Magnetic field structure and ACR acceleration at the termination shock

Anomalous cosmic rays are thought to be accelerated at the termination shock of the solar wind. Voyager 1 observations in the outer heliosphere, upstream of the termination shock and, recently, in the heliosheath impose important constraints on anomalous cosmic rays (ACR) acceleration at the heliospheric termination shock. Here we summarize the three-dimensional, vector anisotropies (radial, azimuthal and latitudinal) of the ACR and show how they depend on the large-scale structure of termination shock. The observations can be understood if the shape of the termination shock is not spherical, both globally and on a small scale. We also discuss the effects of radial motion of the termination shoe. This can alter the magnetic structure and change the intensity and energy spectra both upstream of the shock and in the heliosheath. Recent observations of the magnetic field from the Voyager 1 spacecraft, where, in the heliosheath, the magnetic field has remained directed outward from the Sun for several months suggest that the shock is moving rapidly inward. The implications for the energy spectrum of the ACR are also discussed.