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 1O, 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. Over the past 2 decades, ACRs have been used to study the acceleration and transport of energetic particles within the solar system, deduce the global properties of the heliosphere- the bubble of solar wind that envelopes the solar system, and study the interstellar material itself. It has recently been shown that some of these ACRs have become trapped in Earth's magnetic field, where they form a radiation belt composed of interstellar material (Figure I). Also, ACRs are being used to address a question that has existed ever since the discovery of the solar wind: "How large is the heliosphere?