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

Abstract

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.

Observations of the Origin of Downward Terrestrial Gamma‐Ray Flashes

“…Other issues of note concerning the observations are (a) that the TGFs are broadly rather than narrowly beamed, favoring a tip‐based conducting channel model (Mailyan et al, 2019) and (b) are commonly tilted at substantial angles from vertical. From the TASD footprints and source altitudes, the half angular width of the beaming is on the order of 35° or so (≃2.4 km radial plan spread for a 3.3 km source altitude).…”

“…As summarized in the recent modeling study of TGFs by Mailyan et al (2019), there are two classes of models for TGF production: First, what is termed the relativistic runaway electron avalanche (RREA) or relativistic feedback (RFD) model in which electron avalanches develop in km-scale regions of strong electric fields in storms (Dwyer, 2003). In this model, the avalanching is enhanced by relativistic feedback that increases the avalanche currents by several orders of magnitude (Dwyer, 2012).…”

Geomagnetic Deviation of Relativistic Electron Beams Producing Terrestrial Gamma Ray Flashes

“…On rare occasions, secondary electrons will have a sufficient energy to become runaway electrons themselves. This avalanche process, the so‐called RREA, is associated with a spectral signature, namely, a ∼7 MeV exponential cutoff, that is transferred to the related bremsstrahlung emission and is observed in many TGFs (e.g., Dwyer & Smith, 2005; Mailyan et al., 2016, 2019).…”

Photonuclear Reactions in Lightning: 2. Comparison Between Observation and Simulation Model

“…Cummer et al (2014) gave only altitudes of two upward TGFs, and the estimations (11.8 and 11.9 km) are overlapped in this figure. Mailyan et al (2019) derived the source altitudes with four calculation points of 10, 12, 15, and 20 km. (b) The column density of the atmosphere from an altitude to sea level (solid line) and to a satellite orbit (above 80 km; dashed line), based on International Organization for Standardization (1975).…”

“…Dwyer and Smith (2005) estimated an averaged number of initial electrons above 1 MeV as (0.1-2.0) × 10 17 with an accumulated spectrum recorded by RHESSI. Mailyan et al (2019) also analyzed individual spectra of TGFs recorded by Fermi and estimated the number of avalanche electrons ranging between 6 × 10 15 and 9 × 10 18 . Figure 11a also compares these previous estimations of avalanche electrons with the present result.…”