| Terrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating from
the Earth’s atmosphere in association with thunderstorm activity [e.g., Briggs et
al., JGR, 118, 3805, 2013]. Although TGFs are believed to be produced inside
thunderclouds (below 15 km altitude), the underlying physical mechanisms are still
debated. Large-scale relativistic runaway electron avalanches (RREAs) along with
relativistic feedback caused by positrons and photons have been proposed to occur in
thunderclouds and to produce TGFs [e.g., Dwyer et al., Space Sci. Rev., 173, 133, 2012]. It
has also been found that the production of thermal runaway electrons by stepping
lightning leaders and their further acceleration could explain the TGF spectra and
fluences for intracloud (IC) lightning electric potentials above ∼100 MV [Xu et
al., GRL, 39, L08801, 2012; Celestin et al., JGR, 120, 2015]. In both scenarios,
runaway electron avalanches take place and the related bremsstrahlung produces the
TGF.
The impact of the geomagnetic field on RREAs has been seldom studied (with the notable
exceptions of Lehtinen et al. [JGR, 104, 24699, 1999], Babich et al. [Geom. Aeron., 44, 243,
2004] and Cramer et al. [AGU Fall Meeting, abstract AE33A-0472, San Francisco, USA,
2015]), particularly in view of recent knowledge acquired about TGF sources properties. In
this work, we study the effects of the geomagnetic field on the runaway electron
beam geometry in large-scale RREAs and in the vicinity of lightning leaders and
the corresponding impact on TGF observations using analytical and numerical
means. |