Observational quantification of three-dimensional anisotropies and scalings of space plasma turbulence at kinetic scales

Abstract

A statistical survey of spectral anisotropy of space plasma turbulence is performed using five years measurements from MMS in the magnetosheath. By measuring the five-point second-order structure functions of the magnetic field, we have for the first time quantified the three-dimensional anisotropies and scalings at sub-ion-scales ($<$ 100 km). In the local reference frame $({\hat{L}}_{\perp },{\hat{l}}_{\perp },{\hat{l}}_{\parallel })$ defined with respect to local mean magnetic field ${\mathbf{B}}_0$ (Chen et al. 2012), the "statistical eddies" are found to be mostly elongated along ${\mathbf{B}}_0$ and shortened in the direction perpendicular to both ${\mathbf{B}}_0$ and local field fluctuations. From several $d_i$ (ion inertial length) toward $\sim$ 0.05 $d_i$, the ratio between eddies' parallel and perpendicular lengths features a trend of rise then fall, whereas the anisotropy in the perpendicular plane appears scale-invariant. Specifically, the anisotropy relations for the total magnetic field at 0.1-1.0 $d_i$ are obtained as $l_{\parallel }\simeq 2.44\cdot l_{\perp }^{0.71}$, and $L_{\perp }\simeq 1.58\cdot l_{\perp }^{1.08}$, respectively. Our results provide new observational evidence to compare with phenomenological models and numerical simulations, which may help to better understand the nature of kinetic scale turbulence.

arXiv commentAccepted

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