ИСТИНА

One of the simplest models for a small-scale magnetic field self-generation, which works without such additional conditions as a differential rotation or nonzero helicity of velocity field, is based on the Kazantsev idea. This model describes a growth of magnetic energy in a turbulent delta-correlated flow, if only it can be characterized by large velocities and small magnetic diffusivity, in other words if the flow is defined by a large enough magnetic Reynolds number. Such supercritical regime, when Reynolds number is larger than some critical value, is well-known and studied by numerical, analytical and asymptotic approaches. However, the large Reynolds numbers, usually observed in astrophysics, are hardly reached in the laboratory experiments or in the modern liquid-metal devices. From a traditional point of view the subcritical regimes with small Reynolds numbers should describe a process with exponentially in time decreasing magnetic energy, thus these subcritical cases do not usually attract the special attention. Nevertheless, this point is totally incorrect. In our presentation we show the results of subcritical studying of dynamo in mirror-symmetric and asymmetric flows (the idea of subcritical regime was suggested by Ja.B.Zeldovich). We demonstrate and speculate the possibility of local in time magnetic energy increasing in subcritical velocity fields, the subcritical energy growth with external support, the subcritical small-scale dynamo realized by a large-scale Steenbeck-Krause-Raedler generation and other subcritical cases. We show a dependency of growth rates for such processes on magnetic Reynolds numbers and discuss the moment of a subcritical regime transformation in a supercritical one. The work is supported by RFBR grant N 18-02-00085.