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 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.

The physical processes responsible for a variety of early VLF scattering events have not yet been satisfactorily identified. Properly categorizing the early VLF event type is imperative to understand the causative physical processes involved. In this paper, the onset durations of 26 exceptionally high signal‐to‐noise ratio early VLF scattering events are analyzed, using scattered fields to classify events. New observations of events that exhibit “slow” amplitude changes, but “fast” scattered field changes are presented, which call into question previous analyses of early/slow events. We separately identify and analyze three early VLF events that definitively exhibit slow scattered field behavior. Additionally, we identify a significant number of events which have onset durations between the current definitions of fast and slow. Four events are observed which unambiguously exhibit a rapid initial rotation of the scattered field phasor during the first few seconds of the recovery stage. Possible physical mechanisms are discussed.

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.

A photochemical model has been developed to examine the response of the nighttime mesosphere to electric field heating. Time dynamics of 29 chemical species are accounted for by a set of 156 reactions. Recovery dynamics of electron density enhancements are examined in detail, and the recovery timescales of VLF scattering resulting from the modeled conductivity changes are quantitatively estimated. Both typical recovery (up to 240 s) and long recovery (>300 s) timescales of early VLF scattering events are explainable in terms of the model results. Electron production and loss during recovery is determined by a small set of attachment, detachment, and recombination processes. Based on the model results, we conclude that long recovery VLF scattering proceeds from sufficiently large electron density enhancements that are controlled by slow recombination loss (i.e., when attachment loss is small or balanced by detachment).