Abstract
By following the Kazantsev theory and taking into account both microscopic and turbulent diffusion of magnetic fields, we develop a unified treatment of the kinematic and nonlinear stages of turbulent dynamo and study the dynamo process for a full range of magnetic Prandtl number \(P_m\) and ionization fractions. We find a striking similarity between the dependence of dynamo behavior on \(P_m\) in a conducting fluid and \(\mathcal {R}\) (a function of ionization fraction) in partially ionized gas. In a weakly ionized medium, the kinematic stage is largely extended, including not only exponential growth but a new regime of dynamo characterized by linear-in-time growth of magnetic field strength, and the resulting magnetic energy is much higher than the kinetic energy carried by viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear stage is unaffected by microscopic diffusion processes and has a universal linear-in-time growth of magnetic energy with the growth rate as a constant fraction 3 / 38 of the turbulent energy transfer rate, showing a good agreement with earlier numerical results. Applying the analysis to the first stars and galaxies, we find that the kinematic stage is able to generate a field strength only an order of magnitude smaller than the final saturation value. But the generation of large-scale magnetic fields can only be accounted for by the relatively inefficient nonlinear stage and requires longer time than the free-fall time. It suggests that magnetic fields may not have played a dynamically important role during the formation of the first stars. This chapter is based on Xu and Lazarian (ApJ 833:215, 2016, [1]), Xu and Lazarian (New J Phys 19:065005, 2017, [2]).
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- 1.
Richardson diffusion [39] was initially introduced for hydrodynamic turbulence and is fully consistent with the Kolmogorov theory of turbulence. The explosive separation of magnetic field lines in MHD turbulence conforms to Richardson diffusion, which implies the breakdown of the flux-conservation constraint in MHD turbulence and can be used to recover the Lazarian and Vishniac [38] theory on turbulent reconnection [40, 41].
- 2.
This feature can also be understood from a different perspective called frequency mismatching [21, 59]. For the magnetic fluctuations at scales smaller than the equipartition scale, their Alfvén frequencies \(kV_A\) exceed and mismatch with the turnover rate of the equipartition-scale eddies. As a result, growth of magnetic energy at these scales is no longer possible.
The numerical testing of the Goldreich and Sridhar [61] model of MHD turbulence was influenced by the simulations that suffer from the bottleneck effect [62]. The recent high-resolution MHD simulations in Beresnyak [63] confirmed the Goldreich and Sridhar [61] scaling.
- 3.
The evolution of \(k_p\) was discussed in e.g. Beresnyak et al. [34] in terms of the change of equipartition scale in turbulent shock precursor dynamo. Our study above provides the analytical derivation from the first principles.
- 4.
- 5.
In spite of the same formulae for \(\mathcal {E}_\text {cr}\) and \(t_\text {cr}\), the viscosity involved in cases of fully and partially ionized gases are different.
- 6.
A similar conclusion is true for the magnetic field amplification within present-day super-Alfvénic molecular clouds. In such clouds the kinetic energy exceeds the magnetic energy over a broad range of scales. To amplify the magnetic energy up to equipartition on the scale of cloud size, it requires around 6 turbulent crossing times of the cloud (Eq. 2.98), which is longer than the cloud lifetime of \(1{-}2\) crossing times [92].
- 7.
It is worthwhile noticing that even for the folded magnetic fields, Schekochihin et al. [98] claimed that their interaction with the Alfvénic turbulence may lead to unwinding of the folds and further energy transport to larger scales, until eventual saturation with the Alfvénic spectrum of magnetic energy peaking at the outer scale of turbulence.
- 8.
For the stars formed in the magnetized interstellar medium of the first galaxies, we do not rule out the possible magnetic regulation on their formation process.
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Xu, S. (2019). Small-Scale Turbulent Dynamo. In: Study on Magnetohydrodynamic Turbulence and Its Astrophysical Applications. Springer Theses. Springer, Singapore. https://doi.org/10.1007/978-981-13-7515-6_2
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