Dynamos in small magnetic Prandtl number fluids

Physics – Plasma Physics

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Scientific paper

In liquid metals and dense stellar plasmas, the magnetic Prandtl number Pm--the ratio between viscosity and magnetic diffusivity--is a very small number (10-5-10-9). Consequently the scales of dissipation of velocity and magnetic field are widely separated, and dynamo processes must operate in the inertial range of the turbulence. Two questions naturally arise. First, can the dynamo operate in such a strongly turbulent environment? Second, if it operates, what is the typical strength of the resulting magnetic field? I will argue that dynamo action is possible even at moderate magnetic Reynolds numbers, provided that either the turbulence spectrum is not too flat, or that favorable coherent structures are present. Concerning the strength of the generated dynamo field, I will present numerical results suggesting that a magnetic Prandtl number independent regime is reached even for values of Pm only slightly smaller than unity. If this result were valid in general it would imply that numerical models with Pm near unity could be adequate to describe many aspects of the dynamics of fluids with Pm very small.

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