Energetic Particle Intensities and Anisotropies near the Solar Wind Termination Shock

We address the physics of intensity and anisotropy variations in energetic particles near the solar wind termination shock. We show that rapid, large-amplitude, and highly anisotropic particle events seen upstream of the shock can be understood in terms of the passage across the spacecraft of different filled and unfilled magnetic flux tubes. These are created by large-scale magnetic irregularities that are advected passed the spacecraft and compress as they cross the termination shock. We find significant streaming anisotropies of 50 keV-1 MeV protons in the unshocked solar wind, with front-to-back intensity ratios exceeding a factor of 2, for 5 day averaged values. In contrast, downstream of the shock, our results indicate that the particle distributions are much more uniform and isotropic. However, there is a large-scale, nonzero azimuthal particle anisotropy downstream of the shock. This is caused by the nonradial heliosheath plasma flow necessitated by the nonspherical shock. Our results are consistent with recent Voyager 1 observations.