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Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind

Šafránková, J; Němeček, Z; Němec, F; Montagud-Camps, V; Verscharen, D; Verdini, A; Ďurovcová, T; (2021) Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind. The Astrophysical Journal , 913 (2) , Article 80. 10.3847/1538-4357/abf6c9. Green open access

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Abstract

We present a large statistical study of the fluctuation anisotropy in minimum variance (MV) frames of the magnetic field and solar wind velocity. We use 2, 10, 20, and 40 minute intervals of simultaneous magnetic field (the Wind spacecraft) and velocity (the Spektr-R spacecraft) observations. Our study confirms that magnetic turbulence is a composite of fluctuations varying along the mean magnetic field and those changing in the direction perpendicular to the mean field. Regardless of the length scale within the studied range of spacecraft-frame frequencies, ≈90% of the observed magnetic field fluctuations exhibit an MV direction aligned with the mean magnetic field, ≈10% of events have the MV direction perpendicular to the background field, and a negligible portion of fluctuations has no preferential direction. On the other hand, the MV direction of velocity fluctuations tends to be distributed more uniformly. An analysis of magnetic compressibility and density fluctuations suggests that the fluctuations resemble properties of Alfvénic fluctuations if the MV direction is aligned with background magnetic field whereas slow-mode-like fluctuations have the MV direction perpendicular to the background field. The proportion between Alfvénic and slow-mode-like fluctuations depends on plasma β and length scale: the dependence on the solar wind speed is weak. We present 3D numerical MHD simulations and show that the numerical results are compatible with our experimental results.

Type: Article
Title: Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind
Open access status: An open access version is available from UCL Discovery
DOI: 10.3847/1538-4357/abf6c9
Publisher version: http://dx.doi.org/10.3847/1538-4357/abf6c9
Language: English
Additional information: This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10128940
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