Lightning Observatory In Gainesville Research Articles

Observations of very unusual behavior of lightning discharges

Based on recent observations at the Lightning Observatory in Gainesville (LOG), Florida, we present three examples of very unusual behavior of cloud-to-ground (CG) lightning flashes: (a) One +CG and one tower-terminated -CG separated by 11 km, were briefly coupled, so that positive charge was in effect drawn from the ground at the position of –CG and injected into the ground at the position of +CG. That unusual interaction was accomplished by the initiation of M-component incident wave in the +CG channel by the leader of the first stroke of the –CG. (b) A branched positive leader intermittently extended via bidirectional transients separated by inactive intervals of about 5 ms, on average. One of the bidirectional transients, moving negative charge in the backward direction, triggered a downward negative leader, resulting in a three-stroke –CG. (c) A subsequent-stroke negative leader, moving through virgin air, entered the lower part of residual (non-luminous) first-stroke channel, in addition to creating a new ground termination about 950 m away. Studying unusual lightning behavior helps to improve our understanding of the variability of basic lightning processes and to identify new potential lightning hazards to various objects and systems.

Comparison of far electric field waveforms produced by rocket-triggered lightning strokes and subsequent strokes in natural lightning

• We measured the electric field waveforms produced by both RTL and NL at comparable far distances using same instrumentation, where the radiation field is dominant. Compared to previous work done by Mallick & Rakov (2014), we increase the sample size of RTL data and compare with NL data. • The majority of the parameters show comparable values. The distributions for 10%-to-90% risetime and HPW show less scatter for RTL than NL. The plateau type slow front occurs in RTL strokes more often than in NL strokes. The initial electric field peak and the various time parameters of field waveforms are very low. • For subsequent strokes in NL and field propagation over land, HPW values are typically of the order of 10 μs. • For 184 subsequent strokes in NL and 139 strokes in RTL, we found that the field zero-crossing time for RTL is slightly smaller than for NL. • The smaller electric field pulse widths that we measured for RTL compared to NL could be due to: 1. A faster current decay with height in RTL return strokes. 2. The bottom part of the channel for RTL is straighter than for NL. Using wideband electric field records obtained at the Lightning Observatory in Gainesville (LOG), Florida, we examined in detail the characteristics of far electric field waveforms produced by two categories of lightning return strokes. The first category includes return strokes of any order in lightning flashes triggered by a rocket extending a grounded wire toward the overhead thundercloud, which we label RTL (rocket-triggered lightning) strokes. The second category includes return strokes of order 2 and higher (that is, only subsequent strokes) in natural cloud-to-ground lightning flashes, which we label NL (natural lightning) strokes. Properties of RTL and NL strokes are expected to be similar. A total of 139 RTL strokes from years 2013 to 2016 and 184 NL strokes from years 2013 to 2015 were examined in this study. All strokes transported negative charge to ground and occurred at similar distances from LOG. Similarities and dissimilarities of waveform parameters for the two categories of strokes and their possible explanations are discussed in this paper. This is the first detailed comparison of electric field waveforms of RTL and NL strokes recorded at approximately the same distance (some tens of kilometers) and with the same instrumentation.

New insights into the lightning discharge processes

This review covers selected results of recent observations of lightning discharges performed across the entire electromagnetic spectrum (radiofrequency, optical, and energetic radiation) at the Lightning Observatory in Gainesville, Florida. The most important results include (a) characterization of the preliminary-breakdown, stepped-leader, and return-stroke processes in high-intensity (⩾50 kA) negative lightning discharges, (b) the first high-speed video images of bidirectional leader that made contact with the ground and produced a return stroke, (c) discovery of negative stepped leader branches colliding with the lateral surface of neighboring branches of the same leader, (d) new data on the occurrence context and properties of compact intracloud discharges, and (e) observation of a terrestrial gamma-ray flash that occurred during a bipolar cloud-to-ground lightning discharge. The results serve to improve our understanding of the physics of lightning with important implications for lightning modeling, lightning protection, and high-energy atmospheric physics studies.

Ground‐Based Observation of a TGF Occurring Between Opposite Polarity Strokes of a Bipolar Cloud‐To‐Ground Lightning Flash

AbstractWe present the ground‐based observation of a terrestrial gamma‐ray flash (TGF) that occurred between positive (second) and negative (third) strokes of a five‐stroke bipolar cloud‐to‐ground lightning flash. Those two strokes shared the same channel to ground at a distance of 200 m or so from the Lightning Observatory in Gainesville (LOG), Florida. Earlier TGF observations at ground level in Florida (a total of four) were either associated with the initial continuous current (ICC) of rocket‐triggered lightning flashes or occurred during the relatively steady current following the return‐stroke current peak in natural lightning flashes; that is, in the presence of current‐carrying channel to ground. The TGF presented here occurred in a different context: at the early (in‐cloud) stage of negative leader entering the remnants of the channel previously created by the positive stroke. The TGF had a duration of 35 μs and consisted of 18 pulses with amplitudes ranging from 114 to 912 keV. The overall flash context in which the TGF occurred was as follows. The first (143‐kA) stroke was negative and terminated on ground 1.4 km from LOG, the second (12‐kA) stroke was positive and forged a new path to ground about 200 m from LOG, and the third (28‐kA) stroke (TGF producer) was negative and followed the path of the second (positive) stroke. The fourth (43‐kA) stroke was negative and created a new termination on ground about 900 m from LOG, and the fifth (4.5‐kA) stroke was also negative and followed the fourth‐stroke channel.

Evolution of an Upward Negative Lightning Flash Triggered by a Distant +CG From a 257‐m‐Tall Tower, Including Initiation of Subsequent Strokes

AbstractUsing the high‐speed optical and electric field records, in conjunction with Earth Networks Total Lightning Network and radar data, we examined in detail the morphology and evolution of an upward negative flash containing six downward leader/upward return stroke sequences terminated on a 257‐m tower in Florida. The upward flash was induced (triggered) by a single‐stroke 50‐kA +CG that occurred about 45 km from the tower. The in‐cloud part of +CG was optically detected to extend toward the tower and appeared to stop at a height of about 3 km above the tower top. The six leader‐return‐stroke sequences were each initiated by a bidirectional leader utilizing the remnants of branches created during the initial stage. Electric field signatures of bidirectional leaders were similar to K‐changes. The upper end of the return‐stroke channel in all six cases exhibited branching and appeared to extend to higher altitudes or/and move closer to Lightning Observatory in Gainesville with increasing stroke order.

Compact intracloud discharges: New classification of field waveforms and identification by lightning locating systems

We analyzed wideband electric field waveforms of 1022 compact intracloud discharges (CIDs) that transported negative charge upward and were recorded in 2016 by the Lightning Detection and Waveform Storage System (LDWSS) operating at the Lightning Observatory in Gainesville (LOG), Florida. According to the NLDN, all the examined events occurred within 500 km of the LOG. CIDs, which are the strongest natural producers of HF-VHF radiation on Earth may represent a serious threat to airborne vehicles. They were distinguished from other lightning processes using their characteristic features in both the time-domain waveform and the frequency spectrum. Eleven types of CID electric field waveshapes were identified, seven of which have not been reported as separate types before. More than half of the observed CIDs showed pronounced ringing on the opposite-polarity overshoot of their bipolar electric field waveforms, which is indicative of a bouncing wave process. Two major lightning locating systems (NLDN and ENTLN) each reported most of our CIDs as CGs (essentially all as +CGs). The percentage of misclassified CIDs was higher for higher-intensity events: for peak current ranges of 0–20 kA, 20–50 kA, and ≥50 kA NLDN misclassified 10%, 97%, and 100% of CIDs, respectively. The corresponding percentages for ENTLN were 2%, 63%, and 100%.

Upgrading a Low-Cost System for Measuring Lightning Electric Field Waveforms

Recent upgrades of the low-cost lightning detection and waveform storage system (LDWSS) are presented. The system has been developed for research and operational use in the Amazon region of Brazil and it has been in operation in this region since November 1, 2017. The original version of the LDWSS operated for 10 months at the Lightning Observatory in Gainesville, Florida, where it was calibrated and tested. Two main modifications implemented include the improvement of the effective dynamic range and the capability to detect lightning in different frequency bands. The improvements were achieved by using two pre-amplifiers working in parallel, a new processing (smart saving) algorithm, and programmable analog filters. As a result, a single, low-cost lightning electric field measuring system allows one to record both close and far lightning events with good amplitude resolution and without saturation, and study both wideband and narrow-band (for example, very low frequency) field signatures of lightning, without modifying the hardware.

A Low-Cost System for Measuring Lightning Electric Field Waveforms, its Calibration and Application to Remote Measurements of Currents

A low-cost system for measuring lightning electric field waveforms (Lightning Detection and Waveform Storage System or LDWSS) is presented. The system has been developed for research and operational use in the Amazon region of Brazil. In order to quantify the magnitudes of recorded fields, the LDWSS was calibrated relative to the electric field measuring system operating at the Lightning Observatory in Gainesville (LOG), Florida. The various factors that can influence electric field measurements are discussed. As an example of application of the system, peak current of a return stroke in lightning triggered using the rocket-and-wire technique at Camp Blanding, Florida, was inferred using the corresponding electric field waveform recorded by LDWSS and compared with the directly measured current. Additionally, peak currents inferred from LDWSS field measurements were compared with those reported by the U.S. National Lightning Detection Network.

A study of the ground-attachment process in natural lightning with emphasis on its breakthrough phase

Synchronized high-speed (124 or 210 kiloframes per second) video images and wideband electromagnetic field records of the attachment process were obtained for 4 negative strokes in natural lightning at the Lightning Observatory in Gainesville, Florida. The majority of imaged upward connecting leaders (UCLs) and upward unconnected leaders, inferred to be mostly initiated from trees, exhibited a pulsating behavior (brightening/fading cycles). UCLs, whose maximum extent ranged from 11 to 25 m, propagated in virgin air at speeds ranging from 1.8 × 105 to 6.0 × 105 m/s with a mean of 3.4 × 105 m/s. Within about 100 m of the ground, the ratio of speeds of the downward negative leader and the corresponding positive UCL was about 3–4 for 2 events and 0.5 for 1 event. The breakthrough phase (final jump) was imaged for 2 events. The initial length of the common streamer zone (CSZ) was estimated to be about 30–40 m. For 2 events, speeds of positive and negative leaders developing toward each other inside the CSZ were found to be between 2.4 × 106 and 3.7 × 106 m/s. For 1 event, opposite polarity leaders were observed to accelerate inside the CSZ. The current at the end of the breakthrough phase, lasting on average 4.7 μs, was estimated to be approximately one-half of the overall current peak. Thus, about one-half of the current peak traditionally attributed to the return-stroke process is actually associated with two leaders extending toward each other to collision inside the CSZ.

Evaluation of ENTLN Performance Characteristics Based on the Ground Truth Natural and Rocket‐Triggered Lightning Data Acquired in Florida

AbstractThe 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 data set (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 data set (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 study of National Lightning Detection Network responses to natural lightning based on ground truth data acquired at LOG with emphasis on cloud discharge activity

AbstractThe U.S. National Lightning Detection Network (NLDN) detection efficiency (DE) and classification accuracy (CA) for cloud discharge (IC) activity (identified here by a sequence of non‐return‐stroke‐type electric field pulses not accompanied by channels to ground) were evaluated using optical and electric field data acquired at the LOG (Lightning Observatory in Gainesville), Florida. Our ground truth “IC events” include 26 “isolated IC events” (complete IC flashes), 58 “IC events before first return stroke,” and 69 “IC events after first return stroke.” For the total of 153 IC events, 33% were detected by the NLDN, and the classification accuracy was 86%. For complete IC flashes, the detection efficiency and classification accuracy were 73% and 95%, respectively, and the average number of NLDN‐reported cloud pulses was 2.9 per detected event. For 24 preliminary breakdown pulse trains in CG flashes, the detection efficiency and classification accuracy were 46% and 82%, respectively. We have additionally estimated the DE and CA for return strokes in CG flashes. Irrespective of stroke order and polarity, the DE was 92% (339/367), and the CA was also 92% (312/339). The DEs for negative first and subsequent strokes were 98% and 90%, respectively.

Initiation and propagation of cloud-to-ground lightning observed with a high-speed video camera.

Complete evolution of a lightning discharge, from its initiation at an altitude of about 4 km to its ground attachment, was optically observed for the first time at the Lightning Observatory in Gainesville, Florida. The discharge developed during the late stage of a cloud flash and was initiated in a decayed branch of the latter. The initial channel section was intermittently illuminated for over 100 ms, until a bidirectionally extending channel (leader) was formed. During the bidirectional leader extension, the negative end exhibited optical and radio-frequency electromagnetic features expected for negative cloud-to-ground strokes developing in virgin air, while the positive end most of the time appeared to be inactive or showed intermittent channel luminosity enhancements. The development of positive end involved an abrupt creation of a 1-km long, relatively straight branch with a streamer corona burst at its far end. This 1-km jump appeared to occur in virgin air at a remarkably high effective speed of the order of 106 m/s. The positive end of the bidirectional leader connected to another bidirectional leader to form a larger bidirectional leader, whose negative end attached to the ground and produced a 36-kA return stroke.

A Study of Preliminary Breakdown and Return Stroke Processes in High-Intensity Negative Lightning Discharges

Using an automated data processing algorithm, we examined electric field records of 5498 negative cloud-to-ground flashes reported by the U.S. National Lightning Detection Network (NLDN) within 50 to 500 km of the Lightning Observatory in Gainesville (LOG), Florida. Out of the 5498 flashes, 3496 (64%) had detectable preliminary breakdown (PB) pulse trains. Only 3077 flashes with a single PB pulse train and NLDN-reported first-return-stroke (RS) peak current ≥50 kA were selected for detailed analysis. The arithmetic mean values of PB pulse train duration, PB-RS interval, and PB/RS pulse peak ratio were 2.7 ms, 8.8 ms, and 0.15, respectively. The PB-RS interval was found to decrease with increasing RS peak current (Spearman correlation coefficient was statistically significant and equal to −0.80). The range-normalized PB pulse peak exhibited statistically significant positive correlation with the RS peak current, with Spearman correlation coefficient being 0.48. Thus, it appears that the high-intensity (≥50 kA) negative lightning is characterized by shorter (and, by inference, faster) stepped leaders and more pronounced PB pulse trains.

Optical and electric field signatures of lightning interaction with a 257-m tall tower in Florida

Simultaneous electric field and high-speed video camera records of two flashes, labeled 1593 and 1594, were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Flash 1593 was an upward negative flash whose upward positive leader initiated from a 257-m high tower, located 8.8km from LOG. It contained six leader/return-stroke sequences, all of which developed along the upward leader path and terminated on the 257-m tower. Flash 1594 was a three-stroke downward bipolar flash whose first stroke was positive and terminated on a 60-m tower. The two subsequent strokes were negative and terminated on the same 257-m tower as the six strokes of flash 1593. All strokes terminated on the 257-m tower exhibited very similar narrow bipolar electric field waveforms with damped oscillatory tails. Characteristics of those electric field waveforms are examined and compared to similar observations found in the literature.

Attachment process in subsequent strokes and residual channel luminosity between strokes of natural lightning

AbstractWe examined high‐speed video and electric field records of 23 subsequent strokes following the previous stroke channel in three natural negative lightning flashes, which were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Five strokes exhibited faintly luminous formations (FLFs) occurring in a single, pre‐return‐stroke frame and ranging from 130 to 908 m in length between the lower end of downward leader and the prospective strike point. The FLFs were inferred to be not streamers (as in first strokes) but manifestations of an increase in conduction current in the defunct channel between the leader and ground in response to the increasing electric field produced by the descending leader. Further, in eight strokes, we observed residual channel luminosity persisting over many frames (for 4.7 to 18 ms), through the pre‐return‐stroke frame. The residual luminosity was apparently associated with a stronger channel heating and a larger channel radius (and hence a lower temperature decay rate), both associated with the relatively long preceding continuing current. Presence of either FLF or residual channel luminosity did not appear to significantly influence the mode and velocity of propagation of the descending leader.

A terrestrial gamma-ray flash recorded at the Lightning Observatory in Gainesville, Florida

A terrestrial gamma-ray flash (TGF) observed at ground level is presented. It was recorded at the Lightning Observatory in Gainesville, Florida, on June 13, 2014. Ground-based observations of TGFs are very rare. To date, only two positively identified ones are found in the literature. Our TGF was associated with a single-stroke negative cloud-to-ground discharge. It had a duration of 16μs and consisted of 6 pulses, two of which exceeded the upper measurement limit of 5.7MeV. The pulses apparently corresponded to individual photons, which is a characteristic feature of TGFs. The TGF began 191μs after the return-stroke electric field peak. The stepped leader duration was as short as 3.9ms. There was essentially no energetic radiation detected during the leader process. The NLDN-reported return-stroke peak current was as high as 224kA. The characteristics and occurrence context of the LOG-recorded TGF are compared to those of the two similar events found in the literature. In all three cases there was evidence of a channel carrying appreciable current to ground at the time of TGF, and the associated (preceding or concurrent) cloud-to-ground discharge processes were unusually intense.

Characterization of negative cloud-to-ground lightning in Florida

We examined characteristics of negative cloud-to-ground lightning flashes using their electric field waveforms acquired at the Lightning Observatory in Gainesville (LOG), Florida in the summers of 2013 and 2014. Flash multiplicity, interstroke interval, flash duration, and first to subsequent stroke field peak ratio are determined for 478 flashes containing 2188 strokes and compared with previous results obtained in Florida and in other regions. We found that the average number of strokes per flash is 4.6 and the percentage of single-stroke flash is 12%. The geometric means of interstroke interval, flash duration, and first to subsequent stroke field peak ratio are 52ms, 223ms, and 2.4, respectively. About one-third (34%) of multiple-stroke flashes have at least one subsequent stroke whose field peak is greater than that of the first stroke. The geometric mean of normalized electric field peak shows a relatively weak tendency to decrease with increasing stroke order. We also found that the detectability of preliminary breakdown (PB) pulse train is affected by the signal/noise ratio, type of storm, peak current and distance. For 222 PB pulse trains, statistics on pulse duration, time interval between PB pulse train and return stroke (PB–RS interval), and PB–RS field peak ratio (PB/RS ratio) are presented. Very short PB–RS (≤6ms) intervals were observed to be associated with very high return-stroke peak currents and stepped-leader speeds.

When does the lightning attachment process actually begin?

AbstractHigh‐speed video and electric field records of 43 first and 7 new‐ground‐termination subsequent strokes in negative lightning flashes, obtained at the Lightning Observatory in Gainesville, Florida, were examined. Eighteen (36%) of these strokes exhibited faintly luminous formations (FLFs) below the downward leader tip just prior to the return stroke. All the 18 FLFs were connected to the strike object on the ground, with 14 of them being also in contact with the downward leader tip. For 11 located events showing FLFs in contact with the downward leader tip, the FLF 2‐D length ranged from 51 to 200 m and the time interval between the end of exposure of the FLF frame and the return stroke onset ranged from 42 to 106 μs. We inferred that the FLFs mostly consisted of relatively low conductivity streamers, based on the following observations and estimates. (a) The rates at which the 11 FLFs were replaced by channels capable of guiding return stroke waves ranged from 0.7 × 106 m/s to 2.2 × 106 m/s, which is comparable to leader speeds in virgin air (in the same records). (b) The overwhelming majority of downward leaders continued stepping while being connected to ground via FLFs. (c) The average electric field along FLFs is 1 to 2 orders of magnitude higher than expected for hot, leader‐like channels. Since in all cases the FLF determined the strike point, we conclude that its inferred initiation up to a few hundreds of microseconds before the return stroke onset signified the beginning of the lightning attachment process.

A negative cloud-to-ground flash showing a number of new and rarely observed features

An unusual natural lightning flash containing two branched negative strokes to ground was recorded at the Lightning Observatory in Gainesville, Florida, on 8 June 2013. The flash was apparently a bolt from the blue, whose first-stroke leader emerged from the side of the cloud at a height of about 3.5 km above ground level. The first leader showed profuse branching and what appears to be corona-like formations with spatial extent of 100 to 200 m (probably an intensified portion of radial corona sheath) at the upper part of the channel. Leader branching process facilitated by two simultaneous space stems was observed. The corresponding step lengths were estimated to be 14 and 15 m. The first-stroke attachment process involved a streamer zone about 50 m in length. One of the second-leader branches appeared to abruptly change its direction at the beginning of return stroke process.

Parameters of Electric Field Waveforms Produced by Positive Lightning Return Strokes

In 2007-2008, 52 positive cloud-to-ground flashes containing 63 return strokes (52 first, 10 second, and 1 third) were recorded at the Lightning Observatory in Gainesville, Florida. The U.S. National Lightning Detection Network located 51 (96%) of the positive return strokes at distances of 7.8-157 km from the field measuring station and correctly identified 48 (91%) of them as cloud-to-ground discharges. In this study, parameters of positive return-stroke electric field and electric field derivative waveforms are examined. The geometric mean (GM) value of initial electric field peak normalized to 100 km for 48 strokes is 18 V/m. The GM peak electric field derivative normalized to 100 km for 27 strokes is 9.0 V/m/μs. The GM zero-to-peak risetime and 10-to-90% risetime for 62 strokes are 6.9 μs and 3.4 μs, respectively, and the GM slow front duration is 5.0 μs. The GM zero-crossing time for 42 strokes is 45 μs and the GM opposite polarity overshoot for 33 strokes is 13% of the peak. These characteristics are compared with those of negative return strokes in Florida found in the literature.