Fluidization of collisionless plasma turbulence

TL;DR

This paper demonstrates that collisionless plasma turbulence exhibits fluid-like broad power-law spectra due to suppression of Landau damping by nonlinear effects, challenging traditional views of dissipation in such plasmas.

Contribution

It introduces the concept of 'fluidization' in collisionless plasmas, showing turbulence behaves like fluid turbulence because of nonlinear suppression of phase mixing.

Findings

Collisionless plasma turbulence mimics fluid turbulence spectra.

Stochastic echoes suppress phase mixing and Landau damping.

Collisionless plasmas are effectively dissipationless at large scales.

Abstract

In a collisionless, magnetized plasma, particles may stream freely along magnetic-field lines, leading to phase "mixing" of their distribution function and consequently to smoothing out of any "compressive" fluctuations (of density, pressure, etc.,). This rapid mixing underlies Landau damping of these fluctuations in a quiescent plasma-one of the most fundamental physical phenomena that make plasma different from a conventional fluid. Nevertheless, broad power-law spectra of compressive fluctuations are observed in turbulent astrophysical plasmas (most vividly, in the solar wind) under conditions conducive to strong Landau damping. Elsewhere in nature, such spectra are normally associated with fluid turbulence, where energy cannot be dissipated in the inertial scale range and is therefore cascaded from large scales to small. By direct numerical simulations and theoretical arguments, it is shown here that turbulence of compressive fluctuations in collisionless plasmas strongly resembles one in a collisional fluid and does have broad power-law spectra. This "fluidization" of collisionless plasmas occurs because phase mixing is strongly suppressed on average by "stochastic echoes", arising due to nonlinear advection of the particle distribution by turbulent motions. Besides resolving the long-standing puzzle of observed compressive fluctuations in the solar wind, our results suggest a conceptual shift for understanding kinetic plasma turbulence generally: rather than being a system where Landau damping plays the role of dissipation, a collisionless plasma is effectively dissipationless except at very small scales. The universality of "fluid" turbulence physics is thus reaffirmed even for a kinetic, collisionless system.