Production of Low-Energy Cosmic-Ray Electrons

Abstract

The production of cosmic-ray electrons of characteristically low energies is investigated. Secondary sources, other than that of meson decay, are considered, and constraints are placed on both secondary and primary sources. (1) Calculations are made of the intensity of low-energy knock-on and beta-decay electrons which are secondary to cosmic-ray interactions. In particular, knock-on production is calculated in the 100-KeV to 50-BeV kinetic-energy interval. Interstellar losses due to ionization, leakage from the galaxy, and synchrotron, bremsstrahlung, and inverse Compton effects are considered, as well as those due to plasma excitation, the red shift and synchrotron, bremsstrahlung, and inverse Compton effects in the intergalactic medium. The intensity of low-energy relativistic electrons from these sources is not negligible compared with the low energy $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\rightarrow}e$ intensity, but it is shown not to account for the observed interplanetary electron intensity. (2) Energy inputs to the injected secondary electrons by a possible solar electric field of low magnitude and by a possible galactic Fermi acceleration are investigated. It is shown that at least one such input is necessary if the observed low-energy interplanetary electron intensity is to be attributed to secondary production alone. A heliocentric field which does allow for a fit to the low-energy data cannot, however, account for the high-energy BeV electrons found to be in excess of those from $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\rightarrow}e$ production. The Fermi acceleration shown to be necessary to provide a fit is greater than that usually postulated for cosmic-ray protons, and also requires that the ratio of escape losses to acceleration $\frac{\ensuremath{\lambda}}{\ensuremath{\alpha}}$ be much smaller than is usually assumed for protons. This distinction is acceptable only if one postulates a significant difference between interstellar proton and electron propagation. (3) The observation that the velocity spectrum of electrons in the energy-per-unit-mass region of 7-25 closely approximates that of the cosmic-ray protons, and the necessity of constraints on the secondary-electron hypothesis outlined above, suggest that most of the low-energy electrons are of primary origin. The similarity between this conclusion and the conclusion (based on the measurement of the charge ratio of electrons) that the higher energy electrons are mostly primary is discussed.