The VHF Broadband Digital Interferometer developed by Osaka University has been improved to allow continuous sampling over the entire duration of a lightning flash and to utilize a generalized ...cross‐correlation technique for determining the lightning source directions. Time series waveforms of 20–80 MHz signals received at three orthogonally located antennas are continuously digitized over multisecond intervals, as opposed to sequences of short‐duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When postprocessed using cross correlation, the measurements provide angular uncertainties less than 1° and time resolution better than 1 μs. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000–80,000 radiation sources are located for a typical lightning flash. In this study, two‐dimensional interferometer observations of a classic bilevel intracloud flash are presented and combined with three‐dimensional Lightning Mapping Array observations to produce a quasi 3‐D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3‐D progression speed of negative leaders associated with intracloud K‐leaders.
Key Points
Cross‐correlation algorithm for continuously sampled DITF data
Quasi 3‐D conversion of 2‐D interferometric maps of lightning
Measured 3‐D velocities of K‐events as a function of time
A color photograph has been obtained of a negative lightning leader in clear air at 10.3 km altitude. The individual leader steps are resolved as relatively straight segments of at least ~200 m in ...length, between sharp kinks (nodes) in the channel. Each node is accompanied by a group of streamers of ~100 m in length. One node has an unconnected secondary leader with streamers at both ends. Lightning Mapping Array observations show that the leader was part of an intracloud (IC) flash. The observation shows that steps of negative leaders near 10 km altitude are an order of magnitude longer than values reported in the literature for negative leaders near sea level. Since negative leaders propagate at comparable velocities at low and high altitudes, stepping occurs at a lower rate in IC flashes, which can explain why RF emissions from IC flashes are more intermittent than those from cloud‐to‐ground flashes.
Key Points
Steps of negative leaders near 10 km altitude are a minimum of 200 m in lengthLeader steps at high altitude are completed at a lower rate than at low altitudeFaint leader and streamer phenomena can be imaged by digital still cameras
Six Lightning Mapping Array (LMA) stations were deployed in April 2010 around Eyjafjallajökull volcano in southern Iceland. Single‐station LMA observations were made during the first explosive period ...(14–18 April), and three‐dimensional LMA observations were made during the second explosive period (5–22 May). The single‐station observations revealed that continuous RF electrical activity caused by high rates of small vent discharges occurred during the first explosive period, but not the second, indicating that the strength of vent charging varied between the first and second explosive periods. During the second explosive period, very little lightning was detected between 5 and 10 May, while moderate rates of lightning were detected between 11 and 21 May, signaling that another change occurred on 11 May that affected plume electrification. The data do not make clear if it was changing eruptive activity or changing meteorological activity that resulted in the sudden onset of lightning. The plume charge structure during the second explosive period was inferred from the three‐dimensional lightning data, showing that the dominant charge structure varied between a positive monopole and a negative‐over‐positive dipole. The predominance of a low‐altitude region of positive charge and the observation that electrical activity was concentrated near the vent indicate that net positive vent charging was dominating the electrification.
Key Points
Monopolar and dipolar charge structures were inferred
Vent charging was the dominant mechanism
Volcanic ejecta carried net positive charge
On 3 August 2010 an extensive lightning flash was triggered over Langmuir Laboratory in New Mexico. The upward positive leader propagated into the storm's midlevel negative charge region, extending ...over a horizontal area of 13 × 13 km and 7.5 km altitude. The storm had a normal‐polarity tripolar charge structure with upper positive charge over midlevel negative charge. Lightning Mapping Array (LMA) observations were used to estimate positive leader velocities along various branches, which were in the range of 1–3 × 104 m s−1, slower than in other studies. The upward positive leader initiated at 3.4 km altitude, but was mapped only above 4.0 km altitude after the onset of retrograde negative breakdown, indicating a change in leader propagation and VHF emissions. The observations suggest that both positive and negative breakdown produce VHF emissions that can be located by time‐of‐arrival systems, and that not all VHF emissions occurring along positive leader channels are associated with retrograde negative breakdown.
Key Points
Positive breakdown can produce weak, impulsive VHF emissions
Positive leaders can be mapped using VHF time‐of‐arrival techniques
In this study, we analyze 44 terrestrial gamma‐ray flashes (TGFs) detected by the Fermi Gamma‐ray Burst Monitor (GBM) occurring in 2014–2016 in conjunction with data from the U.S. National Lightning ...Detection Network (NLDN). We examine the characteristics of magnetic field waveforms measured by NLDN sensors for 61 pulses that occurred within 5 ms of the start‐time of the TGF photon flux. For 21 (out of 44) TGFs, the associated NLDN pulse occurred almost simultaneously with (that is, within 200 μs of) the TGF. One TGF had two NLDN pulses within 200 μs. The median absolute time interval between the beginning of these near‐simultaneous pulses and the TGF flux start‐time is 50 μs. We speculate that these RF pulses are signatures of either TGF‐associated relativistic electron avalanches or currents traveling in conducting paths “preconditioned” by TGF‐associated electron beams. Compared to pulses that were not simultaneous with TGFs (but within 5 ms of one), simultaneous pulses had higher median absolute peak current (26 kA versus 11 kA), longer median threshold‐to‐peak rise time (14 μs versus 2.8 μs), and longer median peak‐to‐zero time (15 μs versus 5.5 μs). A majority (77%) of our simultaneous RF pulses had NLDN‐estimated peak currents less than 50 kA indicating that TGF emissions can be associated with moderate‐peak‐amplitude processes. The lightning flash associated with one of the TGFs in our data set was observed by a Lightning Mapping Array, which reported a relatively high‐power source at an altitude of 25 km occurring 101 μs after the GBM‐reported TGF discovery‐bin start‐time.
Key Points
TGF‐associated RF pulses have a wide range of NLDN‐estimated peak currents, which are proportional to their magnetic field peak amplitudes
Median rise time and peak‐to‐zero time for TGF‐simultaneous RF pulses were significantly longer than those for nonsimultaneous pulses
A TGF that was associated with relatively high‐power VHF radiation is described in detail
A GPS‐based system has been developed that accurately locates the sources of VHF radiation from lightning discharges in three spatial dimensions and time. The observations are found to reflect the ...basic charge structure of electrified storms. Observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown, recently identified as the source of trans‐ionospheric pulse pairs (TIPPs) observed by satellites from space. The bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges. The latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm. Peak VHF radiation from the energetic discharges is observed to be typically 30 dB stronger than that from other lightning processes and to correspond to source power in excess of 100 kW over a 6 MHz bandwidth centered at 63 MHz.
During the night of 22–23 October 2012, together with the Hydrology cycle in the Mediterranean eXperiment (HyMeX) Special Observation Period 1 campaign, optical observations of sprite events were ...performed above a leading stratiform Mesoscale Convective System in southeastern France. The total lightning activity of the storm was monitored in three dimensions with the HyMeX Lightning Mapping Array. Broadband Extremely Low Frequency/Very Low Frequency records and radar observations allowed characterizing the flashes and the regions of the cloud where they propagated. Twelve sprite events occurred over the stratiform region, during the last third of the lightning activity period, and well after the coldest satellite‐based cloud top temperature (−62°C) and the maximum total lightning flash rate (11 min−1). The sprite‐producing positive cloud‐to‐ground (SP + CG) strokes exhibit peak current from 14 to 247 kA, Charge Moment Changes (CMC) from 625 to 3086 C km, and Impulsive CMC (iCMC) between 242 and 1525 C km. The +CG flashes that do not trigger sprites are initiated outside the main convective core, have much lower CMC values, and in average, shorter durations, lower peak currents, and shorter distances of propagation. The CMC appears to be the best sprite predictor. The delay between the parent stroke and the sprite allows classifying the events as short delayed (<20 ms) and long delayed (>20 ms). All long‐delayed sprites, i.e., most of the time carrot sprites, are produced by SP + CG strokes with low iCMC values. All SP + CG flashes initiate close to the convective core and generate leaders in opposite directions. Negative leaders finally propagate toward lower altitudes, within the stratiform region that coincides with the projected location of the sprite elements.
Key Points
The sprite‐producing lightning flashes have a long propagation within the stratiform region
The CMC of the stroke is confirmed as a good predictor of the sprite production
Long‐delayed sprites are associated with current moment waveforms of low amplitude and long duration
Acoustic, VHF, and electrostatic measurements throw new light onto the origin and production mechanism of the thunder infrasound signature (<10 Hz) from lightning. This signature, composed of an ...initial compression followed by a rarefaction pulse, has been the subject of several unconfirmed theories and models. The observations of two intracloud flashes which each produced multiple infrasound pulses were analyzed for this work. Once the variation of the speed of sound with temperature is taken into account, both the compression and rarefaction portions of the infrasound pulses are found to originate very near lightning channels mapped by the Lightning Mapping Array. We found that none of the currently proposed models can explain infrasound generation by lightning, and thus propose an alternate theory: The infrasound compression pulse is produced by electrostatic interaction of the charge deposited on the channel and in the streamer zone of the lightning channel.
Key Points
Recent theories on infrasound pulse generation are tested against observations
A mechanism is proposed which may explain infrasound pulse generation
Correction for the speed of sound variation is developed for acoustic location
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.
Key Points
First Lightning Mapping Array VHF images of Florida triggered lightning
Primary negative charge source for FL triggered lightning may be at 3‐6 km
VHF sources obtained from positive impulsive currents less than 10 A
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