High-frequency heating of the solar wind triggered by low-frequency turbulence

TL;DR

This paper proposes that the helicity barrier effect in turbulence explains how low-frequency solar wind turbulence can generate high-frequency ion-cyclotron waves, unifying two main theories of solar wind heating and matching spacecraft observations.

Contribution

It introduces a new unified theory linking low-frequency turbulence and high-frequency ion heating via the helicity barrier, supported by six-dimensional simulations and observational data.

Findings

Simulations show energy growth and small-scale generation due to the helicity barrier.

The turbulence and ion distributions match Parker Solar Probe measurements.

The theory links plasma expansion to ion-electron heating ratios and wind speed bimodality.

Abstract

The fast solar wind's high speeds and nonthermal features require that significant heating occurs well above the Sun's surface. Two leading theories seem incompatible: low-frequency "Alfv\'enic" turbulence, which transports energy outwards and is observed ubiquitously by spacecraft but struggles to explain the observed dominance of ion over electron heating; and high-frequency ion-cyclotron waves (ICWs), which explain the nonthermal heating of ions but lack an obvious source. Here, we argue that the recently proposed "helicity barrier" effect, which limits electron heating by inhibiting the turbulent cascade of energy to the smallest scales, can unify these two paradigms. Our six-dimensional simulations show how the helicity barrier causes the large-scale energy to grow in time, generating small parallel scales and high-frequency ICW heating from low-frequency turbulence. The resulting turbulence and ion distribution function also closely match in-situ measurements from Parker Solar Probe and other spacecraft, explaining, among other features, the decades-long puzzle of the steep "transition range" observed in magnetic fluctuation spectra. The theory predicts a causal link between plasma expansion and the ion-to-electron heating ratio. Given the observational association between wind speed and expansion, we argue that the helicity barrier could play a key role in regulating the bimodal speed distribution of the solar wind.