Prof. Jonathan (Jono) Squire, University of Otago

"The helicity barrier: how low-frequency turbulence triggers high-frequency heating of the solar wind"

Abstract: Turbulence, as the agent responsible for converting mechanical energy into heat, controls the thermodynamic properties of many astrophysical plasmas. For low collisionality plasmas, a wide array of channels are available — ions might be heated more than electrons (or vice versa), or the heating may be anisotropic with respect to the magnetic field. In the solar corona and fast solar wind, observations show that heating is perpendicular, affecting heavy ions more than protons, and protons more than electrons. This detailed heating partition has been a puzzle for low-frequency Alfvénic-turbulence models, which otherwise work well to explain fast-wind heating: theories generally predict electron heating dominates in the highly anisotropic, low-beta limit most relevant to low solar-wind conditions. In contrast, a different mechanism, high-frequency ion-cyclotron waves (ICWs), can naturally explain the heating, but it has proved difficult to explain the source of ICWs. In this talk, I will explain how a bizarre effect in plasma turbulence — termed the “helicity barrier” — may resolve this conundrum. The helicity barrier halts the flux of turbulent energy to the smallest scales when the stirring at large scales is dominated by waves propagating in one direction (imbalanced). This inhibits electron heating, causing the energy in magnetic fields to build up in time until it generates ICWs, thus preferentially heating ions. In our 6D hybrid-kinetic simulations, the resulting turbulence bears detailed resemblance to a wide array of in-situ measurements from the solar wind, capturing the steep “transition range,” observed magnetic-helicity signatures, and key features of the ion distribution function. I will finish by discussing some interesting implications of the effect for global features of the solar wind, such as its bimodal speed distribution, as well as applications to other astrophysical plasmas, such as black-hole accretion flows.

(The paper on this work, recently accepted for publication in Nature Astronomy, can be found at https://arxiv.org/abs/2109.03255)