Observational Quantification of Three-dimensional Anisotropies and Scalings of Space Plasma Turbulence at Kinetic Scales
Résumé
A statistical survey of spectral anisotropy of space plasma turbulence is performed using five years of measurements from the Magnetospheric Multiscale mission 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 ${{\boldsymbol{B}}}_{0}$ , the "statistical eddies" are found to be mostly elongated along ${{\boldsymbol{B}}}_{0}$ and shortened in the direction perpendicular to both ${{\boldsymbol{B}}}_{0}$ and local field fluctuations. From several di (ion inertial length) toward ∼0.05 di, 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 di 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.
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