The "helicity plug": halting nonlinear energy transfer from MHD to kinetic scales in plasma turbulence

Abstract:

By direct numerical simulation and theoretical arguments, we argue that the turbulent flow of energy towards smaller scales cannot proceed through the ion-kinetic transition in imbalanced magnetized turbulence at low beta. This “plug” in the cascade causes the energy to build up in time because it has no channel through which to dissipate. It creates a sharp, well-defined ion-kinetic transition in the spectrum at scales somewhat larger than ρ_i, above which the turbulence follows Kolmogorov’s ∼k^−5/3 law, and below which it drops off precipitately because of the energy-transfer “plug”. At yet smaller scales, a flatter spectrum is again observed due to small leakage through the plug. The resulting turbulence bears a strong similarity to recent measurements from the Parker Solar Probe.