Violation of the zeroth law of turbulence through the helicity barrier effect

In the standard picture of plasma turbulence, energy injected at large scales cascades conservatively to small scales, eventually thermalizing to heat particles via a variety of mechanisms. This leads to the "zeroth law of turbulence," whereby the macroscopic statistics of the flow and magnetic field are independent of the details of how energy is microscopically dissipated into heat. The helicity barrier is a newly discovered effect that upends this notion for low-beta imbalanced Alfvenic turbulence, as occurs in the solar wind and corona. Because of a conserved generalized helicity invariant, the energy cascade is "stuck" around the ion gyroscale, which causes the turbulent energy to build up in time (if it is continuously forced). The outcome is quite dramatic for the solar wind and for plasma heating in general because it changes the thermodynamics properties of the plasma based on how it is stirred at large scales. The effect explains a number of long-standing puzzles from in-situ solar-wind observations, including the steep "transition range" seen in magnetic spectra, magnetic helicity signatures, and ion distribution functions. It also links two previously well-studied coronal heating mechanisms (turbulence and ion-cyclotron waves), suggesting an important role of the helicity barrier in the coronal heating problem. More generally, it demonstrates how the complex, nonlinear microphysics of collisionless plasmas can have a strong influence on the macroscopic properties of astrophysical processes.