Three-Dimensional Anisotropy and Scaling Properties of Solar Wind Turbulence at Kinetic Scales in the Inner Heliosphere: Parker Solar Probe Observations

We utilize the data from the Parker Solar Probe mission at its first perihelion to investigate the three-dimensional (3D) anisotropies and scalings of solar wind turbulence for the total, perpendicular, and parallel magnetic-field fluctuations at kinetic scales in the inner heliosphere. By calculating the five-point second-order structure functions, we find that the three characteristic lengths of turbulence eddies for the total and the perpendicular magnetic-field fluctuations in the local reference frame $({\hat{L}}_{\perp },{\hat{l}}_{\perp },{\hat{l}}_{| | })$ defined with respect to the local mean magnetic field Blocal feature as l∣∣ > L⊥ > l⊥ in both the transition range and the ion-to-electron scales, but l∣∣ > L⊥ ≈ l⊥ for the parallel magnetic-field fluctuations. For the total magnetic-field fluctuations, the wave-vector anisotropy scalings are characterized by ${l}_{| | }\propto {{l}_{\perp }}^{0.78}$ and ${L}_{\perp }\propto {{l}_{\perp }}^{1.02}$ in the transition range, and they feature as ${l}_{| | }\propto {{l}_{\perp }}^{0.44}$ and ${L}_{\perp }\propto {{l}_{\perp }}^{0.73}$ in the ion-to-electron scales. Still, we need more complete kinetic-scale turbulence models to explain all these observational results.