Multispacecraft Analysis of the Properties of Magnetohydrodynamic Fluctuations in Sub-Alfvénic Solar Wind Turbulence at 1 au
Résumé
We present observations of three-dimensional magnetic power spectra in wavevector space to investigate the anisotropy and scalings of sub-Alfvénic solar wind turbulence at magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft. The magnetic power distributions are organized in a new coordinate determined by wavevectors ( $\hat{{\boldsymbol{\kappa }}}$ ) and background magnetic field ( ${\widehat{{\boldsymbol{b}}}}_{0}$ ) in Fourier space. This study utilizes two approaches to determine wavevectors: the singular value decomposition method and multispacecraft timing analysis. The combination of the two methods allows an examination of the properties of magnetic field fluctuations in terms of mode compositions without any spatiotemporal hypothesis. Observations show that fluctuations (δ B ⊥1) in the direction perpendicular to $\hat{{\boldsymbol{\kappa }}}$ and ${\widehat{{\boldsymbol{b}}}}_{0}$ prominently cascade perpendicular to ${\widehat{{\boldsymbol{b}}}}_{0}$ , and such anisotropy increases with wavenumbers. The reduced power spectra of δ B ⊥1 follow Goldreich-Sridhar scalings: $\hat{P}({k}_{\perp })\propto {k}_{\perp }^{-5/3}$ and $\hat{P}({k}_{\parallel })\propto {k}_{\parallel }^{-2}$ . In contrast, fluctuations within the $\hat{k}{\hat{b}}_{0}$ plane show isotropic behaviors: perpendicular power distributions are approximately the same as parallel distributions. The reduced power spectra of fluctuations within the $\hat{k}{\hat{b}}_{0}$ plane follow the scalings $\hat{P}({k}_{\perp })\propto {k}_{\perp }^{-3/2}$ and $\hat{P}({k}_{\parallel })\propto {k}_{\parallel }^{-3/2}$ . Comparing frequency-wavevector spectra with theoretical dispersion relations of MHD modes, we find that δ B ⊥1 are probably associated with Alfvén modes. On the other hand, magnetic field fluctuations within the $\hat{k}{\hat{b}}_{0}$ plane more likely originate from fast modes based on their isotropic behaviors. The observations of anisotropy and scalings of different magnetic field components are consistent with the predictions of current compressible MHD theory. Moreover, for the Alfvénic component, the ratio of cascading time to the wave period is found to be a factor of a few, consistent with critical balance in the strong turbulence regime. These results are valuable for further studies of energy compositions of plasma turbulence and their effects on energetic particle transport.
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