Constraining faint terrestrial gamma-ray flashes with stacking analyses

We searched for gamma-ray emission from lightning using a satellite (the Reuven Ramaty Solar Spectroscopic Imager (RHESSI)) and an instrument on an aircraft (the Airborne Detector for Energetic Lightning Emissions (ADELE)). Both instruments have detected terrestrial gamma-ray flashes (TGFs) via direct searches for statistically significant bursts of gamma-rays. In our new analysis, we instead identified times when the instruments were near known lightning discharges based on VLF radio data (from the Worldwide Lightning Location Network (WWLLN) in the case of RHESSI and from three North American networks in the case of ADELE). We then stacked together the gamma-ray signals for each instrument, with times adjusted to be relative to the time of radio emission for each discharge (corrected for light propagation time to the spacecraft in the case of RHESSI). The resulting stacked gamma-ray time profile is sensitive to an average level of gamma-ray emission far lower than what can be recognized above background for a single TGF.

The summed signal from small, untriggered TGFs is remarkably weak, and preliminary evidence suggests that it comes mostly from distant, bright TGFs observed outside the main bremsstrahlung beam, not from a population of subluminous TGFs near the spacecraft. Under the assumption of a broken power-law differential distribution of TGF intensities, we find that the index must break (harden or cut off) just below the current sensitivity limit of satellites like RHESSI and Fermi, and that less than 1% of lightning can produce a TGF that belongs to the same distribution as those that are observable.