Magnetic field spectral evolution in the inner heliosphere

TL;DR

This study analyzes how magnetic turbulence spectra evolve with distance from the Sun using Parker Solar Probe and Solar Orbiter data, revealing scale-dependent changes and differences based on wind properties.

Contribution

It provides new insights into the radial evolution of magnetic turbulence spectra, especially the transition from a -3/2 to -5/3 power-law scaling with distance.

Findings

Inertial range extends and steepens with distance from the Sun.

Spectra with large magnetic energy excesses steepen more significantly.

Slower wind streams exhibit steeper spectra, linked to lower Alfvénicity.

Abstract

Parker Solar Probe and Solar Orbiter data are used to investigate the radial evolution of magnetic turbulence between $0.06 ~ \lesssim R ~\lesssim 1$ au. The spectrum is studied as a function of scale, normalized to the ion inertial scale $d_{i}$. In the vicinity of the Sun, the inertial range is limited to a narrow range of scales and exhibits a power-law exponent of, $\alpha_{B} = -3/2$, independent of plasma parameters. The inertial range grows with distance, progressively extending to larger spatial scales, while steepening towards a $\alpha_{B} =-5/3$ scaling. It is observed that spectra for intervals with large magnetic energy excesses and low Alfv\'enic content steepen significantly with distance, in contrast to highly Alfv\'enic intervals that retain their near-Sun scaling. The occurrence of steeper spectra in slower wind streams may be attributed to the observed positive correlation between solar wind speed and Alfv\'enicity.