Observations of Energetic Radiation From Triggered Lightning
Abstract
During the summer of 2002, x-ray observations were made less than 30 m from rocket-triggered lightning at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, using an instrument designed to measure energetic radiation during thunderstorms. The instrument consisted of a 5" NaI(Tl)/photomultiplier tube (PMT) scintillation detector, plus a control detector, identical in every way but with no scintillator. The data acquisition was triggered externally by the current measured at the 11 m tall launch tower, which usually corresponds to the occurrence of the return strokes. On 2002 July 20 and 25, 5 rockets were launched from the tower under thunderstorm conditions, resulting in a total of 5 triggered lightning flashes containing at least 24 return strokes terminated on the launch tower. During these lightning events, large amounts of energetic radiation (>>10 keV) in the form of x-rays and gamma-rays and/or energetic electrons were observed just prior to the majority (83%) of the recorded return strokes, depositing into the detector, on average, tens of MeV per stroke. The pulse shapes from the PMT preamplifiers were analyzed and found to be completely consistent with real x-rays and gamma-rays and not consistent with electrical noise in the system. Furthermore, no such signals were observed on the control detector. Also, based upon background measurements, the chance that the energetic radiation events were produced by accidental coincidences with naturally occurring background signals is completely ruled out. We conclude that these new observations provide strong evidence that electrons are accelerated to relativistic energies during triggered lightning discharges with a high occurrence rate and producing significant amounts of x-rays and gamma-rays. The timing of the energetic radiation, arriving 0-200 microseconds just prior to the return stroke, suggests that the mechanism that produces this radiation may be associated with the dart leader phase of the lightning. To date, the only viable models for such high-energy radiation involve the generation of runaway electrons in large electric fields. However, the field strengths and length scales involved in lightning processes provide a significant challenge to such models. These results have important implications for studying runaway electrons in air and the physics of lightning. Furthermore, triggered lightning makes detailed study of the phenomena practical, since in order to observe energetic radiation, the lightning must occur within a few hundred meters of the detectors due to the atmospheric attenuation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.A21E..10D
- Keywords:
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- 3300 METEOROLOGY AND ATMOSPHERIC DYNAMICS;
- 3304 Atmospheric electricity;
- 3324 Lightning;
- 3359 Radiative processes;
- 3394 Instruments and techniques