Turbulence in Fluids and Turbulence in Collisionless Plasmas

Turbulence in ordinary fluids (such as air or water) is relatively well understood: over many decades a large body of knowledge has been built up with the use of observations, wind-tunnel experiments, and sophisticated computer simulations. In contrast, turbulence in collisionless plasmas (such as the solar wind or the interstellar medium) has basic questions about its dynamics, driving, and dissipation that are unanswered. We will compare turbulence in fluids with turbulence in collisionless plasmas based on the physics of fluids and the physics of plasmas. In ordinary fluids, particle collisions act to exchange momentum and give the fluid its fluid behavior. In collisionless plasmas particle collisions are gone, but the presence of a magnetic field in the plasma gives the plasma some fluid behavior. But the magnetic field complicates the dynamics of turbulence in a plasma by adding a wave nature to any turbulent fluctuation. In fluids, particle collisions also produce a viscosity which acts to dissipate turbulence. In collisionless plasma there is no viscous dissipation: instead wave-particle interactions can act to dissipate turbulence. Finally, the magnetic field in the plasma produces a preferred direction, and the physics of a turbulent fluctuation depends on its orientation with respect to the magnetic-field direction.