Terrestrial gamma-ray flashes (TGFs) are brief bursts of energetic gamma-rays produced in the atmosphere and observed by satellites in low-Earth orbit.
Terrestrial gamma-ray flashes (TGFs) are brief bursts of energetic gamma-rays produced in the atmosphere and observed by satellites in low-Earth orbit. First discovered in 1994 (Fishman et al., 1994), these bursts are now known to be associated with lightning (Inan, 2005; Cummer et al., 2005; Stanley et al., 2006; Inan et al., 2006; Cohen et al., 2010; Inan et al., 1996; Cohen et al., 2006), produced in the middle atmosphere (Carlson et al., 2007; Dwyer and Smith, 2005), and consist of photons with individual energies ranging from <10 keV to >40 MeV and total event energy 10 kJ. A sample TGF light curve is shown in Figure 1.
In spite of much study over the past 10 years, the underlying mechanism by which lightning is associated with TGFs is still a mystery. The energetic photons in a TGF are known to be produced by energetic electrons. Such energetic electrons must be accelerated by electric fields. Though low-energy electrons feel strong frictional forces, relativistic electrons feel much less friction. As a result, in a strong electric field, populations of relativistic electrons can grow like an avalanche, suggesting that a strong electric field and an initial population of relativistic electrons will suffice to produce a TGF. This is not a very simple picture, however, as there are several possible sources of the initial population of relativistic electrons, several ways in which lightning can contribute an electric field, and the population of relativistic electrons must be large and contain sufficiently high-energy electrons to produce a TGF.
Initial populations of energetic electrons may come from cosmic rays, energetic atomic nuclei from outside the solar system that continually bombard the Earth and produce energetic electrons in the atmosphere as a result. These energetic electrons are not common enough, however: if cosmic rays are the only source of energetic electrons, the electric fields necessary to produce an observable TGF are too great to be produced in the atmosphere. Complicating things, however, is “relativistic feedback,” where a population of energetic electrons grows so big that it produces enough energetic secondary particles to start a second generation of energetic electrons. When it occurs, this process grows quite large quite rapidly and the large populations of energetic electrons that result are a good candidate for TGF production. One further possible source of the initial population of energetic electrons is lightning itself, where the confined electric fields near the channel can be strong enough to directly accelerate free low-energy electrons to high enough energies.
Whatever the source, these initial seed energetic electrons must be accelerated to very high energies to account for TGF emission. The electric fields involved in this acceleration may appear above the thundercloud as part of a quasi-electrostatic field after a large lightning discharge, but the necessary lightning is unreasonably intense. They may also be produced by an electromagnetic pulse by a fast lightning return stroke, but the necessary speed and intensity of the return stroke renders this possibility unlikely as well. The electric fields normally present in an active thunderstorm are generally too low to directly accelerate electrons to high energies, while the electric fields near lightning channels tend to lack the necessary electric potential. The unknown source of the electric fields involved in TGF is the subject of much debate.
Our research at Stanford focuses on analysis of lightning activity associated with TGFs and on computer models of possible TGF production mechanisms.
Though lightning was suggested as relevant to TGFs even in the first paper, Stanford VLF studies of electrical activity associated with TGFs was the first clear association of electrical activity with TGF emission(Inan et al., 1996). Since then, Stanford has been very active in the study of TGF-associated lightning (see citations below). With the advent of the new high-efficiency global lightning detection network (GLD360) and the high-resolution detection of TGFs by the Fermi spacecraft, the VLF group will continue to improve our observations of TGF-associated lightning.
On the theoretical side, VLF group theorists are working to try to understand TGF production mechanism. The quasi-electrostatic fields produced after lightning discharges (Figure 2, left, Lehtinen et al. (1996)), the electromagnetic pulses radiated by rapid lightning return strokes (Figure 2, middle, Inan and Lehtinen (2005)), and the direct production of radiation by the lightning channel itself (Figure 2, right, Carlson et al. (2009)) are all under consideration.
None of these mechanisms provides a completely convincing picture of TGF production, so further work is underway to refine the models and work out the remaining details.
This material is based upon work supported by the National Science Foundation under Grant No. ATM-0836326.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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