Fluidization of collisionless plasma turbulence
Résumé
In a collisionless, magnetized plasma, particles may stream freely alongmagnetic field lines, leading to ?phase mixing? of their distributionfunction and consequently, to smoothing out of any ?compressive?fluctuations (of density, pressure, etc.). This rapid mixing underliesLandau damping of these fluctuations in a quiescent plasma?one of themost fundamental physical phenomena that makes plasma different from aconventional fluid. Nevertheless, broad power law spectra of compressivefluctuations are observed in turbulent astrophysical plasmas (mostvividly, in the solar wind) under conditions conducive to strong Landaudamping. Elsewhere in nature, such spectra are normally associated withfluid turbulence, where energy cannot be dissipated in the inertial-scale range and is, therefore, cascaded from large scales to small. Bydirect numerical simulations and theoretical arguments, it is shown herethat turbulence of compressive fluctuations in collisionless plasmasstrongly resembles one in a collisional fluid and does have broad powerlaw spectra. This ?fluidization? of collisionless plasmas occurs,because phase mixing is strongly suppressed on average by ?stochasticechoes,? arising due to nonlinear advection of the particle distributionby turbulent motions. Other than resolving the long-standing puzzle ofobserved compressive fluctuations in the solar wind, our results suggesta conceptual shift for understanding kinetic plasma turbulencegenerally: rather than being a system where Landau damping plays therole of dissipation, a collisionless plasma is effectivelydissipationless, except at very small scales. The universality of?fluid? turbulence physics is thus reaffirmed even for a kinetic,collisionless system.