Three dimensional Pickup Ion velocity distribution functions observed at 1 AU with STEREO/PLASTIC : Anisotropic features and density enhancements in pitch angle distributions as tools to investigate transport effects

The solar wind (SW) is a magnetized plasma which emerges from the Sun and forms the Heliosphere which is surrounded by the local interstellar medium (LISM). Interstellar neutrals cross the boundary between LISM and Heliosphere and may become ionized. After ionization, these newly created ions are picked up by the magnetic field embedded into the SW and are hence called pickup ions (PUIs). PUIs can also be created from inner-heliospheric sources. However, this work focuses on the interstellar component.

From the point of ionization to the point of in-situ measurement by a spacecraft, PUIs undergo transport in the SW. Initially, the consensus was that the pitch angle distribution of PUIs is scattered swiftly. However, newer studies observed anisotropic features in the velocity distribution function (VDF).

With the capability of the PLASTIC instrument aboard the STEREO spacecraft to measure He+ PUI VDFs in three dimensions (3d), this work aims to find observational constraints for the theory of PUI transport. Yet, the observation of 3d velocity vectors with STEREO/PLASTIC from which the VDF is derived is dominated by instrumental effects which this work thoroughly investigates and discusses. Especially, asymmetric detection efficencies are found.

In the observed 3d VDFs which are firstly analysed as 3d velocity histograms and secondly as pitch angle distributions both grouped by the angular direction of the local magnetic field, anisotropic features are found at magnetic field azimuthal angles of ±90° to the radial direction. The instrumental limitations of these observations are discussed. In addition, the 3d VDFs are investigated as a function of SW conditions and orbital coordinates of the spacecraft.

In future studies, the observation of 3d PUI VDFs may provide new constraints for the modelling of PUI transport and this study's methods can be directly applied to Solar Orbiter/HIS.


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