Evolution of secondary electron spectrum during cosmic-ray discharge in the universe

Abstract

We recently found that streaming cosmic rays (CRs) induce a resistive electric field that can accelerate secondary electrons produced by CR ionization. In this work, we study the evolution of the energy spectrum of secondary electrons by numerically solving the one-dimensional Boltzmann equation and Ohm's law. We show that the accelerated secondary electrons further ionize a gas, that is, the electron avalanche occurs, resulting in increased ionization and excitation of the gas. Although the resistive electric field becomes weaker than one before the CR discharge, the weak resistive electric field weakly accelerates the secondary electrons. The quasi-steady state is almost independent of the initial resistive electric field, but depends on the electron fraction in the gas. The resistive electric field in the quasi-steady state is larger for the higher electron fraction, which makes the number of secondary electrons that can ionize the gas larger, resulting in a higher ionization rate. The CR discharge could explain the high ionization rate that are observed in some molecular clouds.