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

• Published in: Journal of geophysical research. Space physics Vol. 124; no. 8; pp. 7170 - 7183
• Main Authors: Mailyan, B. G., Xu, W., Celestin, S., Briggs, M. S., Dwyer, J. R., Cramer, E. S., Roberts, O. J., Stanbro, M.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-08-2019; American Geophysical Union/Wiley; American Geophysical Union (AGU)
• Subjects: data; Earth Sciences; Electron avalanche; Gamma ray bursts; Gamma ray flashes; Geophysics; Leader currents; Lightning; Photon beams; Physics; Positive feedback; Relativism; Relativistic effects; runaway electrons; Sciences of the Universe; Space Physics; Spacecraft; Spectral analysis; Spectrum analysis; terrestrial gamma‐ray flashes; Thunderstorms
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2019JA026912

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.
Key Points
Fermi GBM individual TGF spectra are analyzed using lightning leader models of various leader potentials, beaming tilts, and source altitudes
Lightning leader models with potentials of 200 MV and tilted beams often best fit the data
A few, exceptionally soft TGFs can be well explained using 60‐MV lightning leaders with tilted beams