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

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.