Anomalous cosmic rays (ACRs) are cosmic rays with unexpectedly low energies. They are thought to be created at the edge of our solar system, where the solar wind collides with the interstellar medium at the border between the heliosphere and the IM. Galactic cosmic rays which hit the shock front of the solar wind may be decelerated, resulting in its transformation into a lower-energy anomalous cosmic ray.

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Anomalous cosmic ray hydrogen spectra from Voyager 1 and 2