Anomalous cosmic rays: Interstellar interlopers in the heliosphere and magnetosphere



Since the beginning of the space age, it was known that two main sources of energetic particles pervade interplanetary space: galactic cosmic rays (GCRs), which originate in sources outside the solar system, and solar energetic particles (SEPs), associated with transient events on the Sun.

But over 20 years ago, instruments on the Pioneer 10, IMP 5, and IMP 7 spacecraft discovered a third component of energetic particles known as “anomalous cosmic rays,” that represents a sample of nearby interstellar material.