Energy Loss of a Charged Particle Traversing Ionized Gas and Injection Energies of Cosmic Rays

Abstract

The energy loss of a charged particle traversing an electron gas is calculated by taking the direct collisions with free electrons and the plasma excitation into account. The contributions from ions and neutral atoms are added and the resultant energy loss is given as a function of the degree of ionization. The energy loss is found to increase with the degree of ionization, to which excess loss is approximately proportional. The critical injection energy of cosmic rays, based on the Fermi mechanism of acceleration, is given in Table 1. Its value is about fifty percent larger for the degree of ionization of ten percent than for a neutral medium and nearly four times larger for a half ionized medium. In the case of the solar production of cosmic rays the critical injection energy is below one MeV per nucleon, so that the secondary particles generated by nuclear reactions could be accelerated to cosmic ray energies. The mechanism of acceleration in a coronal region is proposed in the Appendix to explain the time delay of the cosmic ray increase compared with the solar eruption as well as the energy spectrum of solar cosmic rays.