Geomagnetically trapped protons from cosmic-ray albedo neutrons

Abstract

Albedo neutrons from galactic cosmic rays are shown to account for the gross features of the observed geomagnetically trapped protons. This source leads to an equilibrium intensity (at altitudes up to several thousand kilometers and at low latitudes) that compares quite favorably with observations, in the shape of the energy spectrum, in the spatial distribution, and in the absolute intensity. In this analysis we take account of the anisotropic emergence of neutrons (above 50 Mev) and of nuclear interactions in the oxygen exosphere (above 300 Mev). The anisotropy of the albedo in effect steepens the neutron energy spectrum significantly. More detailed features, such as the angular distribution, may also be computed from the neutron albedo theory, but no observations are available for comparison at present. The phenomenon responsible for the absence of trapped protons beyond about 2 earth radii is not yet understood. A number of nonadiabatic mechanisms are compared. Injection resulting from the neutron albedo from solar high-energy particles and the consequent transfer of charged albedo particles from polar latitudes to middle latitudes is examined.