Power Anisotropy, Dispersion Signature and Turbulence Diffusion Region in the 3D Wavenumber Domain of Space Plasma Turbulence

We explore the multifaceted important features of turbulence (e.g., anisotropy, dispersion, and diffusion) in the three-dimensional (3D) wavenumber domain (k _∥, k _⊥,1, k _⊥,2), by employing the k-filtering technique to high-quality measurements of fields and particles from the Magnetospheric Multiscale Mission (MMS) multi-spacecraft constellation. We compute the 3D power spectral densities (PSDs) of magnetic and electric field fluctuations (marked as PSD(δ B ( k )) and PSD ( δ E 〈 v i 〉 ′ ( k ) ) , respectively), both of which show a prominent spectral anisotropy in the sub-ion range. We give the first 3D image of the bifurcation between the power spectra of the electric and magnetic fluctuations, by calculating the ratio between PSD ( δ E 〈 v i 〉 ′ ( k ) ) and PSD(δ B ( k )), the distribution of which is related to the nonlinear dispersion relation. We also compute the ratio between electric spectra in different reference frames defined by the ion bulk velocity, PSD ( δ E local v i ′ ) / PSD ( δ E 〈 v i 〉 ′ ) , to visualize the turbulent ion diffusion region (T-IDR) in wavenumber space. The T-IDR has an anisotropy and a preferential direction of wavevectors, which is generally consistent with the plasma wave theory prediction based on the dominance of kinetic Alfvén waves. This work demonstrates the worth of the k-filtering technique in diagnosing turbulence comprehensively, especially when the electric field is involved.