A shell model for a large-scale turbulent dynamo |
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| Authors: | Giuseppina Nigro |
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| Institution: | 1. Dipartimento di Fisica and Centro Nazionale Interuniversitario Struttura della Materia , Università della Calabria , Unità di Cosenza, I-87030 Arcavacata di Rende , Cosenza, Rende 87036 , Italy giusy.nigro@gmail.com |
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| Abstract: | This article addresses the interesting and important problem of large-scale magnetic field generation in turbulent flows, using a self-consistent dynamo model recently developed. The main idea of this model is to consider the induction equation for the large-scale magnetic field, integrated consistently with the turbulent dynamics at smaller scales described by a magnetohydrodynamic shell model. The questions of dynamo action threshold, magnetic field saturation, magnetic field reversals, nature of the dynamo transition and the changes of small-scale turbulence as a consequence of the dynamo onset are discussed. In particular, the stability curve obtained by the model integration is shown in a very wide range of values of the magnetic Prandtl number not yet accessible by direct numerical simulation but more realistic for natural dynamos. Moreover, from our analysis it is shown that the large-scale dynamo transition displays a hysteretic behaviour and therefore a subcritical nature. The model successfully reproduces magnetic polarity reversals, showing the capability to generate persistence times which are increasing for decreasing magnetic diffusivity. Moreover, when the system reaches a statistically stationary dynamo state, where the large-scale magnetic field can abruptly reverse its polarity (magnetic reversal state) or not, keeping the same polarity (steady state), it shows an unmistakable tendency towards the energy equipartition for the turbulence at small scale. |
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| Keywords: | Magnetic dynamo Turbulent alpha dynamo effect Turbulence Magnetohydrodynamic shell model |
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