The Gamma‐ray Burst Monitor (GBM) onboard the Fermi spacecraft has observed many tens of sufficiently bright events, which are suitable for individual analysis. In our previous study, we fit individual, bright terrestrial gamma‐ray flashes (TGFs) with Relativistic Runaway Electron Avalanche (RREA) models for the first time. For relativistic‐feedback‐based models, the TGF‐producing electrons, which are seeded internally by a positive feedback effect, are usually accelerated in a large‐scale field with fully developed RREAs. Alternatively, lightning leader models may apply to either a large‐scale thunderstorm fields with fully developed RREAs or to inhomogeneous fields in front of lightning leaders where RREAs only develop partially. The predictions of the latter, inhomogeneous models for the TGF‐beaming geometry show some differences from estimations of the relativistic feedback models in homogeneous fields. In this work, we analyze a large sample of 66 bright Fermi GBM TGFs in the framework of lightning leader models, making comparisons with previous results from the homogeneous‐field RREA models. In most cases, the spectral analysis does not strongly favor one mechanism over the other, with 59% of the TGF events being best fit with the fully developed RREA mechanism, which corresponds to high‐potential leader models. The majority of the GBM‐measured TGFs can be best fit if the source altitude is below 15 km and 70% of events best fit by leader models cannot be satisfactorily modeled unless a tilted photon beam is used. For several spectrally soft TGFs, the tilted beam low‐potential leader model can best fit the data.

We present an event that was detected by Fermi Gamma-ray Burst Monitor on 4 February 2014 as the spacecraft was flying over Madagascar. We interpret the three pulses during this event (herein known as 140204581) as the following: the first pulse as a terrestrial gamma-ray flash, the second as a 2 ms long terrestrial electron beam (TEB) 0.5 ms after the terrestrial gamma-ray flash, and the last pulse as the TEB mirror pulse 90 ms after the TEB. The nature of these events were confirmed using both the World Wide Lightning Location Network and the Earth Networks Total Lightning Network, which detected the same simultaneous sferic underneath the spacecraft and in the magnetic footprint. Several models were fit to the data, and results show that the vertical narrow beam model was found to be inconsistent with the data.

The Fermi Gamma‐ray Burst Monitor (GBM) has detected terrestrial gamma‐ray flash (TGF) pulses and TGF pairs with separations from submilliseconds to several minutes. Enhancements in the TGF rate are reobserved on successive orbits. We report on the distribution of TGF pulse separations observed with Fermi GBM. Additionally, a detailed analysis is performed on TGFs that have temporal associations within 3.5 ms with radio atmospheric signals (sferics) from the World Wide Lightning Location Network or the Earth Networks Total Lightning Network. Sferics are typically associated with lightning discharges, but the relativistic runaway electron avalanche process, according to models, should also produce radio emissions. The separations times between TGFs show a gap from 10 ms to 1 s that we interpret as showing differing origins for the separations below 10 ms and those above 1 s. Analysis of paired TGFs with separation time ≥1 s reveals 51 pairs with a sferic association with both members. The majority of these pairs have members originating from separate cells in thunderstorm systems, but 10 pairs have geolocations consistent with originating from the same cell. The minimum separation time of pairs from the same cell is 10 s, with an average separation time of 56 s. This leads to implications on TGF generation models and the recharge time of the large‐scale electric field at the source of the production of the TGF. This separation time would result in a constraint on the recharge time of the large‐scale electric field before an additional TGF can be produced.

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 similar to 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 mu s. 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.

In this study, we analyze 44 terrestrial gamma‐ray flashes (TGFs) detected by the Fermi Gamma‐ray Burst Monitor (GBM) occurring in 2014–2016 in conjunction with data from the U.S. National Lightning Detection Network (NLDN). We examine the characteristics of magnetic field waveforms measured by NLDN sensors for 61 pulses that occurred within 5 ms of the start‐time of the TGF photon flux. For 21 (out of 44) TGFs, the associated NLDN pulse occurred almost simultaneously with (that is, within 200 μs of) the TGF. One TGF had two NLDN pulses within 200 μs. The median absolute time interval between the beginning of these near‐simultaneous pulses and the TGF flux start‐time is 50 μs. We speculate that these RF pulses are signatures of either TGF‐associated relativistic electron avalanches or currents traveling in conducting paths “preconditioned” by TGF‐associated electron beams. Compared to pulses that were not simultaneous with TGFs (but within 5 ms of one), simultaneous pulses had higher median absolute peak current (26 kA versus 11 kA), longer median threshold‐to‐peak rise time (14 μs versus 2.8 μs), and longer median peak‐to‐zero time (15 μs versus 5.5 μs). A majority (77%) of our simultaneous RF pulses had NLDN‐estimated peak currents less than 50 kA indicating that TGF emissions can be associated with moderate‐peak‐amplitude processes. The lightning flash associated with one of the TGFs in our data set was observed by a Lightning Mapping Array, which reported a relatively high‐power source at an altitude of 25 km occurring 101 μs after the GBM‐reported TGF discovery‐bin start‐time.

Submitted for the APR17 Meeting of The American Physical Society Beaming Properties of Energetic Electrons and Photons Inside Thunderstorms1 ERIC CRAMER, MICHAEL BRIGGS, Univ of Alabama Huntsville — It has been well established that thunderstorm environments allow relativistic runaway electron avalanches (RREAs) to develop under the influence of strong electric fields. This process can be seeded by external sources, such as cosmic-ray secondary electrons. The resulting bremsstrahlung x-rays and gamma rays that are emitted, propagate through the atmosphere and into space where they are detected by orbiting spacecraft, e.g. NASA Fermi. These high energy radiation blasts are known as Terrestrial Gamma-ray Flashes (TGFs). Using a Monte Carlo particle simulation, we show beaming characteristics of these electrons and photons such as the angular distribution, energy spectra, and the radial distribution from the thunderstorm source to the observation point of orbiting spacecraft. These features are related to the thunderstorm electric field, Earths geomagnetic field, and the potential inside the thundercloud region. Observations of TGFs made by the Gamma-ray Burst Monitor (GBM) will also be discussed, as well as a future multipoint CubeSat mission targeted to measure the beaming geometry of the gamma rays. 1This material is based upon work supported by the National Science Foundation under Grant Number 1524533 Eric Cramer Univ of Alabama Huntsville Date submitted: 30 Sep 2016 Electronic form version 1.4

We have done a thorough simulation analysis on the variability of the photon spectra produced with (due to Relativistic Runaway Electron AvalancheRREA) and without (Modification of Spectra) the avalanche multiplication process. Despite some measurements obviously showing a variability of the spectra, numerous theoretical studies consider RREA spectrum independent on the electric field. However, analytical calculations by Cramer et al. (2014) have shown that RREA spectrum under low electric fields is not constant and stops being exponential. Using the Relativistic Electron Avalanche Model code, we model various layouts of the electric field configuration and study the predicted photon spectra. The primary focus of the present paper is to study the photon energy spectra, as gamma rays are more often observed by ground-based detectors. The simulation analysis of photon spectra potentially can help to deduce electric fields in thunderclouds.

The motivation of this work is to understand the effects of terrestrial gamma ray flashes (TGFs) on the ozone layer. One of the main ozone‐destroying mechanisms is the production of NOx in the stratospheric region. NOx from lightning has been considered as a possible cause of ozone depletion, but probably little of this NOx is transported from the tropopause to the stratosphere. Since the energetic particles of TGFs travel from ≈12 km to space, the resulting ionization can produce NOx directly in the stratosphere. In order to quantify the production of stratospheric NOx from TGFs, we use the Runaway Electron Avalanche Model to simulate a typical setup of the acceleration region inside a thundercloud. The photons are then transported through the Earth's atmosphere, where they deposit some of their energy as ionization in the ozone layer. We then calculate the number of NOx molecules produced by considering the average energy required to produce one electron‐ion pair. Finally, the effect of TGF NOx production is estimated using the global annual rate of TGFs. It is estimated that the NOx production of TGFs is completely negligible compared to other sources, and therefore, TGFs have no effect on the ozone layer.

Terrestrial gamma ray flashes (TGFs) are submillisecond flashes of energetic radiation that are believed to emanate from intracloud lightning inside thunderstorms. This emission can be detected hundreds of kilometers from the source by space‐based observatories such as the Fermi Gamma‐ray Space Telescope (Fermi). The location of the TGF‐producing storms can be determined using very low frequency (VLF) radio measurements made simultaneously with the Fermi detection, allowing additional insight into the mechanisms which produce these phenomena. In this paper, we report 37 TGFs originating from tropical storm systems for the first time. Previous studies to gain insight into how tropical cyclones formed and how destructive they can be include the investigation of lightning flash rates and their dependence on storm evolution. We find TGFs to emanate from a broad range of distances from the storm centers. In hurricanes and severe tropical cyclones, the TGFs are observed to occur predominately from the outer rainbands. A majority of our sample also show TGFs occurring during the strengthening phase of the encompassing storm system. These results verify that TGF production closely follows when and where lightning predominately occurs in cyclones. The intrinsic characteristics of these TGFs were not found to differ from other TGFs reported in larger samples. We also find that some TGF‐producing storm cells in tropical storm systems far removed from land have a low number of WWLLN sferics. Although not unique to tropical cyclones, this TGF/sferic ratio may imply a high efficiency for the lightning in these storms to generate TGFs.

Introduction: On Earth, relativistic electrons can accelerate inside thunderstorms due to large scale electric fields, producing x-ray and gamma ray radiation as they propagate [1]. Figure 1 shows the effective drag force acting on an electron as it moves through Earth’s atmosphere at STP [2]. The solid curve is due to inelastic collisions with air molecules and the dashed curve includes the effects of bremsstrahlung emission. In-situ measurements have found maximum electric fields near the breakeven field (Eb), which suggests that this process is common inside thunderstorms [3]. For a specific value of the electric field, > Eb, runaway electrons exist between energies εth and εmax. Above the critical value of the electric field (Ec), all thermal electrons can run away.

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.

The electric field change (E‐change) data presented herein provide a complementary view of a known terrestrial gamma ray flash (TGF), namely, TGF1 from Cummer et al. ( ). The main E‐change pulse coincident with TGF1 was likely an initial breakdown (IB) pulse of an intracloud (IC) flash since it had the typical characteristics of such an IC IB pulse: a bipolar shape with a positive leading peak and a duration of 47 μs. The IB pulse was especially energetic, with an estimated zero‐to‐peak amplitude between 36 and 47 V/m range normalized to 100 km (compared to an average IC IB pulse amplitude of ~1.5 V/m). The positive peak of the IB pulse occurred 19 μs after the beginning of the pulse. The first and most energetic gamma ray (7.6 MeV) detected in TGF1 occurred at 9 ± 10 μs after the beginning of the IB pulse. Thus, at the earliest the first detected gamma ray may have been produced just before or at the beginning of the IB pulse; at the latest, it may have been produced at the peak of the IB pulse.