Abstract
It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.
Highlights
Despite the ubiquity of thunderstorms, lightning, and related electrical phenomena, many important electromagnetic processes in our atmosphere are poorly understood
Recent work by several groups appears to be in good agreement for key parameters that describe relativistic runaway electron avalanches (RREAs) in our atmosphere, including the avalanche threshold field, the avalanche length, the propagation speed of the avalanche, and the lateral and longitudinal diffusion coefficients
Several authors have claimed that RREAs, initiated by cosmic-rays, result in anomalously large conductivity increases, much larger than would be calculated using the flux of energetic runaway electrons and standard ionization calculations (Gurevich and Milikh 1999; Gurevich et al 1999, 2004a)
Summary
In 1992, Gurevich, Milikh and Roussel-Dupré showed that when Møller scattering (electronelectron elastic scattering) is included, the runaway electrons described by Wilson will undergo avalanche multiplication, resulting in a large number of relativistic runaway electrons for each energetic seed electron injected into the high-field region (Gurevich et al 1992; Gurevich and Zybin 2001). The work by Symbalisty et al (1998) was superseded by the work of Babich et al (2001a), who improved the formulation of the ionization processes, bringing the avalanche rates into better agreement with the Lehtinen et al (1999) results, the avalanche rates found by the more sophisticated ELIZA Monte Carlo code still disagreed. Babich et al (2007b) and Carlson et al (2008) investigated the seeding process by atmospheric cosmic-rays
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