between steps on high-speed video are generally below the noise threshold of the streak film, which shows primarily the newly formed steps.

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the process for stepped, dart, and dart-stepped leaders in an anomalous rocket-triggered lightning flash that terminated on a 10 m grounded utility pole. The initiation of the first return stroke was found to occur at a height of 23 ± 3 m above the top of the utility pole and was associated with three “slow front” dE/dt pulses. A time of 1.5 µs later, a fast rise in luminosity at 18 ± 2 m was associated with a “fast transition” dE/dt pulse. The first return stroke propagated bidirectionally from its initiation height, as did subsequent return strokes from their initiation heights of 8 ± 1 m to 16 ± 2 m above the top of the utility pole. The initial upward speed of the first return stroke was 1.4 × 108 m/s, while its initial downward speed was 2.2 × 107 m/s. The channel bottom luminosity of the first return stroke rose more slowly to a two or more times larger amplitude than that of the subsequent stroke luminosities. In contrast, the National Lightning Detection Network-derived first-return-stroke peak current is smaller than that of the second and the third strokes, and our electric field records at 45 km show similar behavior for the initial field peaks of the first and subsequent strokes.

[1] Correlated Lightning Mapping Array and vertical-scan radar images are presented for three rocket-and-wire triggered lightning flashes that occurred sequentially within 17 min in the presence of a decaying multicellular convective storm system over north-central Florida. The initial stage (IS) of each flash propagated generally vertically to the altitude of the 0°C melting level, about 5 km, and then subsequently propagated for many kilometers horizontally along the melting level contour. Radar images suggest that the propagation paths of the IS channels below and above the melting level were heavily influenced by precipitation gradients. Flash UF 11-24 exhibited a 12.6 km unbranched IS channel, the longest unbranched channel observed in the study by a factor of three. During flash UF 11-25 (119 ms following the cessation of the measured IS current at ground and prior to the first return stroke), a natural cloud-to-ground discharge, perhaps induced by the IS, initiated between 2.5 and 4 km altitude and struck ground 5 to 7 km from the launching facility. The IS of flash UF 11-26 propagated upward through a descending precipitation packet and apparently induced a naturally appearing bi-level intracloud discharge via an upward-negative leader that initiated within the IS breakdown region 3.5 km from the launching facility. The upward-negative leader propagated from 5.6 to 9.3 km altitude in a time of 11 ms. The electrical current measured at ground during the IS of flash UF 11-26 exhibited a 57 ms polarity reversal, transferring 19 C of positive charge to ground.

[1] An “anomalous” rocket-and-wire triggered lightning flash, a flash whose leaders do not follow the triggering wire remnants to ground, is characterized via high-speed video images at 10 and 300 kilo-frames per second, still camera images, 66–72 MHz source locations from a Lightning Mapping Array, channel-base current, and electric field and electric field derivative (dE/dt) measurements. This is the first anomalous flash of about 410 classically triggered flashes in north-central Florida. The flash began with an upward positively charged leader (UPL) initiating from the tip of the upward-moving triggering wire about 280 m above ground level. All but the bottom 17 m of wire exploded (became luminous) 37.6 ms after UPL initiation. A stepped leader initiated, likely from the top of the wire remnants, 282 m above ground level about 1.3 ms after the wire explosion and propagated downward for 2.1 ms, attaching to the top of a grounded utility pole 117 m southwest of the launching facility. The line charge density on the stepped leader is estimated to be of the order of 10−3 C m−1. Contrary to previously reported “anomalous” flashes in France and New Mexico (roughly 16% and 31%, respectively, of their triggered flashes), in our event, there was not a tens of milliseconds current-zero period preceding the stepped leader, there was no observed downward dart leader in the UPL channel prior to the stepped leader to ground, and there was a failed attempt to reestablish current in the exploded-wire channel between the UPL and ground.

Lightning regularly generates ELF radio atmospherics (sferics) in the 5–500 Hz frequency range. The processes that produce ELF sferics have been studied for more than 50 years. Rocket-triggered lightning experiments at the International Center for Lightning Research and Testing (ICLRT) located at Camp Blanding, Florida provide a unique data set for comparing the source characteristics of a lightning return stroke to the ELF sferic measured at great (>3,000 km) distances. In this paper, we present experimental observations of rocket-triggered lightning observed at the ICLRT, including the lightning channel-base current and lightning mapping array sources, together with observations of the ELF sferics detected at Sondrestromfjord, Greenland and at Stanford, California. These observations are critically compared with model predictions (using a modified version of the Long Wavelength Propagation Capability code). We demonstrate that the effective length of the lightning channel varies between return strokes and has a detectable influence on the amplitude of the ELF sferic observed at great distances. Additionally, we demonstrate that the lateral distribution of vertical sources (within the cloud) can reproduce the relative differences in sferic amplitudes observed at Sondrestromfjord and Stanford.

Ionospheric disturbances produced by lightning discharges are capable of scattering sub-ionospherically propagating VLF waves, leading to amplitude and/or phase perturbations of the received VLF signal. A specific class of perturbation is known as the "early" VLF event, which occurs within ∼20 msec of the causative lightning discharge and indicates a direct and immediate effect on the overlying ionosphere. Typical early VLF events recover over the course ∼10–100 seconds. A very interesting sub-class of early VLF events, known as long recovery early VLF events, exhibits recoveries on the order of several minutes to hours. In this paper, we analyze over 50 long recovery events observed in North America together with VLF observations of the causative lightning discharge. Not only are the causative lightning return strokes (of both polarities) unusually strong, but they also exhibit unusual waveform characteristics. We critically compare the observed characteristics of lightning associated with long recovery events to the characteristics of natural lightning observed at the International Center for Lightning Research and Testing at Camp Blanding, Florida.

[1] We characterize the geometrical and electrical characteristics of the initial stages of nine Florida triggered lightning discharges using a Lightning Mapping Array (LMA) and measured channel-base currents. We determine initial channel and subsequent branch lengths, average initial channel and branch propagation speeds, and channel-base current at the time of each branch initiation. The channel-base current is found to not change significantly when branching occurs, an unexpected result. The initial stage of Florida triggered lightning typically transitions from vertical to horizontal propagation at altitudes of 3–6 km, near the typical freezing level of 4 km and several kilometers below the expected center of the negative cloud-charge region at 7–8 km. The data presented potentially provide information on thunderstorm electrical and hydrometeor structure and discharge propagation physics. LMA source locations were obtained from VHF sources of positive impulsive currents as small as 10 A, in contrast to expectations found in the literature.

The correlations between channel-bottom light intensity and channel-base current of all discharge processes of a rocket-and-wire-triggered lightning flash, including initial continuous current (ICC) pulses, ICC pulse background continuing current (IBCC), return strokes, M components, and M component background continuing currents (MBCC), have been investigated. A rough linear correlation has been found between the current squared and the light intensity for ICC pulses (including peaks of different ICC pulses), IBCC, the initial rising stage (IRS) of return strokes (including current peaks of different strokes), and MBCC. The slopes of the correlation regression lines for the current squared versus light intensity of ICC pulses and IBCC are similar, but they are about 2–3 times smaller than the slopes of MBCC and 5–7 times smaller than the slopes of the IRS of return strokes. In contrast, a rough linear correlation has been found between the current and the light intensity for the later slow decay stage of return strokes and for the M components. The slopes of the correlation regression lines of the current versus the light intensity for these latter two processes are found to be similar. No simple correlation has been found between the current and the light intensity for the initial fast decay stage (IFDS) of return strokes. The duration of the IFDS of return strokes generally lasts from several microseconds to several tens of microseconds and is more or less directly proportional to the corresponding peak return stroke current squared. A time delay ranging from 12 µs to 300 µs has been found between the current and the light intensity of all ICC pulses and M components. The time delay decreases as the corresponding peak current increases.

Lightning Mapping Array (LMA) data are used to compare the propagation paths of seven rocket-triggered lightning flashes to the inferred charge structure of the thunderstorms in which they were triggered. This is the first LMA study of Florida thunderstorm charge structure. Three sequentially (within 16 min) triggered lightning flashes, whose initial stages were the subject of Hill et al. (2013), are reexamined by comparing the complete flashes to the preceding natural lightning to demonstrate that the three rocket-triggered flashes propagated through an inferred negative charge region that decreased from about 6.8 to about 4.4 km altitude as the thunderstorm dissipated. Two other flashes were also sequentially triggered (within 9 min) in a thunderstorm that contained a convectively intense region ahead of a stratiform region, with similar observed results. Finally, two unique cases of triggered lightning flashes are presented. In the first case, the in-cloud portion of the triggered lightning flash, after ascending to and turning horizontal at 5.3 km altitude, just above the 0°C level, was observed to very clearly resemble the geometry of the in-cloud portion of the preceding natural lightning discharges. In the second case, a flash was triggered relatively early in the storm's lifecycle that did not turn horizontal near the 0°C level, as is usually the case for triggered lightning in dissipating storms, but ascended to nearly 7.5 km altitude before exhibiting extensive horizontal branching.

Electric field derivative (dE/dt) pulse waveforms from dart-stepped-leaders in rocket-and-wire triggered lightning, recorded a distance of 226 m from the channel base, are characterized. A single dE/dt pulse associated with a leader step consists of a fast initial rise of the same polarity as the following return stroke followed by an opposite polarity overshoot of smaller amplitude and subsequent decay to background level, without superimposed secondary pulses. A “slow front” often precedes the fast initial rise. For 47 single dE/dt leader pulses occurring during the final 15 µs of 24 dart-stepped-leaders, the pulse mean half-peak width was 76 ns, mean 10-to-90% risetime 73 ns, and mean range-normalized peak amplitude 2.5 V/m/µs. For integrated dE/dt, the mean half-peak width was 214 ns and the mean range-normalized peak amplitude 0.21 V/m. Most dart-stepped-leader dE/dt pulses are more complex than a single pulse. Interpulse interval and average peak amplitude range normalized to 100 km were measured for both single and complex dE/dt pulses during the final 15 µs of 10 dart-stepped-leaders preceding triggered return strokes with peak currents ranging from 8.1 to 31.4 kA. The average range-normalized dE/dt and numerically integrated dE/dt (electric field) peak amplitude increased from 0.9 to 4.9 V/m/µs and 0.13 to 0.47 V/m, respectively, with increasing return stroke peak current while the interpulse interval remained relatively constant at about 2 µs. Strong positive linear correlations were found between both average range-normalized peak pulse amplitude and interstroke interval versus the following return stroke peak current.

[1] Lightning Mapping Array source locations, channel base currents, and electric field waveforms are presented for a lightning flash triggered in the rainbands of 2012 tropical storm Debby. The National Lightning Detection Network reported no natural cloud-to-ground discharges within 60 km of the North Florida triggering site for at least 20 h before and 8 h after the triggered flash. Additionally, local electric field mill and wideband antenna networks show no close cloud or cloud-to-ground flashes. The triggering rocket was launched with negative charge overhead producing an electric field at the ground of 5 kV m−1 and in coordination with X-band, dual-polarimetric radar observations of streamers of enhanced precipitation descending from the melting level as they approached the site. The Debby flash consisted of an initial stage (IS) followed by eleven leader/return stroke sequences. The flash exhibited all the processes of normal triggered and natural cloud-to-ground lightning: leader/return stroke sequences, continuing currents, K events, and M components. Additionally, the flash exhibited several exceptional characteristics: three return stroke peak currents greater than 25 kA, one very long, 352 ms, continuing current that transferred about 35 C of charge to ground, and a relatively short, 202 ms, IS containing no initial continuous current pulses. Following a near-vertical upward positive leader attaining 2.8 km height, the IS branched and propagated horizontally at 3.5 km altitude. The flash, exhibiting strokes and continuing current, then ascended to and propagated horizontally at 5.5 km, extending about 25 km south and 15 km east. The 0°C level was near 4.5 km above sea level. It follows from the above that clouds that are not producing natural lightning can represent a triggered lightning hazard to launch vehicles and aircraft.