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

星际拾取离子（PUIs）是太阳风（SW）中非太阳起源的离子。它们的空间分布和速度分布函数（VDF）系统地受到局地星际介质和星际输运性质的影响。PUI输运的描述依赖于假设，即PUI VDF是回旋的，各向同性迅速，并且SW速度是VDF的参考框架。然而，最近的研究对PUI传输的参考框架提出了质疑，并强调了使用de-Hoffman-Teller （dHT）框架的重要性。此外，PUI vdf的各向异性特征已被报道。我们计划研究输运框架在PUI输运中的作用，从STEREO/PLASTIC观测中计算dHT速度，并将PUI vdf观测与之联系起来，并将结果与模拟预期进行比较。最近，用ACE/SWICS观测了三维PUI vdf。我们将观察三维PUI vdf，以进一步发现各向异性的痕迹，并将它们与太阳风的不同状态联系起来。通过与ACE/SWICS、Ulysses/SWICS和最近发射的太阳轨道器的观测结合，我们将探测PUI vdf的空间分布和形状特征与位置的关系。太阳轨道器提供了一个前所未有的机会来观察太阳风中的粒子传输。此外，这项研究是及时的。它将为美国宇航局选择的IMAP任务提供关键观测。

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.