The First Fermi‐GBM Terrestrial Gamma Ray Flash Catalog

Abstract

We present the first Fermi Space Telescope Gamma Ray Burst Monitor (GBM) catalog of 4,144 terrestrial gamma ray flashes (TGFs), detected since launch in 11 July 2008 through 31 July 2016. We discuss the updates and improvements to the triggered data and off-line search algorithms, comparing this improved detection rate of ∼800 TGFs per year with event rates from previously published TGF catalogs from other missions. A Bayesian block algorithm calculated the temporal and spectral properties of the TGFs, revealing a delay between the hard (>300 keV) and soft (≤300 keV) photons of around 27 μs. Detector count rates of "low-fluence" events were found to have average rates exceeding 150 kHz. Searching the World-Wide Lightning Location Network data for radio sferics within ±5 min of each TGF revealed a clean sample of 1,314 World-Wide Lightning Location Network locations, which were used to to accurately locate TGF-producing storms. It also revealed lightning and storm activity for specific regions, as well as seasonal and daily variations of global lightning patterns. Correcting for the orbit of Fermi, we quantitatively find a marginal excess of TGFs being produced from storms over land near oceans (i.e., narrow isthmuses and small islands). No difference was observed between the duration of TGFs over the ocean and land. The distribution of TGFs at a given local solar time for predefined American, Asian, and African regions were confirmed to correlate well with known regional lightning rates. Figure 17. The hardness ratio (E ≤ 300/E ≤ 300) for each terrestrial gamma ray flash (TGF). The mean ratio of the TGFs in this sample seem to be just slightly above the pivot energy (∼316 keV).

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

“…In this paper, we analyze the TGF spectra using the data from the BGO detectors, which have a broad spectral response (0.2-40 MeV) and large effective area (∼160 cm 2 ; Tierney et al, 2013). We focus on bright TGFs which have very low frequency (VLF) radio associations, as described in Mailyan et al (2016) and Roberts et al (2018). The photon counts are assigned to spectral channels according to 128 pseudo-logarithmically spaced energy channels, and time-tagged with relative timing resolution of 2 μs.…”

“…These include an updated onboard triggering algorithm and off-line search algorithms of the high time resolution data Roberts et al, 2018). These include an updated onboard triggering algorithm and off-line search algorithms of the high time resolution data Roberts et al, 2018).…”

“…Until recently, there was an opinion that the spectrum of the accelerated electrons in fully developed RREAs should not depend on the electric field, and the electron spectrum can be described by an exponential function with a cutoff at 7.3 MeV (see, e.g., Dwyer & Babich, 2011). This spectral dependence is promising for obtaining improved fits of the GBM-observed data (Roberts et al, 2018). Consequently, photon spectra produced by different large-scale homogeneous electric fields can differ.…”

Characteristics of Radio Emissions Associated With Terrestrial Gamma‐Ray Flashes

“…After implementation of new “ground‐search” algorithms in 2012, the TGF detection rate improved to about 800 events per year, making the GBM the most efficient current‐generation TGF observatory (Briggs et al, ). The Fermi‐GBM TGF catalog (e.g., Roberts et al, ) provides the photon counts per 2 μs for each of the two BGO detectors, cumulative photon counts for the 12 NaI detectors per 2 μs (10 μs if NaI detectors are saturated), spacecraft position, TGF start‐time measured at the spacecraft altitude, and the duration of the discovery‐bin. The discovery‐bin is the time‐window in the ground‐search algorithm corresponding to the most significant joint Poisson probability of occurrence of the identified TGF (Briggs et al, ).…”

Observationally Weak TGFs in the RHESSI Data

“…The implementation of the continuous time tagged event data mode and its subsequent offline TGF search have increased the detection rate of TGFs about tenfold (Briggs et al, ). After November 2012 the Fermi team implemented this change for the entire orbit, detecting ∼800 TGFs per year (Roberts et al, ).…”