Evidence for an Inverse Cascade of Magnetic Helicity in the Inner Heliosphere

TL;DR

This study uses Parker Solar Probe data to confirm a persistent inverse cascade of magnetic helicity from the Sun to Mercury, revealing a radial sign change and challenging previous assumptions about turbulence randomness.

Contribution

It provides the first observational evidence of an inverse cascade of magnetic helicity in the inner heliosphere, extending previous theoretical predictions.

Findings

Confirmed inverse cascade from Sun to Mercury

Discovered radial sign change of magnetic helicity density

Challenged the idea of turbulence randomness in the inner heliosphere

Abstract

To elucidate the cascade direction of the solar wind turbulence, we analyzed magnetic helicity density spectra from the Parker Solar Probe data across more than 500 heliocentric distances. For the first time, we confirmed a persistent inverse cascade extending from the Sun to Mercury's orbital vicinity. This finding challenges the conventional hypothesis that the magnetic helicity density within the inner heliosphere is random. Furthermore, our analysis revealed a radial sign change of the spectral magnetic helicity density at a frequency whose value decreases logarithmically with distance. These results provide new insights into the evolution of solar wind turbulence in the inner heliosphere.