Terrestrial gamma‐ray flashes in the Fermi era: Improved observations and analysis methods

Abstract

[1] A new data mode and new analysis methods are used to detect Terrestrial Gamma-ray Flashes (TGFs) with the Fermi Gamma-ray Burst Monitor (GBM) 10 times more frequently than previously. In 1037 h of observations at times and over regions for which TGFs are expected, 384 new TGFs were found in addition to the 39 TGFs and two Terrestrial Electron Beam events already detected without the new data mode and methodology. Cosmic ray showers were found to be an important background; they show characteristic signatures in the data of both GBM and the Fermi Large Area Telescope Calorimeter that enable their removal, leaving a sample estimated to consist of 98% TGFs. The sample includes shorter TGFs than previously found with GBM. The true duration distribution likely contains additional short TGFs because their detection by GBM is limited by detector dead time. One-third of this sample has matches with locations from the World Wide Lightning Location Network (WWLLN)-maps of these locations show the geographic and meteorological features more clearly than maps of spacecraft locations. The intrinsic TGF rate is evaluated using the lightning rate maps of the Lightning Imaging Sensor, accounting for the detection efficiency of GBM as a function of spacecraft-source offset, from which we estimate a global TGF rate of 400,000 per year. With continuous production of data in the new mode we estimate that GBM will detect 850 TGFs per year.

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

“…The limit of sensitivity of Fermi is ≈ 0.053 photons cm −2 ms −1 (= 19 photons/716 cm 2 /500 µs) and it corresponds to a limit radius of R lim F ≈ 795 km. This value is consistent with the maximum distance between the TGF source positions and Fermi footprints given using WWLLN (World Wide Lightning Location Network) associations (Briggs et al, 2013;Fitzpatrick et al, 2014). We can also determine R lim R = 694 km, which is reasonably close to the largest distance found between RHESSI's position and the WWLLN match of the TGF source location (Nisi et al, 2014).…”

“…The two main objectives of IDEE are to study Lightning-induced Electron Precipitations (LEP) . c Briggs et al (2013). (Voss et al, 1984;Inan et al, 2007) and the electrons beams associated to TGFs, known as TEBs.…”

“…As presented in Briggs et al (2013), the effective area that should be used to detect TGF is 160 cm 2 for each BGO detector. Our calculation from the response matrices show that it should be about 33 cm 2 for each NaI detector, giving a total σ TGF F of about 716 cm 2 for Fermi-GBM.…”

“…The TGF detection methods used by RHESSI, Fermi-GBM and AGILE-MCAL are described respectively in Gjesteland et al (2012); Briggs et al (2013); Marisaldi et al (2014). Concerning XGRE, a TGF event can be triggered if a given number of counts is reached within a 10 µs window.…”

“…For Fermi, by looking to publicly available catalogue (http://fermi.gsfc.nasa. gov/ssc/data/access/gbm/tgf/), we could estimate that about N F = 650 TGFs yr −1 were detected after the offline searching method was set up (Briggs et al, 2013). All these values are summarised in Table 2.…”