Anomalous cosmic rays: Messengers from the outer heliosphere

In this review much of the current knowledge about the physics of anomalous cosmic rays is surveyed. These energetic particles are, most probably, accelerated at the so-called heliospheric shock which terminates the supersonic solar wind expansion. After a presentation of the general scenario embracing a description of the heliosphere. the basic paradigm explaining the existence of anomalous cosmic rays and the relevant observations, the main problems connected to the physics of this particular cosmic-ray component are identified and discussed in detail. To this end, the characteristics of the progenitor as well as descendant particle populations, i.e. interstellar neutral atoms. pick-up ions, and energetic neutral atoms. and those of galactic cosmic rays are described from an observational as well as a theoretical perspective, as far as they are related to heliospheric research. The relevance of heliospheric physics is pointed out as a vital link between basic (plasma) physics and (extra-heliospheric) astrophysics as well as a test bed for astrophysical concepts.