Interpreting Solar Wind Turbulent Spectra beyond Taylor’s Hypothesis

In this Letter we apply a methodology, recently proposed by Bourouaine & Perez (BP19), to interpret solar wind turbulent power spectra beyond the Taylor approximation (TA). The turbulent power spectra were measured using Helios spacecraft data near 0.6 au. We use the model proposed in BP19 to reproduce the field-perpendicular power spectrum E(k⊥) of antisunward Alfvénic fluctuations in the plasma frame (where k⊥ is the field-perpendicular wavenumber) from the corresponding measured frequency power spectrum ${P}_{\mathrm{sc}}(\omega ,{\theta }_{b})$ along the sampling angle θb, which is the angle between the local magnetic field and the sampling direction. Here ω = 2πf 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⊥) is the same and independent of the considered sampling angle θb. This finding is consistent with the fact that the analyzed turbulence is strong and highly anisotropic with ${k}_{\parallel }\ll {k}_{\perp }$ (where ${k}_{\parallel }$ 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⊥ for each angular frequency ω is present. This broadening can be described in the context of the methodology proposed in BP19.