Helium pickup ion 3D velocity distributions at interplanetary shocks

3D velocity distribution functions (VDFs) of He⁺ pickup ions (PUIs) were used to trace local physical processes around interplanetary shocks. He⁺ PUIs are measured by PLasma And SupraThermal Ion Compostion (PLASTIC) instrument onboard the Solar Terrestrial Relations Observatory (STEREO) in an unprecedented cadence and angular/velocity resolutions with mass and charge state unambiguously determined. We focused on the VDFs in terms of the interplanetary magnetic field orientation in the solar wind frame and indipendently evaluated the acceleration, heating, and pitch-angle scattering for both perpendicular and parallel shocks with notable differences. For the perpendicular shock: (a) Reflected and energized particles were found in the upstream, and the PUI properties were isolated between upstream and downstream, (b) Strong heating is observed in the sheath region, (c) Suprathermal particles are found in the sheath region and attributed to the compression within the solar wind, and (d) Universal pitch-angle scattering were found through out the ICME. For the parallel shock: (a) Locaized heating and acceleration were found around the shock location, (b) Harder suprathermal particle distributions were found near the shock, suggesting diffusive shock processes, (c) Weak or no sheath heating was observed, and (d) Pitch angle scattering were correlated with magnetic field power spectral density at the Helium cyclotron.