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

TL;DR

This study analyzes Parker Solar Probe data to reveal strong anisotropy in magnetic field spectra at kinetic scales in solar wind turbulence, showing how spectral indices vary with angle and are linked to plasma modes.

Contribution

It provides new insights into the anisotropic nature of magnetic spectra at kinetic scales and connects spectral index variations to plasma mode characteristics in solar wind turbulence.

Findings

Magnetic spectra are strongly anisotropic at kinetic scales, with maximum power perpendicular to the magnetic field.

Spectral index varies from -3.2 to -5.8 depending on the angle to the magnetic field.

Anisotropy of spectral indices is explained by the plasma modes involved in the cascade.

Abstract

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 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 magnetic spectra varies from -3.2 to -5.8 when the angle theta_B changes from 90 to 180 (or 0) deg, 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.