Mach Number Dependence of Turbulent Magnetic Field Amplification: Solenoidal versus Compressive Flows
We study the growth rate and saturation level of the turbulent dynamo in magnetohydrodynamical simulations of turbulence, driven with solenoidal (divergence-free) or compressive (curl-free) forcing. For models with Mach numbers ranging from 0.02 to 20, we find significantly different magnetic field geometries, amplification rates, and saturation levels, decreasing strongly at the transition from subsonic to supersonic flows, due to the development of shocks. Both extreme types of turbulent forcing drive the dynamo, but solenoidal forcing is more efficient, because it produces more vorticity.
Federrath, C.; Chabrier, G.; Schober, J.; Banerjee, R.; Klessen, R. S.; Schleicher, D. R. G., 2011, Physical Review Letters, 107, 114504
Movie of the turbulent magnetic field structure [ 26MB mp4 ]
The movie shows the turbulent magnetic field structure in four computer models representing vastly different physical conditions. The Sun for example, is only weakly compressible and characterised by subsonic flows (top left), while the plasma in the early Universe was likely dominated by strong compressions and highly supersonic turbulence (bottom right).