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The Magnetic Prandtl number (Prm) is a dimensionless quantity occurring in magnetohydrodynamics which approximates the ratio of momentum diffusivity (viscosity) and magnetic diffusivity. It is defined as: : where: * Rem is the magnetic Reynolds number * Re is the Reynolds number * ''ν'' is the momentum diffusivity (kinematic viscosity) * ''η'' is the magnetic diffusivity At the base of the Sun's convection zone the Magnetic Prandtl number is approximately 10−2, and in the interiors of planets and in liquid-metal laboratory dynamos is approximately 10−5. As another example, a magnetic Prandtl number of order unity is a widely accepted value for the thin accretion discs. Nevertheless this complicates the problem of launching jets: strong concentrations of magnetic field are required at the inner regions of the disc in order to launch jets, but a magnetic Prandtl number of order unity causes the field to diffuse away faster than it accretes inward. ==See also== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Magnetic Prandtl number」の詳細全文を読む スポンサード リンク
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