Measurements of Ionizing Radiation Generated in Thunderstorms

Measurements and theoretical studies show that thunderclouds can act as particle accelerators in nature. Terrestrial gamma ray flashes (TGFs) are bursts of gamma rays with energies ranging from below 10 keV to above 40 MeV, which last for microseconds to several milliseconds, and coincide with lightning produced in Earth's atmosphere. The gamma-ray spectra of TGFs are consistent with bremsstrahlung from energetic electrons going upward into space. More recently, similar phenomena but beamed downward have been discovered by ground-based observations: short bursts, “downward TGFs”, with durations of milliseconds and long bursts, “thunderstorm ground enhancements” (TGEs) or gamma ray glows, with durations up to several minutes. These findings have established a new academic field called “high-energy atmospheric physics”. The origin, and exact mechanisms creating the thunderstorm radiation (TGFs and TGEs), are still unclear but it has been suggested that TGFs are caused by intense electric fields produced above or inside thunderstorms. To explain TGF, electrons which are traveling at speeds very close to the speed of light collide with atomic nuclei in the atmosphere and release their energy in the form of X-rays (bremsstrahlung). Large populations of energetic electrons can be formed by avalanche growth driven by electric fields, a phenomenon called relativistic runaway electron avalanche (RREA). Airplane crew and passengers flying near thunderstorms could therefore be exposed to “dangerously” high levels of radiation in the form of short terrestrial y-ray flashes and little longer y-ray glows. To clarify the origin of TGFs, and to evaluate the absorbed dose of thunderstorm radiation, the project CRREAT (Research Centre of Cosmic Rays and Radiation Events in Atmosphere) measures both the lightning and the ionizing radiation during thunderstorms using detection instrument mounted on the roofs and inside of cars, and with ionizing radiation detectors installed on high mountains. One other important research area of CRREAT is measurements and simulations of the radiation field at aviation altitudes and at low earth orbits (LEO), since this poses a health risk for crew members and passenger's onboard aircraft and spacecraft. This paper gives an overview of possible sources of ionizing radiation and phenomena during thunderstorm.