Acceleration at the termination shocks: the pitch-angle perspective

We use a recently developed anisotropic transport model to investigate observed pitch-angle distributions (PADs) of energetic protons near the solar wind termination shock. Voyager data is represented in the form of expansion in spherical harmonics in the frame aligned with the measured mean magnetic field, permitting a direct comparison with the model. We find that observed PADs are consistent with model predictions for particle acceleration at a highly oblique shock wave. The termination shock obliquity angle is derived from first-order anisotropy measurements. Additionally, second-order anisotropy is used to obtain energy dependence of the particle mean free paths. We demonstrate that analyzing second-order anisotropy is crucial for interpreting data correctly. It is also shown that certain measured distributions characterized by anomalously large second moments can be interpreted as oppositely propagating beams in regions multiply-connected to the spacecraft.