Gamma-Ray Localization of Terrestrial Gamma-Ray Flashes

Terrestrial gamma-ray flashes (TGFs) are very short bursts of high-energy photons and electrons originating in Earth’s atmosphere. We present here a localization study of TGFs carried out at gamma-ray energies above 20 MeV based on an innovative event selection method. We use the AGILE satellite Silicon Tracker data that for the first time have been correlated with TGFs detected by the AGILE Mini-Calorimeter. We detect 8 TGFs with gamma-ray photons of energies above 20 MeV localized by the AGILE gamma-ray imager with an accuracy of $\sim 5–10°$ at 50 MeV. Remarkably, all TGF-associated gamma rays are compatible with a terrestrial production site closer to the subsatellite point than 400 km. Considering that our gamma rays reach the AGILE satellite at 540 km altitude with limited scattering or attenuation, our measurements provide the first precise direct localization of TGFs from space.

Figure 1

Histogram of the arrival times with respect to the TGF MCAL start time $T_0$ for the gamma-rays detected by the AGILE gamma-ray imaging detector (GRID).

Figure 2

3D view of the tracks reconstructed for the two GRID events detected in association with TGF 12809-19. The MCAL detector with the detected events is shown at the bottom of the figure. The AGILE pointing direction is orthogonal to the MCAL detection planes, toward the top of the figure.

Figure 3

Location of the satellite footprints (solid circles) and of the projection of the associated tracker events to the Earth ellipsoid (open circles). The inset is a zoom over the region of TGF 12809-19. The dashed ellipses represent the $1\sigma$ error circle for the associated tracker events.

Figure 4

(a) Scatter plot of the GRID events projection with respect to the AGILE footprint. The dashed circles are 300 and 600 km in radius. The full AGILE visibility region has a 2600 km radius. (b) Occurrence density vs distance from footprint (each bin has been divided by the subtended area in ${\mathrm{km}}^2$).