[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] We present observations of a rocket-and-wire triggered lightning flash obtained with high-speed video cameras recording 5400 and 50000 frames per second (frame times 185 ms and 20 ms) with time-synchronized current and electric field measurements. Transient leader channels were observed with precursor current pulses occurring before the development of the sustained upward positive leader that initiated the initial continuous current. The sustained upward positive leader stepped with a constant speed of 5.6 Â 10 4 m s À1 over its initial 100 m. The wire destruction occurred discontinuously over a time of 7 ms about 45 ms after sustained upward leader inception, with a small change in channel current. Downward leaders, upward connecting leaders, and filamentary streamers were imaged in the bottom 50 m of the channel. We present the first images of a negative step forming in lightning, apparently involving a space stem similar to steps in meter-length negative laboratory sparks.

[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 evaluated performance characteristics of the U.S. National Lightning Detection Network (NLDN) using rocket-triggered lightning data acquired in 2004-2009 at Camp Blanding, Florida. A total of 37 negative flashes that contained leader/return stroke sequences (a total of 139) were triggered during these years. For all the return strokes, locations of channel terminations on the ground were known exactly, and for 122 of them currents were measured directly using noninductive shunts. The NLDN recorded 105 Camp Blanding strokes in 34 flashes. The resultant flash and stroke detection efficiencies were 92% and 76%, respectively. The median absolute location error was 308 m. The median NLDN-estimated peak current error was −6.1%, while the median absolute value of current estimation error was 13%. Strokes in "classical" triggered flashes are similar to regular subsequent strokes (following previously formed channels) in natural lightning, and hence the results presented here are applicable only to regular negative subsequent strokes in natural lightning. The flash detection efficiency reported here is expected to be an underestimate of the true value for natural negative lightning flashes, since first strokes typically have larger peak currents than subsequent ones.

[1] Using an eight-station time of arrival (TOA) network composed of NaI(Tl) scintillation detectors and wideband electric field derivative (dE/dt) antennas covering approximately 1 km 2 on the ground, we have located both the sources of X-ray emissions and electric field changes produced during the leader phase of both downward negative natural and rocket-triggered lightning strokes. We show that the sources of X rays and leader step electric field changes are co-located in space within 50 m and that the located X rays are emitted 0.1 to 1.3 ms after the origin of the leader step electric field changes. Citation: Howard, J.,