Multi-spacecraft observations of 3D velocity distribution functions of pickup ions as a tool to investigate transport phenomena in the heliosphere

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Interstellar pickup ions (PUIs) are ions of non-solar origin in the solar wind (SW). Their spatial distribution and their velocity distribution function (VDF) are systematically influenced by properties of the local interstellar medium and transport in the SW. The description of the PUIs’ transport relies on assumptions, i.e. that the PUI VDF is gyrotropic, isotropized swiftly and that the the SW velocity is theVDFs frame of reference. However, recent studies question this reference frame for PUI transport and highlight the importance of using the de-Hoffman-Teller (dHT) frame. Moreover, anisotropic featuers have been reported in PUI VDFs. We plan to investigate the role of the transport frame in PUI transport, to compute the dHT velocity from STEREO/PLASTIC observations and relate PUI VDFs observations to it and to compare the results to simulation expectations. Recently, observations of three dimensional (3d) PUI VDFs with ACE/SWICS have been presented. We will observe 3d PUI VDFs to find further traces of anisotropy and relate them to different regimes in the solar wind. Through combination of observations with ACE/SWICS, Ulysses/SWICS and the recently launched Solar Orbiter we will probe the spatial distribution and relate features of the shape of PUI VDFs to location. Solar Orbiter provides an unprece-dented opportunity to observe particle transport in the solar wind. Moreover, this study is timely. It will add key observations to those that NASA’s selected IMAP mission will make.