The energy spectrum of X‐rays from rocket‐triggered lightning

Abstract

Although the production of X‐rays from natural and rocket‐triggered lightning leaders have been studied in detail over the last 10 years, the energy spectrum of the X‐rays has never been well measured because the X‐rays are emitted in very short but intense bursts that result in pulse pileup in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS‐S (ARIS Spectrometer). It consists of seven 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ thick aluminum box). Using X‐ray pulses preceding 48 return strokes in 8 rocket‐triggered lightnings, we found that the spectrum of X‐rays from leaders is too soft to be consistent with Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.

Modeling of X‐ray Images and Energy Spectra Produced by Stepping Lightning Leaders

“…The mechanism of relativistic runaway electron avalanches (RREA) (Gurevich et al, 1992) has so far provided a very good agreement with satellite measurements of gamma ray radiation from intracloud lightning discharges: terrestrial gamma ray flashes (Dwyer & Smith, 2005), and the gamma ray radiation measured at mountain tops (e.g., Chilingarian et al, 2010). However, it has been suggested that this 10.1002/2016JD026410 mechanism is inconsistent with the energy spectra of X-rays measured during natural CG or triggered lightning flashes (e.g., Arabshahi et al, 2015;Dwyer, 2004;Schaal et al, 2012). Alternatively, Celestin and Pasko (2011) have shown theoretically how these X-ray bursts can originate from the bremsstrahlung radiation of thermal runaway electrons produced during the negative corona flash stage of stepping lightning leaders.…”

“…Finally, the X-ray energy spectra that would be measured by TERA are obtained by folding the averaged energy distribution with the response matrix of unshielded TERA detectors. The formulas used for the calculation of energy deposition (Arabshahi et al, 2015) and the convolution of the photon energy distribution with the detector response matrix are presented in Appendix A.…”

“…The total energy deposition is then divided by the area of the ring in order to compare with TERA measurements. The energy that is deposited into TERA detectors by an incident photon with an energy p is calculated using the following formula (Arabshahi et al, 2015):…”

Electrostatic field changes and durations of narrow bipolar events

“…The fast breakdown mechanism might be a relativistic runaway electron avalanche [e.g., Gurevich and Zybin , ] (though this mechanism has been deemed unphysical by Arabshahi et al . []), but based on Rison et al . [], it appears to be due to fast positive breakdown.…”

“…To examine the physical mechanism behind the long duration of NBEs, we hypothesize that each NBE consists of a fast breakdown event followed by a much smaller prolonged current. The fast breakdown mechanism might be a relativistic runaway electron avalanche [e.g., Gurevich and Zybin, 2005] (though this Journal of Geophysical Research: Atmospheres 10.1002/2016JD024789 mechanism has been deemed unphysical by Arabshahi et al [2015]), but based on Rison et al [2016], it appears to be due to fast positive breakdown. The mechanism of the prolonged current is also unknown.…”