Anisotropies of density and magnetic field fluctuations from inertial to kinetic scales in solar wind turbulence

Magnetic- and density-field fluctuations in the solar wind extend over a broad range of spatial and temporal scales. At inertial (MHD) scales, magnetic-field fluctuations are dominated by Alfvénic and/or 2D turbulence, while compressive magnetic fluctuations are associated with slow and fast MHD modes. Density fluctuations at these scales arise primarily from a mixture of entropic, slow-mode, and fast-mode contributions in the transition range near ion characteristic scales, the nature of these fluctuations changes as MHD descriptions break down and kinetic effects become important. At sub-ion scales, both magnetic-field and density fluctuations are governed by fully kinetic processes. Their coupling reflects the dominance of kinetic Alfvén wave like fluctuations, leading to enhanced compressibility and altered phase relationships between density and magnetic fields. Across all these regimes, density fluctuations—tightly linked to magnetic-field variability—play a key role in the scattering of radio waves from astrophysical sources both within and beyond the heliosphere, providing a powerful diagnostic of solar-wind turbulence across scales.

In this paper, we describe observations from two long solar wind intervals measured by the BMSW instrument onboard the Spektr-R spacecraft, which provides ion density measurements at a cadence of 32 ms. Because the Spektr-R magnetometer was not operational, we analyze magnetic-field measurements from the THEMIS-C and Wind spacecraft. The analysis of density fluctuations shows that at large (inertial) scales the fluctuations are nearly isotropic, while in the kinetic range they become strongly anisotropic. In contrast, magnetic-field fluctuations display pronounced anisotropy in both the inertial and kinetic ranges. We discuss the differing anisotropic properties of density and magnetic-field fluctuations and the complications they introduce in interpreting multi-spacecraft measurements.