Terrestrial gamma-ray flashes and neutron pulses from direct simulations of gigantic upward atmospheric discharge

Gamma-ray pulses are calculated from 2D numerical simulations of upward atmospheric discharge in selfconsistent electric field using multi-group approach to the kinetics of relativistic runaway electrons. Calculated numbers and spectra of $\gamma$-rays are consistent with the data on terrestrial $\gamma$-ray flashes observed aboard spacecrafts. The calculated runaway electron flux is concentrated mainly within the domain of the Blue Jet fluorescence. This proves that exactly this domain is a source of the $\gamma$-ray flashes. One $\gamma$-ray flash generates $\sim10^{14}\text{--}10^{15}$ photonuclear neutrons.