Fluidization of collisionless plasma turbulence

Romain Meyrand
Laboratoire de Physique des Plasmas, Ecole Polytechnique, F-91128 Palaiseau, France
Two textbook physical processes compete to thermalize turbulent fluctuations in collisionless plasmas: Kolmogorov’s “cascade” to small spatial scales, where dissipation occurs, and Landau’s damping, which transfers energy to small scales in velocity space via “phase mixing”, also leading to dissipation. By direct numerical simulations and theoretical arguments, I will show during this presentation that in a magnetized plasma, another textbook process, plasma echo, brings energy back from phase space and on average cancels the effect of phase mixing. Energy cascades effectively as it would in a fluid system and thus Kolmogorov wins the competition with Landau for the free energy in a collisionless turbulent plasma. This reaffirms the universality of Kolmogorov’s picture of turbulence and explains, for example, the broad Kolmogorov-like spectra of density fluctuations observed in the solar wind.