Radial evolution of the wave-vector anisotropy of solar wind turbulence between 0.3 and 1 AU

Abstract

We present observations of the power spectral anisotropy in wave-vector space of solar wind turbulence, and study how it evolves in interplanetary space with increasing heliocentric distance. For this purpose we use magnetic field measurements made by the Helios-2 spacecraft at three positions between 0.29 and 0.9 AU. To derive the power spectral density (PSD) in $(k_{\parallel },k_{\perp })$-space based on single-satellite measurements is a challenging task not yet accomplished previously. Here we derive the spectrum ${\mathrm{PSD}}_{2D}$(${\mathrm{k}}_{\parallel }$, ${\mathrm{k}}_{\perp }$) from the spatial correlation function ${\mathrm{CF}}_{2D}({\mathrm{r}}_{\parallel },{\mathrm{r}}_{\perp })$ by a transformation according to the projection-slice theorem. We find the so constructed PSDs to be distributed in k-space mainly along a ridge that is more inclined toward the ${\mathrm{k}}_{\perp }$ than ${\mathrm{k}}_{\parallel }$ axis, a new result which probably indicates preferential cascading of turbulent energy along the ${\mathrm{k}}_{\perp }$ direction. Furthermore, this ridge of the distribution is found to gradually get closer to the ${\mathrm{k}}_{\perp }$ axis, as the outer scale length of the turbulence becomes larger while the solar wind flows further away from the Sun. In the vicinity of the ${\mathrm{k}}_{\parallel }$ axis, there appears a minor spectral component that probably corresponds to quasi-parallel Alfvénic fluctuations. Their relative contribution to the total spectral density tends to decrease with radial distance. These findings suggest that solar wind turbulence undergoes an anisotropic cascade transporting most of its magnetic energy towards larger ${\mathrm{k}}_{\perp }$, and that the anisotropy in the inertial range is radially developing further at scales that are relatively far from the ever increasing outer scale.

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• Imprints of expansion onto the local anisotropy of solar wind turbulence
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• Kinetic Alfvén turbulence below and above ion-cyclotron frequency
• Observational quantification of three-dimensional anisotropies and scalings of space plasma turbulence at kinetic scales
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• The Fluid-like Behavior of Kinetic Alfvén Turbulence in Space Plasma
• Isotropic Scaling Features Measured Locally in the Solar Wind Turbulence with Stationary Background Field
• A Model for Dissipation of Solar Wind Magnetic Turbulence by Kinetic Alfvén Waves at Electron Scales: Comparison with Observations
• Parker Solar Probe Observations of High Plasma Beta Solar Wind from Streamer Belt
• Spectral Anisotropy of Elsässer Variables in Two Dimensional Wave-vector Space as Observed in the Fast Solar Wind Turbulence
• Sunward-propagating Alfvénic fluctuations observed in the heliosphere
• The uncertainty of local background magnetic field orientation in anisotropic plasma turbulence
• The Temperature, Electron, and Pressure Characteristics of Switchbacks: Parker Solar Probe Observations
• Forward Modeling of Reduced Power Spectra From Three-Dimensional k-Space
• Magnetic Energy Transfer and Distribution between Protons and Electrons for Alfvénic Waves at Kinetic Scales in Wavenumber Space
• 3-D feature of self-correlation level contours at $10^{10}\mathrm{cm}$ scale in solar wind turbulence
• Chaotic Motion of Ions In Finite-amplitude Low-frequency Alfvén Waves
• Properties and Radial Evolution of Solar Wind Turbulence Near Mercury's Orbit