Evidence for the helicity barrier from measurements of the turbulence transition range in the solar wind

TL;DR

This study uses Parker Solar Probe data to identify a helicity barrier in solar wind turbulence, which influences energy dissipation and heating, especially near the Sun, diverging from standard turbulence models.

Contribution

It provides observational evidence for a helicity barrier in solar wind turbulence and links its formation to specific plasma parameters, advancing understanding of solar wind heating.

Findings

The magnetic energy spectrum shape varies with plasma parameters.

The barrier forms at low ion plasma beta (<0.5).

The barrier becomes more prominent with higher cross helicity.

Abstract

The means by which the turbulent cascade of energy is dissipated in the solar wind, and in other astrophysical systems, is a major open question. It has recently been proposed that a barrier to the transfer of energy can develop at small scales, which can enable heating through ion-cyclotron resonance, under conditions applicable to regions of the solar wind. Such a scenario fundamentally diverges from the standard picture of turbulence, where the energy cascade proceeds unimpeded until it is dissipated. Here, using data from NASA's Parker Solar Probe, we find that the shape of the magnetic energy spectrum around the ion gyroradius varies with solar wind parameters in a manner consistent with the presence of such a barrier. This allows us to identify critical values of some of the parameters necessary for the barrier to form; we show that the barrier appears fully developed for ion plasma beta of below $\simeq0.5$ and becomes increasingly prominent with imbalance for normalised cross helicity values greater than $\simeq0.4$. As these conditions are frequently met in the solar wind, particularly close to the Sun, our results suggest that the barrier is likely playing a significant role in turbulent dissipation in the solar wind and so is an important mechanism in explaining its heating and acceleration.