Astrophysical magnetic fields and nonlinear dynamo theory

By Axel Brandenburg, K. Subramanian. 2004

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The current understanding of astrophysical magnetic fields is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a self-excited dynamo, which involves flows that can amplify a weak 'seed' magnetic field exponentially fast. Particular emphasis is placed on the nonlinear saturation of the dynamo. Analytic and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale dynamos is the so-called alpha effect which explains the generation of a mean field if the turbulence lacks mirror symmetry, i.e. if the flow has kinetic helicity. Large scale dynamos produce small scale helical fields as a waste product that quench the large scale dynamo and hence the alpha effect. With this in mind, the microscopic theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic fields are reviewed.

Subfields: Astronomy and Astrophysics, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/3acb06ab56401e437c2e07e585b68a458ef853d8

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Citations in the corpus

Adding helicity to inflationary magnetogenesis (2014, neutral): “1) for (i) providing the “seed” fields assumed to be the origin, via a dynamo mechanism, of the observed galactic fields (see [26] and references therein), and (ii) fulfilling the bound on cosmological fields originating by the non-observation of cascade photons in the GeV band from TeV blazars [1–3”
THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, positive): “BothMA andM show some minor systematic decrease, which is caused by dynamo action, amplifying the magnetic field by converting turbulent energy into magnetic energy (Brandenburg & Subramanian 2005).”
ZONAL FLOWS AND LONG-LIVED AXISYMMETRIC PRESSURE BUMPS IN MAGNETOROTATIONAL TURBULENCE (2008, background): “Since rotating shear flows are prone to an inverse cascade of magnetic energy (Brandenburg & Subramanian 2005), large simulation domains are needed to fully capture the saturated state of the MRI.”
A solar surface dynamo (2007, neutral): “Given these conditions, our results do not provide a proper basis for commenting on the much debated question of the dependence of turbulent dynamo action on the value of Prm (e.g., Boldyrev & Cattaneo 2004; Ponty et al. 2005; Schekochihin et al. 2005; Brandenburg & Subramanian 2005).”
Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo (2006, positive): “(2003) and Brandenburg and Subramanian (2005a, b), and critically reviewed by Rädler and Rheinhardt (2007). The second approach to the mean-field coefficients makes use of numerical modeling, in which the mean electromotive force, E, as well as the mean-field, B, are determined numerically as output”
TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7−3946 (2011, background): “The induced eddies amplify the magnetic field through the stretching of magnetic field lines, or, in other words, through the action of the turbulent (or small-scale) dynamo effect (Brandenburg & Subramanian 2005, Cho & Vishniac 2000, Cho et al. 2009).”
The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition (2009, background): “Since we have only found non-relativistic derivations in the literature [31, 55, 73], let us estimate here in some detail the magnetic diffusivity and the magnetic Prandl number for relativistic electrons in the cosmic plasma with temperatures 1 MeV < T < 100 GeV,”
BIPOLAR MAGNETIC REGIONS ON THE SUN: GLOBAL ANALYSIS OF THE SOHO/MDI DATA SET (2011, positive): “Introduction The origin of solar activity with its 11 yr cycle is generally understood in terms of an oscillatory dynamo inside the Sun (cf. Brandenburg & Subramanian 2005; Brandenburg 2005).”
Evolving turbulence and magnetic fields in galaxy clusters (2005, methodology): “The seed field itself can be produced by a wide range of mechanisms (Appendix A; see also Ruzmaikin et al. 1989; Widrow 2002; Brandenburg & Subramanian 2005).”
Statistical theory of magnetohydrodynamic turbulence: recent results (2004, methodology): “For details refer to the original papers and review by Brandenburg and Subramanian [25].”
Magnetogenesis from axion inflation (2016, neutral): “However, the dynamo can only amplify existing magnetic fields [13], not create them ex nihilo.”