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ATMOSPHERIC SCIENCE
In 1994, Fishman et al. observed brief (>1 ms) bursts of intense gamma rays, called terrestrial gamma-ray flashes, with the Compton Gamma-Ray Observatory (CGRO) (1). Prior to this observation, intense transient bursts of gamma rays were only known to occur in an astrophysical context. The observed photon energies of more than 1 MeV suggested that the gamma rays were produced by "bremsstrahlung" radiation from high-energy electrons. (Such radiation is emitted when energetic electrons are scattered by nuclei.) The flashes-the most energetic natural photon phenomena on Earth-are caused by upward beams of electrons ("runaways") that are accelerated by thundercloud fields (2, 3).
On page 1085 of this issue, Smith et al. (4) show, based on data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite, that terrestrial gamma-ray flashes are much more common than previously thought and that the photon energies can reach ~20 MeV The CGRO and RHESSI data may be the first observational evidence of relativistic runaway breakdown, in which seed electrons at relativistic (>1 MeV) energies are accelerated by an electric field, followed by electrical breakdown as a result of their collision with air molecules. Relativistic runaway breakdown can proceed at much lower electric fields than conventional air breakdown (in which ambient thermal electrons must be accelerated to energies sufficient to ionize nitrogen). It may also be an important process in astrophysical plasmas. However, it has never been observed in the laboratory.
Computer models predict (3) that intense, transient electric fields associated with thunderclouds impose a total potential drop between 20- and 80-km altitude of more than 30 MV for large positive cloud-to-ground discharges of over 100 C (see the figure). These fields produce highly nonlinear runaway avalanches, in which accelerated electrons collide with molecules of air to strip even larger numbers of relativistic electrons. This process leads to a rapidly increasing number of relativistic electrons over large regions, with gamma-ray flashes emitted at altitudes of 30 to 70 km. The intense upward-driven relativistic electron beams eventually enter the radiation belts (the near-Earth space populated by...