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

Abstract

We investigate the transition of the solar wind turbulent cascade from MHD to sub‐ion range by means of a detailed comparison between in situ observations and hybrid numerical simulations. In particular, we focus on the properties of the magnetic field and its component anisotropy in Cluster measurements and hybrid 2D simulations. First, we address the angular distribution of wave vector in the kinetic range between ion and electron scales by studying the variance anisotropy of the magnetic field components. When taking into account a single-direction sampling, like that performed by spacecraft in the solar wind, the main properties of the fluctuations observed in situ are also recovered in our numerical description. This result confirms that solar wind turbulence in the sub‐ion range is characterized by a quasi-2D gyrotropic distribution of k-vectors around the mean field. We then consider the magnetic compressibility associated with the turbulent cascade and its evolution from large-MHD to sub‐ion scales. The ratio of field aligned to perpendicular fluctuations, typically low in the MHD inertial range, increases significantly when crossing ion scales and its value in the sub‐ion range is a function of the total plasma beta only, as expected from theoretical predictions, with higher magnetic compressibility for higher beta. Moreover, we observe that this increase has a gradual trend from low to high beta values in the in situ data; this behavior is well captured by the numerical simulations. The level of magnetic field compressibility that is observed in situ and in the simulations is in fairly good agreement with theoretical predictions, especially at high beta, suggesting that, in the kinetic range explored, the turbulence is supported by low-frequency and highly oblique fluctuations in pressure balance, like kinetic Alfvén waves or other slowly evolving coherent structures. The resulting scaling properties as a function of the plasma beta and the main differences between numerical and theoretical expectations and in situ observations are also discussed.

A Novel Method for Estimating the Intrinsic Magnetic Field Spectrum of Kinetic-Range Turbulence

“…We conclude that correlation coefficients between magnetic field components do not trivially translate into correlations between their respective powers. Moreover, the condition δB 2 ⊥ δB 2 is generally satisfied in the inertial range, whereas in the kinetic range, the fluctuations may exhibit significant compressibility δB 2 /δB 2 ⊥ ≈ 1/2, the exact value of which depends on the total plasma beta [44]. Therefore, the dimensionality of the problem is no longer K = 2, and F c will depend on the cross-correlations of all the three components/powers.…”

Solar Orbiter’s encounter with the tail of comet C/2019 Y4 (ATLAS): Magnetic field draping and cometary pick-up ion waves

“…The fourth and fifth panels show the spectrogram of the magnetic compressibility (e.g. Matteini et al 2020) and reduced helicity (e.g. Woodham et al 2018), respectively; in these panels, the upper and lower solid black lines encode the proton and Oxygen O + cyclotron frequencies, respectively.…”

Importance of accurate consideration of the electron inertia in hybrid-kinetic simulations of collisionless plasma turbulence: 1. The 2D limit

“…This is particularly true for the study of the turbulent solar wind plasma 4 which provides an excellent natural laboratory for turbulence studies in a broad range of parameters 5 . In fact, recent in situ spacecraft measurements in the solar wind have provided a wealth of data which extended the observational basis to kinetic scales [6][7][8][9] . Recent observations in the inner solar corona by the Parker Solar Probe have revealed new kinetic scale features of the collisionless turbulence of the solar wind plasma [10][11][12] .…”