[1] The Thunderstorm Energetic Radiation Array (TERA) is located at the University of Florida, Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. The array includes forty-five 7.6-cm-diameter NaI/photomultiplier tube detectors enclosed in 24 separate aluminum boxes that shield the detectors from light, moisture, and RF noise. The array covers the $1 km 2 ICLRT facility, centered on the rocket launch tower, used to trigger lightning. From 2005 to 2007, TERA recorded seven rocket-triggered lightning flashes. In this paper we present an analysis of the X-ray emission of three of these flashes. The X-ray emission is observed to occur during the dart leader phase of each stroke, just prior to the time of the return stroke. Significant X-rays are observed on all the detectors to a distance of 500 m from the lightning channel for times up to 200 ms prior to the start of the return stroke. Using Monte Carlo simulations to model the X-ray propagation, we find that the energetic electrons that emit the X-rays have a characteristic energy of about 1 MeV for one particular dart-stepped leader event. The X-ray emission for all three events has a radial fall off proportional to [exp (Àr/120)]/r and is most consistent with the energetic source electrons being emitted isotropically from the leader. It is also found that the X-ray and energetic electron luminosities of the leader channel decreases with increasing height above the ground. These results help shed light onto the mechanism for producing energetic radiation from lightning. For instance, a characteristic energy of 1 MeV is not consistent with the relativistic runaway electron avalanche mechanism, suggesting that so-called cold runaway electrons, produced by very strong electric fields, dominate the production of the X-rays.

[1] Using an eight-station array of electric field derivative (dE/dt) sensors and colocated NaI X-ray detectors, we have obtained 3-D RF source locations during the leaders and attachment processes of three first strokes initiated by stepped leaders in natural cloud-toground lightning and one stroke initiated by a dart-stepped leader in a rocket-and-wire triggered flash. Stepped leader and dart-stepped leader dE/dt pulses are tracked from a few hundred meters to a few tens of meters above ground, after which pulses of different characteristics than the step pulses are observed to occur at lower altitudes. These postleader pulses include: (1) the ''leader burst,'' a group of pulses in the dE/dt waveform occurring just prior to the slow front in the corresponding return-stroke electric field waveform; (2) dE/dt pulses occurring during the slow front; and (3) the fast-transition or dominant dE/dt pulse that is usually associated with the rapid transition to peak in the return-stroke electric field waveform. Additionally, the timing coincidence between X rays and dE/dt pulses on colocated measurements is used to examine the X-ray production by the postleader processes. Leader bursts (LBs) are the largest X-ray producers of the three postleader processes and exhibit propagation speeds that exceed the preceding stepped leader speeds by more than an order of magnitude. Slow-front (SF) and fast-transition pulses appear to originate from similar physical processes, probably the multiple connections of upward and downward leaders. However, more X-rays are coincident with slow-front pulses than with fast-transition pulses.

[1] We present a detailed investigation of X-ray emission from long laboratory sparks in air at atmospheric pressure. We studied 231 sparks of both polarities using a 1-MV Marx generator with gap lengths ranging from 10 to 140 cm. The X rays generated by the discharges were measured using five NaI/PMT detectors plus one plastic scintillator/PMT detector, all enclosed in 0.32-cm-thick aluminum boxes. X-ray emission was observed to accompany about 70% of negative polarity sparks and about 10% of positive polarity sparks. For the negative sparks, X-ray emission was observed to occur at two distinct times during the discharge: (1) near the peak voltage, specifically, about 1 ms before the voltage across the gap collapsed, and (2) near the time of the peak current through the gap, during the gap voltage collapse. Using collimators we determined that the former emission emanated from the gap, while the latter appeared to originate from above the gap in the space over the high-voltage components. During individual sparks, the total energy of the X rays that was deposited in a single detector sometimes exceeded 50 MeV, and the maximum energy of individual photons in some cases exceeded 300 keV. X-ray emission near the peak voltage was observed for a wide range of electrode geometries, including 12-cm-diameter spherical electrodes, a result suggesting that the X-ray emission was the result of processes occurring within the air gap and not just due to high electric fields at the electrode.

[1] Using recent X-ray and gamma-ray observations of terrestrial gamma-ray flashes (TGFs) from spacecraft and of natural and rocket-triggered lightning from the ground, along with detailed models of energetic particle transport, we calculate the fluence (integrated flux) of high-energy (million electronvolt) electrons, X rays, and gamma rays likely to be produced inside or near thunderclouds in high electric field regions. We find that the X-ray/gamma-ray fluence predicted for lightning leaders propagating inside thunderclouds agrees well with the fluence calculated for TGFs, suggesting a possible link between these two phenomena. Furthermore, based on reasonable meteorological assumptions about the magnitude and extent of the electric fields, we estimate that the fluence of high-energy runaway electrons can reach biologically significant levels at aircraft altitudes. If an aircraft happened to be in or near the high-field region when either a lightning discharge or a TGF event is occurring, then the radiation dose received by passengers and crew members inside that aircraft could potentially approach 0.1 Sv (10 rem) in less than 1 ms. Considering that commercial aircraft are struck by lightning, on average, one to two times per year, the risk of such large radiation doses should be investigated further.

X‐ray observations were made during fourteen 1.5 to 2.0 m high‐voltage discharges in air produced by a 1.5 MV Marx circuit. All 14 discharges generated x‐rays in the ∼30 to 150 keV range. The x‐rays, which arrived in discrete bursts, less than 0.5 microseconds in duration, occurred from both positive and negative polarity rod‐to‐plane discharges as well as from small, 5–10 cm series spark gaps within the Marx generator. The x‐ray bursts usually occurred when either the voltages across the gaps were the largest or were in the process of collapsing. The bursts are remarkably similar to the x‐ray bursts previously observed from lightning. These results should allow for the detailed laboratory study of runaway breakdown, a mechanism that may play a role in thunderstorm electrification, lightning initiation and propagation, and terrestrial gamma‐ray flashes (TGFs).