TY  - JOUR
A1  - Rüdiger, Günther
A1  - Schultz, Manfred
T1  - On the toroidal-velocity antidynamo theorem under the presence of nonuniform electric conductivity
JF  - Astronomische Nachrichten = Astronomical notes
N2  - Laminar electrically conducting Couette flows with the hydrodynamically stable quasi-Keplerian rotation profile and nonuniform conductivity are probed for dynamo instability. In spherical geometry, the equations for the poloidal and the toroidal field components completely decouple, resulting in free decay, regardless of the spatial distribution of the electric conductivity. In cylindrical geometry the poloidal and toroidal components do not decouple, but here also we do not find dynamo excitations for the cases that the electric conductivity only depends on the radius or - much more complex- that it only depends on the azimuthal or the axial coordinate. The transformation of the plane-flow dynamo model of Busse and Wicht (1992) to cylindrical or spherical geometry therefore fails. It is also shown that even the inclusion of axial flows of both directions does not support the dynamo mechanism. The Elsasser toroidal-velocity antidynamo theorem, according to which dynamos without any radial velocity component cannot work, is thus not softened by nonuniform conductivity distributions.
KW  - antidynamo theorem
KW  - MHD
KW  - Taylor-Couette flow
Y1  - 2022
U6  - https://doi.org/10.1002/asna.20224011
SN  - 0004-6337
SN  - 1521-3994
VL  - 343
IS  - 5
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Schultz, Manfred
T1  - Large-scale dynamo action of magnetized Taylor-Couette flows
JF  - Monthly notices of the Royal Astronomical Society
N2  - A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler-Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining a effect and eddy diffusivity. If both degenerated modes with m = +/- 1 are excited with the same power then the global a effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite alpha effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an alpha(2) dynamo. Moreover, an alpha Omega dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm similar or equal to 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, eta(T) similar or equal to eta. By basically lower or larger eddy diffusivities the dynamo instability is suppressed.
KW  - dynamo
KW  - instabilities
KW  - MHD
KW  - magnetic fields
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa293
SN  - 0035-8711
SN  - 1365-2966
VL  - 493
IS  - 1
SP  - 1249
EP  - 1260
PB  - Oxford Univ. Press
CY  - Oxford
ER  -