Spectroscopic Measurements of the Ion Velocity Distribution at the Base of the Fast Solar Wind

Jeffrey, Natasha, Hahn, Michael, Savin, Daniel W. and Fletcher, Lyndsay (2018) Spectroscopic Measurements of the Ion Velocity Distribution at the Base of the Fast Solar Wind. Astrophysical Journal Letters, 855 (1). L13. ISSN 2041-8213

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Official URL: https://doi.org/10.3847/2041-8213/aab08c

Abstract

In situ measurements of the fast solar wind reveal non-thermal distributions of electrons, protons, and minor ions extending from 0.3 au to the heliopause. The physical mechanisms responsible for these non-thermal properties and the location where these properties originate remain open questions. Here, we present spectroscopic evidence, from extreme ultraviolet spectroscopy, that the velocity distribution functions (VDFs) of minor ions are already non-Gaussian at the base of the fast solar wind in a coronal hole, at altitudes of <1.1 R ⊙. Analysis of Fe, Si, and Mg spectral lines reveals a peaked line-shape core and broad wings that can be characterized by a kappa VDF. A kappa distribution fit gives very small kappa indices off-limb of κ ≈ 1.9–2.5, indicating either (a) ion populations far from thermal equilibrium, (b) fluid motions such as non-Gaussian turbulent fluctuations or non-uniform wave motions, or (c) some combination of both. These observations provide important empirical constraints for the source region of the fast solar wind and for the theoretical models of the different acceleration, heating, and energy deposition processes therein. To the best of our knowledge, this is the first time that the ion VDF in the fast solar wind has been probed so close to its source region. The findings are also a timely precursor to the upcoming 2018 launch of the Parker Solar Probe, which will provide the closest in situ measurements of the solar wind at approximately 0.04 au (8.5 solar radii).

Item Type: Article
Uncontrolled Keywords: line: profiles; solar wind; Sun: corona; Sun: heliosphere; Sun: UV radiation – techniques: spectroscopic
Subjects: F300 Physics
F500 Astronomy
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: Elena Carlaw
Date Deposited: 03 Sep 2019 14:35
Last Modified: 11 Oct 2019 12:49
URI: http://nrl.northumbria.ac.uk/id/eprint/40499

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