Solar Orbiter observations of the Kelvin-Helmholtz waves in the solar wind

Kieokaew, R., Lavraud, B., Yang, Y., Matthaeus, W. H., Ruffolo, D., Stawarz, Julia, Aizawa, S., Foullon, C., Génot, V., Pinto, R. F., Fargette, N., Louarn, P., Rouillard, A., Fedorov, A., Penou, E., Owen, C. J., Horbury, T. S., O’Brien, H., Evans, V. and Angelini, V. (2021) Solar Orbiter observations of the Kelvin-Helmholtz waves in the solar wind. Astronomy & Astrophysics, 656. A12. ISSN 0004-6361

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Context. The Kelvin-HeImholtz (KH) instability is a nonlinear shear-driven instability that develops at the interface between shear flows in plasmas. KH waves have been inferred in various astrophysical plasmas, and have been observed in situ at the magnetospheric boundaries of solar-system planets and through remote sensing at the boundaries of coronal mass ejections. Aims. KH waves are also expected to develop at flow shear interfaces in the solar wind. While they were hypothesized to play an important role in the mixing of plasmas and in triggering solar wind fluctuations, their direct and unambiguous observation in the solar wind was still lacking. Methods. We report in situ observations of quasi-periodic magnetic and velocity field variations plausibly associated with KH waves using Solar Orbiter during its cruise phase. They are found in a shear layer in the slow solar wind in the close vicinity of the heliospheric current sheet. An analysis was performed to derive the local configuration of the waves. A 2D magnetohydrodynamics simulation was also set up with approximate empirical values to test the stability of the shear layer. In addition, magnetic spectra of the event were analyzed. Results. We find that the observed conditions satisfy the KH instability onset criterion from the linear theory analysis, and its development is further confirmed by the simulation. The current sheet geometry analyses are found to be consistent with KH wave development, albeit with some limitations likely owing to the complex 3D nature of the event and solar wind propagation. Additionally, we report observations of an ion jet consistent with magnetic reconnection at a compressed current sheet within the KH wave interval. The KH activity is found to excite magnetic and velocity fluctuations with power law scalings that approximately follow k-5/3 and k-2.8 in the inertial and dissipation ranges, respectively. Finally, we discuss reasons for the lack of in situ KH wave detection in past data. Conclusions. These observations provide robust evidence of KH wave development in the solar wind. This sheds new light on the process of shear-driven turbulence as mediated by the KH waves with implications for the driving of solar wind fluctuations.

Item Type: Article
Additional Information: Funding Information: The authors acknowledge the reviewer's constructive and helpful comments. Work at IRAP and LAB was supported by the Centre National de la Recherche Scientifique (CNRS, France), the Centre National d'Etudes Spatiales (CNES, France), the Université Paul Sabatier (UPS), and the Université de Bordeaux. Solar Orbiter data are publicly available at http: // We acknowledge science teams of the Connectivity Tool ( and 3DView (http: // at IRAP and CNES. Y.Y. is supported by grant No. 11902138 from the National Natural Science Foundation of China. The computing resources were provided by the Center for Computational Science and Engineering of Southern University of Science and Technology. D.R. is supported by grant RTA6280002 from Thailand Science Research and Innovation. J.E.S. is supported by the Royal Society University Research Fellowship URF/R1/201286. C.J.O. is funded under STFC grant number ST/5000240/1. Solar Orbiter Solar Wind Analyser (SWA) data are derived from scientific sensors which have been designed and created, and are operated under funding provided in numerous contracts from the UK Space Agency (UKSA), the UK Science and Technology Facilities Council (STFC), the Agenzia Spaziale Italiana (ASI), the CNES, the CNRS, the Czech contribution to the ESA PRODEX programme and NASA. Solar Orbiter SWA work at UCL/MSSL is currently funded under STFC grants ST/T001356/1 and ST/S000240/1.
Uncontrolled Keywords: instabilities, magnetohydrodynamics (MHD), methods: observational, plasmas, Sun: heliosphere, solar wind
Subjects: F300 Physics
F500 Astronomy
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: Rachel Branson
Date Deposited: 19 Jan 2023 13:59
Last Modified: 19 Jan 2023 14:00

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