Terrestrial gamma-ray flash electron beam geometry, fluence, and detection frequency

Terrestrial gamma-ray flashes (TGFs) are associated with emission of detectable beams of electrons into space. In this paper we use simulations of TGF and electron beam emission and escape from the atmosphere to determine how the geometry and fluence of such events depend on the angular distribution of the source photons. Given a photon source, the geometry of the electron beam depends on the geomagnetic latitude of the source but can be well-predicted by tracing a disk at 57 km altitude along the geomagnetic field to satellite orbit. The fluence and geometry are then used to infer the relative detection probabilities of TGF and electron beam in the context of a variety of photon sources and intensities. Analysis of detection probabilities and the relative frequency of TGF and electron beam detections suggests the existence of a population of electron beams emitted by TGFs too faint to be detected as photons.