Anisotropy of Solar-Wind Turbulence in the Inner Heliosphere at Kinetic Scales: PSP Observations

The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of Parker Solar Probe, we present an observation of the anisotropy at kinetic scales in the slow, Alfv\'enic, solar wind in the inner heliosphere. The magnetic compressibility behaves as expected for kinetic Alfv\'enic turbulence below the ion scale. A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of k >> k// is found evident in both transition and kinetic ranges, with the power anisotropy P/P// > 10 in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from t//= -5.7 1.0 to t= -3.7 0.3 in the transition range and k//= -3.12 0.22 to k= -3.12 0.22 in the kinetic range. The corresponding wavevector anisotropy has the scaling of k// ~ k^(0.710.17) in the transition range, and changes tok// ~ k^(0.380.09) in the kinetic range, consistent with the kinetic Alfv\'enic turbulence at sub-ion scales.