Anomalous Cosmic Rays (ACRs) are a low-energy component of interplanetary particles that include the elements H, He, C, N, O, and Ar. They are now known to originate from interstellar neutral particles that have been swept into the heliosphere, ionized by solar UV or charge exchange with the solar wind, convected into the outer heliosphere, and then accelerated to energies of about 10 MeV/n or more. It is commonly assumed that the bulk of ACR acceleration takes place at the solar wind termination shock.
The observation of the anomalous N, O, and Ne ionic charge composition
with the SAMPEX satellite
confirmed the theoretical predictions that ACRs are only partially
charged; more precisely, singly charged ions dominate only at energies
below 20 MeV/n, while at higher energies multiply charged
ions become more abundant.
Being only partially ionized, ACRs have a much greater magnetic rigidity (at a given energy per nucleon) than either GCRs, which are essentially fully stripped, or SEPs, which have charge states characteristic of coronal temperatures. As a result, ACRs can be observed to much lower invariant latitude with a polar orbiting spacecraft like NINA.