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 \emph{Parker Solar Probe} (\emph{PSP}), we present an observation of the anisotropy at kinetic scales in the slow, Alfv'enic, solar wind in the inner heliosphere. \textbf{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 $k_\perp \gg k_\parallel$ is found evident in both transition and kinetic ranges, with the power anisotropy $P_{\perp }\mathrm{/}{P}_{\parallel }>10$ in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from ${\alpha }_{t\parallel }=-5.7\pm 1.0$ to $\alpha_{t\perp}=-3.7\pm 0.3$inthetransitionrangeand$\alpha_{k\parallel}=-3.12\pm 0.22$to$\alpha_{k\perp}=-2.57\pm 0.09$ in the kinetic range. The corresponding wavevector anisotropy has the scaling of $k_\parallel \sim k_\perp^{0.71\pm 0.17}$inthetransitionrange,andchangesto$k_\parallel \sim k_\perp^{0.38\pm 0.09}$ in the kinetic range, consistent with the kinetic Alfv'enic turbulence at sub-ion scales.