Multi-spacecraft analysis of the properties of Magnetohydrodynamic Perturbations in Solar Wind turbulence at 1 au

We present observations of three-dimensional magnetic power spectra in wavevector space to investigate the anisotropy and scalings of sub-Alfvénic solar wind turbulence in low-βp plasma at magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft. The magnetic power distributions are organized in a new coordinate determined by wavevectors (k̂) and background magnetic field (b̂0) in Fourier space. This study utilizes two approaches to determine wavevectors: the singular value decomposition method and multi-spacecraft timing analysis. The combination of both methods allows an examination of magnetic field fluctuation properties in terms of mode compositions without spatiotemporal hypothesis. Observations show that fluctuations (δB⊥1) in the direction perpendicular to k̂ and b̂0 prominently cascade perpendicular to b̂0, and such anisotropy increases with wavenumbers. The reduced power spectra of δB⊥1 follow Goldreich-Sridhar scalings: P̂ (k⊥) ∝ k − 53 ⊥ and P̂ (k‖) ∝ k−2 ‖ . In contrast, fluctuations within k̂b̂0 plane show isotropic behaviors: perpendicular power distributions are approximately the same as parallel distributions. The reduced power spectra of fluctuations within k̂b̂0 plane follow the scalings: P̂ (k⊥) ∝ k − 32 ⊥ and P̂ (k‖) ∝ k − 32 ‖ . Comparing frequency-wavevector spectra with theoretical dispersion relations of MHD modes, we find that δB⊥1 are probably associated with Alfvén modes. On the other hand, magnetic field fluctuations within k̂b̂0 plane more likely originate from fast modes in low-βp plasma 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. These results are valuable for further studies of energy compositions of plasma turbulence and their effects on energetic particle transports.