Characteristics of Solar Wind Fluctuations at and below Ion Scales

Abstract

Kinetic-scale fluctuations in magnetized collisionless plasmas, such as a solar wind, attract attention owing to their vital role in the dynamics of the dissipation of free energy to random particle motion. As the free energy cascades in the inertial range of turbulence, fluctuations at ion characteristic scales become more compressible. Measurements show that these fluctuations possess highly oblique propagation angles with respect to the background magnetic field and follow theoretical predictions for kinetic Alfvén waves (KAWs). We performed alarge (465 cases) statistical study of normalized fluctuations of the density, bulk velocity, and magnetic field around ion gyroscale and concentrated on (i) their compressibility, (ii) the ratio of density and magnetic field fluctuations, and (iii) the ratio of density and velocity fluctuations. We find that observed fluctuations follow the two-fluid prediction for KAWs generally, but the spread of measured values around their theoretical predictions is large. The analysis of measurement uncertainties shows that the difference between the observed and predicted levels of fluctuations cannot be fully explained by these uncertainties and that the nature of solar wind fluctuations is more complex.

Magnetic Field Turbulence in the Solar Wind at Sub‐ion Scales: In Situ Observations and Numerical Simulations

“…In simulations, there is very good matching with the predicted level at higher beta, but an excess of C at low beta; this effect was already observed in Cerri et al (2017) and is confirmed here on a larger range of β. On the other hand, in solar wind observations, the ratio is always lower than 1 (smaller compressibility than predicted by the theory) and close to ∼ 0.75 for all β, in agreement with similar studies on the plasma compressibility (Chen et al, 2013a;Pitňa et al, 2019).…”

“…Our study confirms this scenario and suggests that the same trend is followed for all spectra, almost independently of the plasma beta. A ratio smaller than one and close to ∼ 0.75 is also consistent with similar observational results of the plasma compressibility and based on the ratio between density and perpendicular magnetic fluctuations predicted by linear theory (Chen et al, 2013a;Pitňa et al, 2019). This was interpreted by Chen et al (2013a) as a consequence of the nonlinear behavior of the solar wind fluctuations in the sub-ion range, in agreement with simulations of strong KAW-turbulence (Boldyrev et al, 2013).…”

“…Quite differently, the ratio between the in situ observations and the theoretical C is always below one and around 0.7-0.8 for all β groups in the subion range (see also Figure 10 of Lacombe et al, 2017). This behavior is consistent with the results of Pitňa et al (2019) based . The horizontal dashed line denotes energy equipartition between components (i.e., isotropy).…”

“…It is believed that this is related to a change in the properties of the turbulent fluctuations, which become intrinsically compressive at small scales. It is then by studying in detail their properties that it is possible to shed light on the nature of the fluctuations which support the cascade at kinetic scales (Chen et al, 2013b;Grošelj et al, 2019;Pitňa et al, 2019;Alexandrova et al, 2020).…”

On the Fourier Contribution of Strong Current Sheets to the High‐Frequency Magnetic Power SpectralDensity of the Solar Wind

“…In a cascade picture the high‐frequency magnetic power spectrum of the solar wind is associated with plasma wave modes (e.g., whistler waves; Narita & Gary, ; Gary et al, ; Narita et al, , kinetic Alfvén waves; Podesta & TenBarge, ; Smith et al, ; Podesta, ; Salem et al, ; Chen et al, ; Pitna et al, ), both (Boldyrev et al, ; Gary & Smith, ; Mithaiwala et al, ), and perhaps Bernstein waves (Podesta, ; Verscharen et al, ), the wave‐wave interactions of these plasma modes, and the kinetic damping of these wave modes. In the cascade picture the high‐frequency spectrum represents the amplitudes of plasma waves with various wavenumbers.…”