SH009-0020
The Solar Wind Frame of STEREO/PLASTIC: Contributions of the de-Hoffmann-Teller Frame

Tuesday, 8 December 2020
Poster
Duncan Keilbach1, Lars Berger2, Verena Heidrich-Meisner1 and Robert F Wimmer-Schweingruber3, (1)University of Kiel, Kiel, Germany, (2)Institute for Experimental and Applied Physics (IEAP), Christian-Albrechts-University of Kiel, Kiel, Germany, (3)University of Kiel, Institute for Experimental and Applied Physics, Kiel, Germany
Abstract:
The study of velocity distribution functions (VDFs) of protons and ions in the solar wind (SW) has been an interesting and challenging exercise for many decades and still offers many puzzles to be solved. Yet, until recently Parker Solar Probe and Solar Orbiter started, three-dimensional observations of SW VDFs have been rare. Helios observations showed, that the fast SW proton VDF can be approximated by a bi-Maxwellian core and a beam-distribution, which is aligned to the local magnetic field vector.


For the VDFs of other ion species a differential streaming to the proton core has been observed with differential velocities in the magnitude of fractions of the Alfvén velocity aligned along the local magnetic field. In this way the magnetic field can be regarded as an ordering parameter in the SW. For studies its observing kinematic properties the velocity of its propagation should be referenced: the de-Hoffmann-Teller velocity. This is the velocity of the frame of reference, where the motional electric field vanishes [de Hoffmann & Teller, 1950]. Employing the capability of STEREO/PLASTIC to measure incident angles of incoming ions and in combination with magnetic field observations from STEREO/MAG, we have determined the de-Hoffmann-Teller velocity based on an approach by Němeček et. al (2020).


We discuss the limitations of the PLASTIC instrument w.r.t. the determination of three dimensional ion velocities and which ions are best suited to determine the de-Hoffmann-Teller velocity from. In addition methods to sanity-check the computed velocities against common physical models are presented.

The results of this study are not only applicable to studies which observe differential streaming in the solar wind but also to e.g. interplanetary shocks, acceleration mechanisms and pickup ion VDFs.

References:

de Hoffmann, F., & Teller, E. 1950, PhRv, 80, 692

Němeček, Z., Ďurovcová, T., Šafránková, J., Němec, F., Matteini, L., Stansby, D., Janitzek, N., Berger, L., and Wimmer-Schweingruber, R. F. (2020). What is the Solar Wind Frame of Reference? ApJ, 889(2):163.

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