Spectroscopic measurements of the ion velocity distribution at the base of the fast solar wind

Jeffrey, N. L.S. , Hahn, M., Savin, D. W. and Fletcher, L. (2018) Spectroscopic measurements of the ion velocity distribution at the base of the fast solar wind. Astrophysical Journal Letters, 856(1), L13. (doi: 10.3847/2041-8213/aab08c)

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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:Articles
Additional Information:N.L.S.J. and L.F. gratefully acknowledge the financial support provided by the STFC Consolidated Grant ST/L000741/1, and the grant awarded by the Principal's Early Career Mobility Scheme (University of Glasgow). M.H. and D.W.S. were supported by the NASA Living with a Star Program grant NNX15AB71G and by the NSF Division of Atmospheric and Geospace Sciences SHINE program grant AGS-1459247. Hinode is a Japanese mission developed and launched by ISAS/JAXA, collaborating with NAOJ as a domestic partner, NASA and UKSA as international partners. Scientific operation of the Hinode mission is conducted by the Hinode science team organized at ISAS/JAXA. This team mainly consists of scientists from institutes in the partner countries. Support for the post-launch operation is provided by JAXA and NAOJ (Japan), UKSA (U.K.), NASA, ESA, and NSC (Norway).
Glasgow Author(s) Enlighten ID:Fletcher, Professor Lyndsay and Jeffrey, Dr Natasha
Authors: Jeffrey, N. L.S., Hahn, M., Savin, D. W., and Fletcher, L.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal Letters
Publisher:IOP Publishing
ISSN (Online):2041-8213

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
631581Consolidated grant in solar and astrophysical plasmasLyndsay FletcherScience & Technology Facilities Council (STFC)ST/L000741/1S&E P&A - PHYSICS & ASTRONOMY