Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind

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.

Regulation of Proton–α Differential Flow by Compressive Fluctuations and Ion-scale Instabilities in the Solar Wind

“…Although solar-wind turbulence is a nonlinear process, its fluctuations display many properties that are similar to the properties of linear waves (He et al 2011;Howes et al 2012;Chen et al 2013). A large body of observational work (Yao et al 2011;Chen et al 2012;Howes et al 2012;Klein et al 2012;Verscharen et al 2017;Šafrankova et al 2021) suggests the presence of slow-mode-like fluctuations in the solar wind although the origin of these fluctuations is still unclear. According to linear theory, classical slow-mode waves are strongly damped in a homogeneous Maxwellian plasma, especially at intermediate to high β p and when T e  T p (Barnes 1966).…”