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Abstract

The velocity distribution functions (VDFs) of pickup He+ ions near the interplanetary shocks are expected to be extremely variable due to the specific source particle distributions and the interactions with the shock passage, turbulence, and large scale magnetic structures. However, due to the lack of high-resolution measurements of pickup ion’s energy and phase space densities, their kinetics are not well understood.In this study, variations of He+ pickup ion VDFs are investigated during CME passages accompanied with typical quasi-perpendicular and quasi-parallel shocks observed in the heart of the helium focusing cone. In order to calculate the VDFs of helium ions, pulse height information from each ion event in the PLasma And SupraThermal Ion Compostion (PLASTIC) instrument on the Solar Terrestrial Relations Observatory (STEREO) was utilized.Our study focuses on three regions: (1) VDFs in the shock upstream, (2) VDFs within the CME sheath in the shock downstream, and (3) VDFs in the magnetic cloud. VDFs are analyzed in terms of particle heating/cooling, acceleration/deceleration, and pitch angle diffusion. Overall, the pickup He+ VDFs keep the ring distribution features. Angular diffusion effects were eminent near the shock, while the velocity diffusion was dominant away from the shock. In the shock upstream, shock-reflected particles were observed, while significant angular diffusions were seen. In the shock downstream region, suprathermal tail formations were observed and the spectra were consistent with the solar wind compression origin. In the magnetic clouds, typical fresh pickup ion features were consistently observed without eminent suprathermal tails.