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

Abstract

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, Alfvénic, solar wind in the inner heliosphere. \textbf{The magnetic compressibility behaves as expected for kinetic Alfvénic 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 $P_{\perp }/P_{\parallel }>10$ 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 ${\alpha }_{t\perp }=-3.7\pm 0.3$ in the transition range and ${\alpha }_{k\parallel }=-3.12\pm 0.22$ to ${\alpha }_{k\perp }=-2.57\pm 0.09$ in the kinetic range. The corresponding wavevector anisotropy has the scaling of $k_{\parallel }\sim k_{\perp }^{0.71\pm 0.17}$ in the transition range, and changes to $k_{\parallel }\sim k_{\perp }^{0.38\pm 0.09}$ in the kinetic range, consistent with the kinetic Alfvénic turbulence at sub-ion scales.

4 figures; submitted to apjl

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