Production of Low-Energy Cosmic Rays by Collisions of High-Energy Cosmic Rays with Interstellar Gas.

Abstract

The production rate of protons of energy less than 1 BeV in interstellar space is calculated from empirical cross sections for proton-proton elastic and inelastic collisions in the kinetic energy range 1 to 30 BeV. Representing the cosmic ray intensity by a simple fit to the observed spectrum in the absence of solar activity the production spectrum is found to decrease steadily from zero energy to a value one- tenth as great at 500 MeV. The integrated production rate of 0-500 MeV protons is 10-26 cm-3 sec-1 from elastic collisions, and the contribution from inelastic collisions is estimated to be of lower order. The production spectrum is combined with the slowing lifetimes of protons in the interstellar gas (Meskys and Milford, Bull. Am. Phys. Soc. 8, 305, 1963) to give the density of these 0-500 MeV protons as 10-li cm-3 with an intensity of 10-i cm-2 sec-1. Recently Hayakawa, Nishimura, and Takayanagi (Publ. Astron. Soc. Japan 13, 184, 1961) have suggested that low-energy cosmic rays contribute appreciably to the heating of interstellar gas clouds, with an assumed low-energy cosmic ray intensity of order 100 cm-2 sec-1. From the above calculations it may be seen that high-energy collisions can not supply any significant fraction of such a hypothesized high intensity. Future measurements of low-energy interstellar cosmic rays will indicate what fraction of these can be attributed to the collision mechanism discussed above, and what fraction must be due to other mechanisms, such as stellar injection and interstellar magnetic field acceleration.