Interpreting solar wind turbulent spectra beyond Taylor's Hypothesis

Abstract

In this paper we apply a methodology, recently proposed by Bourouaine and Perez (2019) (BP19), to interpret solar-wind turbulent power spectra beyond Taylor approximation (TA). The turbulent power spectra were measured using \emph{Helios} spacecraft data near 0.6 au. We use the model proposed in BP19 to reproduce the field-perpendicular power spectrum $E(k_{\perp })$ of anti-sunward Alfvénic fluctuations in the plasma frame (where $k_{\perp }$ is the field-perpendicular wavenumber) from the corresponding measured frequency power spectrum $P_{\mathrm{sc}}(\omega ,{\theta }_b)$ along the sampling angle ${\theta }_b$, which is the angle between the local magnetic field and the sampling direction. Here $\omega =2\pi f$ and $f$ is the frequency of the time signal. Interestingly enough, we found that for all corresponding measured frequency power spectrum $P_{\mathrm{sc}}(\omega ,{\theta }_b)$ the reproduced field-perpendicular power spectrum $E(k_{\perp })$ is the same and independent of the considered sampling angle ${\theta }_b$. This finding is consistent with the fact that the analyzed turbulence is strong and highly anisotropic with $k_{\Vert }\ll k_{\perp }$ (where $k_{\Vert }$ is the field-parallel wavenumber). Furthermore, for this specific time signal we found that the commonly used TA is still approximately valid with the important difference that a broadening in $k_{\perp }$ for each angular frequency $\omega$ is present. This broadening can be described in the context of the methodology proposed in BP19.

Accepted for publication in ApJL

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• The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration
• Applicability of Taylor's Hypothesis during Parker Solar Probe perihelia
• The radial variation of the solar wind turbulence spectra near the kinetic break scale from Parker Solar Probe measurements
• On the Interpretation of the Scalings of Density Fluctuations from In-situ Solar Wind Observations: Insights from 3D Turbulence Simulations