The spectroscopy of individual terrestrial gamma‐ray flashes: Constraining the source properties

Abstract

We report on the spectral analysis of individual terrestrial gamma‐ray flashes (TGFs) observed with the Fermi Gamma‐ray Burst Monitor (GBM). The large GBM TGF sample provides 46 events suitable for individual spectral analysis: sufficiently bright, localized by ground‐based radio, and with the gamma rays reaching a detector unobstructed. These TGFs exhibit diverse spectral characteristics that are hidden when using summed analysis methods. We account for the low counts in individual TGFs by using Poisson likelihood, and we also consider instrumental effects. The data are fit with models obtained from Monte Carlo simulations of the large‐scale Relativistic Runaway Electron Avalanche (RREA) model, including propagation through the atmosphere. Source altitudes ranging from 11.6 to 20.2 km are simulated. Two beaming geometries were considered: In one, the photons retain the intrinsic distribution from scattering (narrow), and in the other, the photons are smeared into a wider beam (wide). Several TGFs are well fit only by narrow‐beam models, while others favor wide‐beam models. Large‐scale RREA models can accommodate both narrow and wide beams, with narrow beams suggest large‐scale RREA in organized electric fields while wide beams may imply converging or diverging electric fields. Wide beams are also consistent with acceleration in the electric fields of lightning leaders, but the TGFs that favor narrow‐beam models appear inconsistent with some lightning leader models.

Analysis of Individual Terrestrial Gamma‐Ray Flashes With Lightning Leader Models and Fermi Gamma‐Ray Burst Monitor Data

“…Gjesteland et al (2015) analyzed three bright TGFs observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager above the Mediterranean Sea region. Mailyan et al (2016) found that several events were best fit by wide geometry beams, while some other events were best fit by narrow beams. More recently, Mailyan et al (2016) have done a spectral analysis of more than 40 events observed by the Gamma-ray Burst Monitor (GBM) on-board the Fermi spacecraft (Meegan et al, 2009).…”

“…The likelihood analysis showed that the individual spectra are diverse and that different TGF events are best fit by various While analyzing the TGF data, care should be taken to account for spectral distortions caused by the pulse pileup, as those instrumental effects play an important role for TGFs with high photon rates. There were some indications of TGF producing beams being tilted from satellite TGF measurements as well Mailyan et al, 2016). Mailyan et al (2016) found that several events were best fit by wide geometry beams, while some other events were best fit by narrow beams.…”

“…However, the variation of individual TGF events remains insufficiently investigated, which represents a major goal of this study. Not accounting for these effects may lead to erroneous interpretation of the data (see discussion in Mailyan et al, 2016, and references therein). For their brightest event, it was not possible to get constraints on the source altitude of the TGF within 6-to 14-km region using a 2 method.…”

“…For their brightest event, it was not possible to get constraints on the source altitude of the TGF within 6-to 14-km region using a 2 method. In this work we expand the set of models used in Mailyan et al (2016) to also include lightning leader models in inhomogeneous fields. The likelihood analysis showed that the individual spectra are diverse and that different TGF events are best fit by various While analyzing the TGF data, care should be taken to account for spectral distortions caused by the pulse pileup, as those instrumental effects play an important role for TGFs with high photon rates.…”

TARANIS XGRE and IDEE detection capability of terrestrial gamma-ray flashes and associated electron beams

“…NaI detectors have a high rate of overflow counts, making TGF spectra obtained from them very hard to analyse in practice. On the other hand, these problems are less important for the BGO detectors, allowing correction and study of spectra from single TGF events (Mailyan et al, 2016).…”