Preacceleration of anomalous cosmic rays in the inner heliosphere

Because of the apparent difficulty in accelerating previously unaccelerated pickup ions locally at the solar wind termination shock, a model for anomalous cosmic-ray protons is considered in which the initial acceleration of pickup ions to ∼ 10-20 MeV occurs in the inner heliosphere. These accelerated pickup ions are assumed to be accelerated either by propagating interplanetary shocks or by some other, unspecified process. Voyager 2 Low-Energy Charged Particle (LECP) observations at low energies are used to normalize their spectra. The ions are then transported to the termination shock, where they are further accelerated to anomalous cosmic-ray energies. We use a well-established transport model to simulate this process. In this model, the two-dimensional cosmic-ray transport equation is solved using the assumed (and normalized) source of energetic particles (∼0.01-20 MeV) located at 10 AU. We show that the computed spectra at higher energies (≳100 MeV) are consistent with observed anomalous cosmic-ray fluxes and suggest that interplanetary shocks, especially those in the inner heliosphere, may play an important role in the initial acceleration of pickup ions leading to anomalous cosmic rays.