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, acc elerated at the so-called heliospheric shock which terminates the supersoni c solar wind expansion. After a presentation of the general scenario embrac ing a description of the heliosphere. the basic paradigm explaining the exi stence of anomalous cosmic rays and the relevant observations, the main pro blems 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 n eutral atoms. pick-up ions, and energetic neutral atoms. and those of galac tic cosmic rays are described from an observational as well as a theoretica l perspective, as far as they are related to heliospheric research. The rel evance of heliospheric physics is pointed out as a vital link between basic (plasma) physics and (extra-heliospheric) astrophysics as well as a test b ed for astrophysical concepts.