Properties of the X‐ray emission from rocket‐triggered lightning as measured by the Thunderstorm Energetic Radiation Array (TERA)

Abstract

[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.

The relativistic feedback discharge model of terrestrial gamma ray flashes

“…The Monte Carlo code used in this work was developed at Florida Tech in 2002 by J. R. Dwyer [ Dwyer , 2003, 2004, 2005, 2007, 2008, 2009, 2010; Coleman and Dwyer , 2006; Saleh et al , 2009; Dwyer et al , 2009; Dwyer and Babich , 2011]. This Monte Carlo simulation of relativistic runaway electron avalanches is capable of modeling the development and propagation of runaway electron avalanches in any gaseous medium for both spatially and time‐varying electric and magnetic fields.…”

“…Indeed, X‐rays are now routinely measured from natural cloud‐to‐ground and rocket‐and‐wire‐triggered lightning. In a series of experiments beginning in 2002 at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL, it was found that lightning produces significant X‐ray emission up to about an MeV during the stepped‐leader phase of natural lightning and during the dart‐leader phase of natural and rocket‐and‐wire‐triggered lightning, with the most intense emission often detected immediately before the return stroke during the attachment process [ Dwyer et al , 2004b, 2005; Howard et al , 2008; Saleh et al , 2009; Howard et al , 2010]. The X‐rays are usually observed to arrive in short (<1 μ s) bursts in coincidence with the step formation of the stepped leaders and dart‐stepped leaders.…”

“…Using the luminosity of runaway electrons measured from triggered lightning of about 10 17 electrons per second [ Saleh et al , 2009], with an additional RREA multiplication of 10 4 (consistent with the feedback limit), in 10 −4 s (the duration of a TGF) 10 17 runaway electrons would result, with the correct 7 MeV energy spectrum [ Dwyer et al , 2010]. However, it is not clear how the TGF time scale of 0.1 ms comes about.…”

X-ray Observations at Gaisberg Tower

“…X-ray emissions have been measured successfully over a span of about ten years from triggered lightning and natural lightning in Florida at distances up to about a kilometre from the lightning channel [16][17][18][19]. According to experiments carried out in Gainesville, Florida, the maximum source height of X-rays corresponding to first-stroke stepped leaders and dart-stepped leaders of natural lightning are 800 m and 3.6 km respectively [19].…”

“…The detector used is similar to the detector described in more detail in [18] by Saleh et al Two fibre-optic links (of type Terahertz Technologies V-250, with a bandwidth of 10 MHz, and input voltage peak-to-peak threshold of 1 V) carried the detector signal to a transient recorder inside the EMC cabinet. The X-ray detection system (the detectors and fibre-optic transmitters) was powered by two 12 V batteries also housed inside the same aluminium box as the detectors.…”