Evidence for trapped anomalous cosmic ray oxygen ions in the inner magnetosphere

A series of measurements of 5–30 MeV/nucleon oxygen ions made with track detector stacks on Cosmos satellites show isotropic angular distributions during solar energetic particle events. Solar‐quiet times, on the other hand, have highly anisotropic distributions suggestive of a trapped‐particle component. Detailed Monte Carlo simulations confirm this interpretation and allow us to measure the trapped and cosmic‐ray contributions to the observed fluxes. Our data are fully consistent with anomalous cosmic‐ray ions, rather than radial diffusion from the outer zone, as the source of the trapped particles.