Terrestrial Gamma-Ray Flashes as Powerful Particle Accelerators

Strong electric discharges associated with thunderstorms can produce terrestrial gamma-ray flashes (TGFs), i.e., intense bursts of x rays and $\gamma$ rays lasting a few milliseconds or less. We present in this Letter new TGF timing and spectral data based on the observations of the Italian Space Agency AGILE satellite. We determine that the TGF emission above 10 MeV has a significant power-law spectral component reaching energies up to 100 MeV. These results challenge TGF theoretical models based on runaway electron acceleration. The TGF discharge electric field accelerates particles over the large distances for which maximal voltages of hundreds of megavolts can be established. The combination of huge potentials and large electric fields in TGFs can efficiently accelerate particles in large numbers, and we reconsider here the photon spectrum and the neutron production by photonuclear reactions in the atmosphere.

Figure 1

Light curves, photon energy distributions, and spectra of three intense TGFs detected by AGILE. (Top panels) AGILE-MCAL light curve for events above 350 keV with 0.2 ms time bins. The trigger time is marked as $T_0$. (Middle panels) Photon energy distributions as a function of arrival times. (Bottom panels) MCAL counts spectrum as a function of photon energies: the solid blue curve shows the normalized cumulative spectrum (the same as in Fig. 2) obtained by summing 130 high-quality TGFs detected by AGILE during the period June 2008–January 2010.

Figure 2

The background-subtracted cumulative counts spectrum of the 130 TGFs detected by AGILE-MCAL during the period June 2008–January 2010. The solid curve shows the broken PL fit (see text), and the dashed curve is a pre-AGILE phenomenological model $F(E)\sim E^{-\alpha }{e}^{(-E/E_c)}$, of index $\alpha =0.4\pm 0.2$ and exponential cutoff energy $E_c=6.6\pm 1.2\mathrm{MeV}$.