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Evidence for the Helicity Barrier from Measurements of the Turbulence Transition Range in the Solar Wind

J. R. McIntyre1, C. H. K. Chen1, J. Squire2, R. Meyrand2, and P. A. Simon1

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Phys. Rev. X 15, 031008 – Published 8 July, 2025

DOI: https://doi.org/10.1103/PhysRevX.15.031008

Abstract

Many hot, diffuse astrophysical plasmas are collisionless, with collision times that exceed dynamical timescales of interest. Without a standard viscosity, the mechanisms by which mechanical energy dissipates into heat via a turbulent cascade are complex and nonuniversal, with important consequences for the partition of heat between different species and directions. Recent theory has argued that in some parameter regimes a barrier to the turbulent cascade of energy forms at small scales, profoundly altering the nature of the heating of the plasma by changing how fluctuations dissipate. The solar wind is an environment, when in this parameter regime, where such a barrier would be expected to form. Here, using data from NASA’s Parker Solar Probe, we find that the shape of the magnetic energy spectrum around ion scales varies with solar wind parameters in a manner consistent with the presence of the barrier. This allows us to identify critical values of parameters necessary for the barrier to form; we show that the barrier appears fully developed for ion plasma beta of below ≃0.5 and becomes increasingly prominent with increasing normalized cross helicity (the difference in energy in wave packets propagating parallel and antiparallel to the background magnetic field) for values greater than ≃0.4. 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.

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Popular Summary

One of the big open questions in solar physics is how particles in the solar wind are heated and accelerated as they stream away from the Sun. Recent research suggests that under certain conditions, a barrier to energy transfer forms in the solar wind, preventing energy from reaching smaller scales. This phenomenon could alter the heating process and help clarify some of the wind’s unexplained properties. Using data from NASA’s Parker Solar Probe, we find evidence that this barrier frequently forms, particularly in regions relatively close to the Sun.

The barrier can form only under certain conditions, such as when thermal energy is relatively low compared to magnetic energy. Since fluctuations in the magnetic field are expected to behave differently when the barrier is active versus when it is not, measuring how these fluctuations vary with solar wind conditions relevant to the barrier’s formation—including the thermal-to-magnetic energy ratio—provides a way to test for the barrier’s presence. By analyzing solar wind magnetic field measurements, we find that the fluctuations behave exactly as predicted with changes in solar wind parameters that characterize these conditions. This analysis also allows us to identify specific values for these parameters that are needed for the barrier to form, and we find that these values are common near the Sun.

This small-scale barrier seems to significantly affect how the solar wind heats up. Among other things, it helps explain why protons in the solar wind tend to be hotter than electrons.

Article Text

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Phys. Rev. X 15, 031008– Published 8 July, 2025
  • Received 15 July 2024
  • Revised 19 February 2025
  • Accepted 23 April 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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