Saturation mechanism of the fluctuation dynamo in supersonic turbulent plasmas

Abstract

Magnetic fields in several astrophysical objects are amplified and maintained by a dynamo mechanism, which is the conversion of the turbulent kinetic energy to magnetic energy. A dynamo that amplifies magnetic fields at scales less than the driving scale of turbulence is known as the fluctuation dynamo. We study the properties of the fluctuation dynamo in supersonic turbulent plasmas, which is of relevance to the interstellar medium of star-forming galaxies, structure formation in the universe, and laboratory experiments of laser-plasma turbulence. Using numerical simulations of driven turbulence, we explore the global and local properties of the exponentially growing and saturated (statistically steady) state of the fluctuation dynamo for subsonic and supersonic turbulent flows. First, we confirm that the fluctuation dynamo efficiency decreases with compressibility. Then we show that the fluctuation dynamo-generated magnetic fields are spatially intermittent and the intermittency is higher for supersonic turbulence, but in both cases the level of intermittency decreases as the field saturates. We also find a stronger back-reaction of the magnetic field on the velocity for the subsonic case as compared to the supersonic case. Locally we find that the level of alignment between vorticity and velocity, velocity and magnetic field, and current density and magnetic field in the saturated stage is enhanced in comparison to the exponentially growing phase for the subsonic case, but only the current density and magnetic field alignment is enhanced for the supersonic case. Finally, we show that both the magnetic field amplification (mainly due to weaker stretching magnetic field lines) and diffusion decreases when the field saturates, but the diffusion is enhanced relative to amplification. This occurs throughout the volume in the subsonic turbulence, but primarily in the strong-field regions for the supersonic case. This leads to the saturation of the fluctuation dynamo. Overall both the amplification and diffusion of magnetic fields are affected by the exponentially growing magnetic fields and thus a drastic change in either of them is not required for the saturation of the fluctuation dynamo.