A physical mechanism for the formation of a natural positron belt in the Earth's magnetosphere is considered. It is assumed that a natural source of energetic positrons as well as electrons can be created owing to the decay of charged pions &pgr;¿→&mgr;¿→e¿, which have their origin in nuclear collisions between energetic trapped inner zone protons and heavier atoms (He and O) in the upper atmosphere of the Earth. Simulations of these processes demonstrate that there is a predominant production of positive pions over negative pions, and consequently the decays result in a substantial excess of positrons over electrons at energies greater than tens of MeV. This positron excess is found to be energy-dependent and to decrease with increasing incident proton energy; this excess is essentially absent at proton energies corresponding to cosmic ray primaries of ≥8 GeV. Our numerical computations for the resulting e+/e- fluxes provide ratio values of ~4 at multi-MeV energies and at L=1.2¿0.1. The simulation results presented herein are compared to the existing and recent experimental evidence. ¿ 2001 American Geophysical Union