On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

Abstract

Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled.

The 3rd AGILE Terrestrial Gamma Ray Flash Catalog. Part I: Association to Lightning Sferics

“…The distance bin size is chosen so that the circular area, corresponding to each bin, is constant and equal to 125,664km 2 . In agreement with Cummer et al (), Collier et al (), and Marisaldi et al (), most TGFs are detected within ∼500 km from the subsatellite point, and very few TGF are detected farther away than 800 km. The red WWLLN distribution is not flat due to the nonconstant latitude distribution of the WWLLN detections and AGILE's orbital inclination angle.…”

The 3rd AGILE Terrestrial Gamma‐ray Flashes Catalog. Part II: Optimized Selection Criteria and Characteristics of the New Sample

“…Those high energy counts are however a product of pileup, which affects short and high‐fluence events. The analysis reported in Marisaldi et al () shows that these events are still compatible with a standard RREA spectrum, with maximum energy of few tens of MeV, provided that the instrumental effects are carefully accounted for. However, the events affected by pileup are real TGFs and reliable in all other aspects; they should just be considered carefully when calculating the energy spectra.…”

“…Remarkably, WI TGFs tend to have higher values of count rate: This is explained by the shorter duration of WI events, as explained in Connaughton et al () and L20. Energies greater than 30 MeV can be either due to real photons, pileup (Marisaldi et al, ) or the presence of counts from cosmic rays, and confirm the need of relaxing the criteria on maximum energy. This led to the introduction of an additional, empirical criterion, as a way to remove even more false events: for count rate below 50 kHz, we only allow events with maximum energy up to 30 MeV.…”

Special Classes of Terrestrial Gamma Ray Flashes From RHESSI

“…The electronics simulation includes the effects of deadtime, pileup and pileup rejection, and preamplifier resets, so it simulates the paralysis in the peak of the TGF. The need to pay careful attention to instrumental deadtime in TGFs has been known for a while, but the importance of including the effects of pileup has become clearer more recently, particularly in the reanalysis of AGILE data by Marisaldi et al (2019), which demonstrated that an apparent extra high-energy component in the spectrum could be explained by pileup and deadtime issues. Their procedure was similar to the multistage simulation outlined here.…”