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

Abstract

We explore the multi-faceted important features of turbulence (e.g., anisotropy, dispersion, diffusion) in the three-dimensional (3D) wavenumber domain ($k_{\parallel }$, $k_{\perp ,1}$, $k_{\perp ,2}$), by employing the k-filtering technique to the high-quality measurements of fields and particles from the MMS multi-spacecraft constellation. We compute the 3D power spectral densities (PSDs) of magnetic and electric fluctuations (marked as $\mathrm{PSD}(\delta \mathbf{B}(\mathbf{k}))$ and $\mathrm{PSD}(\delta {\mathbf{E}}_{⟨{\mathbf{v}}_{\mathrm{i}}⟩}^{\prime }(\mathbf{k}))$), both of which show a prominent spectral anisotropy in the sub-ion range. We give the first 3D image of the bifurcation between power spectra of the electric and magnetic fluctuations, by calculating the ratio between $\mathrm{PSD}(\delta {\mathbf{E}}_{⟨{\mathbf{v}}_{\mathrm{i}}⟩}^{\prime }(\mathbf{k}))$ and $\mathrm{PSD}(\delta \mathbf{B}(\mathbf{k}))$, the distribution of which is related to the non-linear dispersion relation. We also compute the ratio between electric spectra in different reference frames defined by the ion bulk velocity, that is $\mathrm{PSD}(\delta {\mathbf{E}}_{\mathrm{local}{\mathbf{v}}_{\mathrm{i}}}^{\prime })/\mathrm{PSD}(\delta {\mathbf{E}}_{⟨{\mathbf{v}}_{\mathrm{i}}⟩}^{\prime })$, to visualize the turbulence 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 (KAW). This work manifests the worth of the k-filtering technique in diagnosing turbulence comprehensively, especially when the electric field is involved.

19 pages, 7 figures, submitted to ApJ

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