Anisotropy of Magnetic Field Spectra at Kinetic Scales of Solar Wind Turbulence as Revealed by the Parker Solar Probe in the Inner Heliosphere

Using the Parker Solar Probe data taken in the inner heliosphere, we investigate the power and spatial anisotropy of magnetic field spectra at kinetic scales (i.e., around sub-ion scales) in solar wind turbulence in the inner heliosphere. We find that strong anisotropy of the magnetic spectra occurs at kinetic scales with the strongest power in the perpendicular direction with respect to the local magnetic field (forming an angle theta (B) with the mean flow velocity). The spectral index of the magnetic spectra varies from -3.2 to -5.8 when the angle theta (B) changes from 90 degrees to 180 degrees (or 0 degrees), indicating that strong anisotropy of the spectral indices occurs at kinetic scales in the solar wind turbulence. Using a diagnosis based on the magnetic helicity, we show that the anisotropy of the spectral indices can be explained by the nature of the plasma modes that carry the cascade at kinetic scales. We discuss our findings in light of existing theories and current development in the field.