Optical emissions associated with terrestrial gamma ray flashes

Abstract

Terrestrial gamma ray flashes (TGFs) are high-energy photon bursts produced by high-energy electrons originating in the Earth's atmosphere through bremsstrahlung processes. In this paper, we present modeling studies on optical emissions resulting from the excitation of air molecules produced by the large population of electrons involved in TGF events based on two possible production mechanisms: relativistic runaway electron avalanches (RREAs) and acceleration of thermal runaway electrons produced by high-potential intracloud lightning leaders. Numerical models developed in this study are first validated through the calculation of fluorescence emissions from air excited by energetic electrons and comparison with available laboratory observations. Detailed discussion of the role of excitation and ionization collisions on the formation of the electron energy distribution is presented. Moreover, using Monte Carlo simulations, we show that electron energy distributions established from the two TGF production mechanisms considered here are inherently different over the full energy range. The strong energy dependence of the capability of electrons to generate excited states responsible for optical emissions from neutral and ionized nitrogen molecules leads to intrinsic differences in optical emissions produced by different mechanisms of TGF production. We also show that TGFs are most likely accompanied by detectable levels of optical emissions and that the distinct optical features are of significant interest for constraining and validating current TGF production models.

Modeling of X‐ray Images and Energy Spectra Produced by Stepping Lightning Leaders

“…Three main collision types for photons with energies between 10 keV and 100 MeV are considered: photoelectric absorption, Compton scattering, and electron‐positron pair production. We emphasize that the set of Monte Carlo models has been validated through calculations of TGF energy spectra based on the RREA theory and a very good agreement has been obtained with previous published results (Celestin et al, ; Dwyer and Smith, ; Xu et al, , ).…”

Atmospheric discharges and particle fluxes

“…Unlike visible lightning, RREA "beams" do not create a hot plasma channel and optical flash; however, the high-energy electrons are capable to generate excited states of atmospheric neutral and ionized nitrogen molecules responsible for the optical emissions (mostly in the ultraviolet region of 300-400 nm). Therefore, TGEs and TGFs are most likely to be accompanied by detectable levels of optical emissions [Xu et al, 2015]. In Figure 10 we present the photos of the skies above Aragats station on 2 June 2014 coincided in time with RRE avalanches development during 2 min of TGE (see Figure 7).…”

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

“…These electrons are initially placed at a location where the corresponding electric field is 50 kV/cm, as reasonable for the field magnitude at the leader tip (Bazelyan & Raizer, 2000, pp. We emphasize that this set of Monte Carlo models has been utilized to study RREA properties (e.g., Celestin et al, 2012), the acceleration of electrons in inhomogeneous fields produced by lightning leaders (e.g., Celestin et al, 2012;Xu et al, 2015), and optical emissions produced by TGFs (Xu et al, 2017); modeling results show good agreements with previously published results (also, see the review and discussion in Dwyer et al, 2012). The Monte Carlo model employed to simulate photon transport in the Earth's atmosphere is similar to that described in Østgaard et al (2008).…”