Growth rate of small-scale dynamo at low magnetic Prandtl numbers Academic Article uri icon

abstract

  • In this study, we discuss two key issues relating to a small-scale dynamo instability at low magnetic Prandtl numbers and large magnetic Reynolds numbers, namely: (i) the scaling for the growth rate of a small-scale dynamo instability in the vicinity of the dynamo threshold; (ii) the existence of the Golitsyn spectrum of magnetic fluctuations in small-scale dynamos. There are two different asymptotics for the small-scale dynamo's growth rate: in the vicinity of the threshold of the excitation of the small-scale dynamo instability, \lambda \propto \ln \left ({Rm} / {Rm}^{ { cr}}\right ) , and when the magnetic Reynolds number is much larger than the threshold of the excitation of the small-scale dynamo instability, λ∝Rm1/2, where Rmcr is the small-scale dynamo instability threshold in the magnetic Reynolds number Rm. We demonstrate that the existence of the Golitsyn spectrum of magnetic fluctuations requires a finite correlation time of the random velocity field. On the other hand, the influence of the Golitsyn spectrum on the small-scale dynamo instability is minor. This is the reason why it is so difficult to observe this spectrum in direct numerical simulations for the small-scale dynamo with low magnetic Prandtl numbers.

publication date

  • July 1, 2012