D. Elstner

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Paul Woods

University College London

Silvia De Bianchi

Università degli Studi di Milano - State University of Milan (Italy)

Mark Hammonds

The University of Tokyo

Isabel Aleman

Universidade de São Paulo

Andi Dirpan

Hasanuddin University

Sergey A Khaibrakhmanov

Saint-Petersburg State University

Hsien Shang

Academia Sinica

Jhon Fredy Mojica Moncada

University of Miami

Edward Dever

Oregon State University

Jana Javornik

University of East London

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Turbulence Effects in Accretion Disk Theory

Dynamo-driven accretion in galaxies

Astronomy and Astrophysics, 1993

Non-linear flat dynamo models are presented using the 2D timestepping code introduced by Elstner ... more Non-linear flat dynamo models are presented using the 2D timestepping code introduced by Elstner et al. (1990). Three effects of feedback of the magnetic field on the inducing motions are considered: α-quenching, »turbulent buoyancy«, and accretion flow. Density is assumed to be the only stratified quantity in the turbulence field. As input parameters we fix the (normalized) rms velocity as well as the (normalized) correlation length of the turbulence. Parameters associated with the definitions of the eddy diffusivity, η T , and the equipartition field, B eq , remain as free and unknown quantities of order unity. In particular, the dimensionless c η in the defining relation η T = c η u'l corr (u' the rms velocity, l corr the correlation length) governs the dynamo regime which for small c η operates far in the non-linearity

Dynamo saturation in direct simulations of the multi-phase turbulent interstellar medium

Astronomische Nachrichten, 2015

The ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies ca... more The ordered magnetic field observed via polarised synchrotron emission in nearby disc galaxies can be explained by a mean-field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical-flux initial conditions are potentially able to influence this dynamo via the occurrence of the magnetorotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean-field dynamo. This is motivated by the observation that different saturation behaviour was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three-dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing-periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean-field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched αΩ dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub-grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields.

D. Elstner

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