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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.

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.

Ground waves and sky waves measured 209 km and 250 km south of six triggered lightning flashes containing 30 strokes that occurred in the half-hour before sunset on 27 August 2015 are presented and analyzed. We use a cross-correlation technique to find the ionospheric effective reflection height and compare our results to previous techniques that calculate effective height based on the time delay between ground wave and sky wave time domain features. From the first flash to the last flash there is, on average, a 1.6 km increase in effective ionospheric height, whereas no change in effective ionospheric height can be discerned along the individual strokes of a given flash. We show to what extent the triggered lightning radiation source can be described (using channel-base current, channel geometry, and channel luminosity versus time and height) and speculate that a well-characterized source could allow a more accurate determination of the electromagnetic fields radiated toward the ionosphere than has been done to date. We show that both channel geometry and the change in return stroke current amplitude and waveshape with channel height (inferred from measured channel luminosity versus height and time) determine the waveshape of the ground wave (and presumably the upward propagating wave that results in the sky wave) and that the waveshape of the ground wave does not appear to be related to the current versus time waveform measured at the channel base other than a roughly linear relationship between the two peak values.

Fourier analysis is applied to time-domain return-stroke luminosity signals to calculate the phase and group velocities and the amplitude of the luminosity signals as a function of frequency measured between 4 m and 115 m during twelve triggered lightning strokes. We show that pairs of time-domain luminosity signals measured at different heights can be interpreted as the input and the output of a system whose frequency-domain transfer function can be determined from the measured time-domain signals. From the frequency-domain transfer function phase we find the phase and group velocities, and luminosity amplitude as a function of triggered lightning channel height and signal frequency ranging from 50 kHz to 300 kHz. We show that higher frequency luminosity components propagate faster than the lower frequency components and that higher frequency luminosity components attenuate more rapidly than lower frequency components. Finally, we calculate time-domain return stroke velocities as a function of channel height using two time-delay techniques: 1) measurement at the 20% amplitude level, and 2) cross-correlation.

Using a high-speed optical imaging system, we have observed the step formation bright pulse discharges occurring at the tip of dart-stepped leaders of rocket triggered lightning with a time resolution of 0.1 µs and a spatial resolution of about 1.4 m. Each of the step formation pulse discharges appeared to initiate at a location immediately below the bottom of its previous pulse discharge and to propagate in bidirectional (upward and downward) waves with a speed on the order of 107 m/s. The downward waves of the pulse discharges tended to slow down significantly after they propagated a distance of about 2 m. Based on the results observed in this study and those published in literatures, we propose a conceptual view of leader step formation.

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.

An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning. Measurements were taken at the International Center for Lightning Research and Testing in Camp Blanding, FL, during the summer of 2014. The array was positioned in an end-fire orientation thus enabling the peak acoustic reception pattern to be steered vertically with a frequency-dependent spatial resolution. On 14 July 2014, a lightning event with nine return strokes was successfully triggered. We present the first acoustic images of individual return strokes at high frequencies (>1 kHz) and compare the acoustically inferred profile with optical images. We find (i) a strong correlation between the return stroke peak current and the radiated acoustic pressure and (ii) an acoustic signature from an M component current pulse with an unusual fast rise time. These results show that acoustic imaging enables clear identification and quantification of thunder sources as a function of lightning channel altitude.

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.

Corona streamers are a critical component of lightning leader step formation and are postulated to produce the very high electric fields at their tips that produce runaway electrons resulting in the observed X-ray bursts associated with leader stepping. Corona emanating from the vicinity of the leader tip between leader steps was analyzed using three sequential high-speed video sequences of dart-stepped leaders in three different triggered lightning flashes during the summers of 2013 and 2014 in northeast Florida. Images were recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) at an altitude of 65 m or less. In each image sequence, the leader propagates downward in consecutive frames, with corona streamers observed to fan outward from the bright leader tip in less than the image frame time of about 1.5 µs. In 21 exposures, corona streamers propagate, on average, 9 m below the bright leader tip.

We present 63 high-speed video frames (108 kilo-frames per second (kfps), 9.26 µs per frame) showing the development of the downward negative stepped leader in the initial stage of an altitude-triggered flash. The downward negative stepped leader initiated from the bottom of the triggering wire at a height of about 128 m above ground and, 553 µs later, it struck a lightning rod located at a distance of about 50 m from the launch tower. During the leader's development, electric field derivative pulses were detected associated with leader stepping. The interpulse intervals ranged from 3 to 27 µs with a mean value of 13 µs. Distinct segments of luminosity were observed ahead of the main leader channel that appear similar to space leaders were observed in the high-speed video frames. A total of eight luminous segments were observed that were 1 m to 6 m in length and were centered at distances from the main leader channel ranging from 3 m to 8 m. The new leader steps that appeared in the frames following the luminous segments were 5 m to 8 m in length. Two of the observed segments apparently never connected to the leader channel and thus failed to produce a new leader step.

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.

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.

We investigate the structure of X-ray emissions from downward triggered lightning leaders using a pinhole-type X-ray camera (XCAM) located at the International Center for Lightning Research and Testing. This study builds on the work of Dwyer et al. (2011), which reported results from XCAM data from the 2010 summer lightning season. Additional details regarding the 2010 data are reported here. During the 2011 summer lightning season, the XCAM recorded 12 out of 17 leaders, 5 of which show downward leader propagation. Of those five leaders, one dart-stepped leader and two chaotic dart leaders are the focus of this paper. These three leaders displayed unique X-ray emission patterns: a chaotic dart leader displayed a diffuse structure (i.e., a wide lateral “spraying” distribution of X-rays), and a dart-stepped leader and a chaotic dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong X-ray emission). These two distinct X-ray emission patterns (compact and diffuse) illustrate the variability of lightning leaders. Using Monte Carlo simulations, we show that the diffuse X-ray source must originate from a diffuse source of energetic electrons or possibly emission from several sources. The compact X-ray sources originate from compact electron sources, and the X-ray source region radius and electric charge contained within the X-ray source region were between 2 and 3 m and on the order of 10–4 C, respectively. For the leaders under investigation, the X-ray source region average currents were determined to be on the order of 102 A.

[1] Using a high speed optical imaging system operated at a time resolution of either 10 ns or 100 ns, we have documented the initiation process of 14 return strokes in four rocket-triggered lightning flashes. Of the 14 strokes, nine occurred following dart leaders and five following dart-stepped leaders. The return strokes are found to initiate at heights ranging from 7.2 ± 1.4 to 21.0 ± 4.6 m above the lightning termination point. Return strokes with larger peak current tend to initiate higher. All the return strokes show initial bidirectional (upward and downward from their initiation height) propagation. We have been able to estimate the initial upward propagation speeds below 60 m for all of the return strokes. The resultant speeds range from 0.4 × 108 to 2.5 × 108 m/s. For the downward propagation speeds, only six strokes among the 14 strokes allow us to perform a reasonable estimation. Those downward speeds range from 0.6 × 108 to 1.9 × 108 m/s.

[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.

Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last 10 years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pileup in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.

We measured current and luminosity at the channel bottom of 12 triggered lightning discharges including 44 return strokes, 23 M-components, and 1 initial continuous current pulse. Combined current and luminosity data for impulse currents span a 10–90% risetime range from 0.15 to 192 µs. Current risetime and luminosity risetime at the channel bottom are roughly linearly correlated (τr,I = 0.71τr,L1.08). We observed a time delay between current and the resultant luminosity at the channel bottom, both measured at 20% of peak amplitude, that is approximately linearly related to both the luminosity 10–90% risetime (Δt20,b = 0.24τr,L1.12) and the current 10–90% risetime (Δt20,b = 0.35τr,I1.03). At the channel bottom, the peak current is roughly proportional to the square root of the peak luminosity (IP = 21.89LP0.57) over the full range of current and luminosity risetimes. For two return strokes we provide measurements of stroke luminosity vs. time for 11 increasing heights to 115 m altitude. We assume that measurements above the channel bottom behave similarly to those at the bottom and find that (1) one return stroke current peak decayed at 115 m to about 47% of its peak value at channel bottom, while the luminosity peak at 115 m decayed to about 20%, and for the second stroke 38% and 12%, respectively; and (2) measured upward return stroke luminosity speeds of the two strokes of 1.10 × 108 and 9.7 × 107 ms−1 correspond to current speeds about 30% faster. These results represent the first determination of return stroke current speed and current peak value above ground derived from measured return stroke luminosity data.

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the attachment process for six strokes in three natural lightning flashes. All strokes initiate at a height above ground and propagate bidirectionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang et al. (2013, 2014). Though the data are quite limited, these natural return strokes suggest a correlation between larger peak current and greater initiation height. Initiation heights determined here span 12–60 m with a typical uncertainty of less than 10 m. The initial upward return stroke luminosity speeds range from (0.8 ± 0.2) to (2.0 ± 0.4) × 108 m/s. Two first return strokes downward luminosity speeds are assessed as (1.6 ± 0.3) × 107 m/s and (1.4 ± 0.3) × 108 m/s. One of the first return strokes appeared to be initiated with a stepping pulse discharge of its leader as an inseparable part of the return stroke.

[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.

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] We investigate individual X-ray bursts from lightning leaders to determine if energetic electrons at the source (and hence X-rays) are emitted isotropically or with some degree of anisotropy. This study was motivated by the work of Saleh et al. (2009), which found the falloff of X-rays in concentric radial annuli, covering all azimuthal directions in each annulus, from the lightning channel to be most consistent with an isotropic electron source. Here we perform a statistical analysis of angular and spatial distributions of X-rays measured by up to 21 NaI/PMT detectors at the International Center for Lightning Research and Testing site for 21 leader X-ray bursts from five leaders (including four dart-stepped leaders and one dart leader). Two procedures were used to complete this analysis. Procedure 1 found the first-order anisotropy, and procedure 2 tested whether or not the angular distribution was consistent with an isotropic distribution. Because higher-order anisotropies could be present in the data, a distribution that is not isotropic does not necessarily have a significant first-order anisotropy. Using these procedures, we find that at least 11 out of 21 X-ray bursts have a statistically significant first-order anisotropy, and hence those 11 are inconsistent with an isotropic emission. The remaining 10 bursts do not have significant first-order anisotropy. However, of those 10 bursts, 9 are inconsistent with isotropic emission, since they exhibit significant higher-order anisotropies. Since Saleh et al. (2009) did not consider anisotropies in the azimuthal direction, these new measurements of anisotropy do not necessarily contradict that work. Indeed, our analysis supports the finding that the X-ray emissions from lightning are inconsistent with a vertically downward beam. The level of anisotropy of the runaway electrons is important because it provides, in principle, information on the streamer zone in front of the leader and the electric field near the lightning leader tip.

During the summer of 1982 we used a collimated narrow slit and photomultiplier tube in conjunction with a broadband electric-field system to study light and electric-field signals from cloud-to-ground lightning flashes in Gainesville, Florida. For three flashes at a range of about 2 km, we observed correlated sequences of light and electric-field pulses from sections of channel about 12 m long and about 80 m above ground for 50–100 μs prior to the first return strokes. These time intervals included all the leader light pulses that were recorded in each case. In one case there were at least nine distinct light pulses. Sometimes there were measurable electric-field pulses preceding the first light pulse. Early or intermediate light pulses were the largest in amplitude. Electric-field pulses generally increased in amplitude with time up to the return stroke. We interpret these observations as the result of leader steps passing through the field of view of the slit. Both light and electric-field pulses occurred at intervals of 5–20 μs, had 10 to 90% risetimes of 0.5–0.9 μs, and had full widths at half maximum of 0.8–1.5 μs. We estimate that stepped-leader speeds within 100 m of ground were between 0.8×106 and 3.9×106 m s−1.