The radial variation of the solar wind turbulence spectra near the kinetic break scale from Parker Solar Probe measurements

TL;DR

This study analyzes how solar wind turbulence spectra change with distance from the Sun using Parker Solar Probe data, revealing variations in spectral indices and the spectral break near the kinetic scale.

Contribution

It provides new measurements of the radial dependence of turbulence spectral indices and the spectral break in the inner heliosphere from Parker Solar Probe data.

Findings

Inertial range power law index varies weakly with radial distance.

Dissipation range index becomes steeper closer to the Sun.

Spectral break position aligns with gyro-resonant damping models.

Abstract

In this study we examine the radial dependence of the inertial and dissipation range indices, as well as the spectral break separating the inertial and dissipation range in power density spectra of interplanetary magnetic field fluctuations using {\it Parker Solar Probe} data from the fifth solar encounter between $\sim$0.1 and $\sim$0.7 au. The derived break wavenumber compares reasonably well with previous estimates at larger radial distances and is consistent with gyro-resonant damping of Alfv\'enic fluctuations by thermal protons. We find that the inertial scale power law index varies between approximately -1.65 and -1.45. This is consistent with either the Kolmogorov (-5/3) or Iroshnikov-Kraichnan (-3/2) values, has a very weak radial dependence with a possible hint that the spectrum becomes steeper closer to the Sun. The dissipation range power law index, however, has a clear dependence on radial distance (and turbulence age), decreasing from -3 near 0.7 au (4 days) to -4 [$\pm$0.3] at 0.1 au (0.75 days) closer to the Sun.