Anomalous cosmic rays and solar modulation

Abstract

We review aspects of anomalous cosmic rays (ACRs) that bear on the solar modulation of energetic particles in the heliosphere. We show that the latitudinal and radial gradients of these particles exhibit a 22-year periodicity in concert with the reversal of the Sun's magnetic field. The power-law index of the low energy portion of the energy spectrum of ACRs at the shock in 1996 appears to be ≤ -1.3, suggesting that the strength of the solar wind termination shock at the helioequatorial plane is relatively weak, with s ≤ 2.8. The rigidity dependence of the perpendicular interplanetary mean free path in the outer heliosphere for particles with rigidities between ∼ 0.2 and 0.7 GV varies approximately as R^2, where R is particle rigidity. There is evidence that ACR oxygen is primarily multiply charged above ∼ 20 MeV/nuc and primarily singly-charged below ∼ 16 MeV/nuc. The location of the termination shock was at ∼ 65 AU in 1987 and ∼ 85 AU in 1994.