The many incarnations of accretion disk dynamos: mixed parities and chaos for large dynamo numbers.
Abstract
We study nonlinear accretion disk dynamos, and demonstrate that different types of magnetic field configurations will arise depending on the dynamo number. Dynamo action is parametrised by the α-effect, whose strength is determined by a dimensionless number Calpha_. Using α-quenching as the only nonlinearity, we find for Calpha_>0 that a steady quadrupolar field is first excited. However this solution disappears before any other field configuration is excited. Thus there is a gap without any magnetic field. This is a feature of the particular nonlinearity used, and has not been observed previously in other systems. At higher dynamo numbers oscillatory dipolar and quadrupolar fields emerge, and later the quadrupolar solution bifurcates to a chaotically varying mixed parity solution. Finally, at extremely high dynamo numbers the chaotic field transforms into a steady mixed parity solution, which is dominated by an even parity component. For Calpha_<0 we first find an oscillatory dipolar field, which bifurcates to a doubly periodic mixed parity field, and later also to a chaotically varying field. Using magnetic buoyancy as the nonlinearity the gap no longer occurs. However we are not able to go to as high dynamo numbers as before. Nevertheless for the entire interval investigated, we only find a steady quadrupolar field for Calpha_>0, and an oscillatory quadrupolar or dipolar field for Calpha_<0.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- December 1994
- Bibcode:
- 1994A&A...292..341T
- Keywords:
-
- Accretion Disks;
- Cataclysmic Variables;
- Chaos;
- Computational Astrophysics;
- Dynamo Theory;
- Magnetic Fields;
- Mathematical Models;
- Quenching (Atomic Physics);
- Stellar Models;
- Buoyancy;
- Diagrams;
- Magnetohydrodynamics;
- Nonlinearity;
- Plasmas (Physics);
- Spatial Dependencies;
- Astrophysics;
- ACCRETION: ACCRETION DISKS;
- MAGNETIC FIELDS;
- MHD;
- CHAOTIC PHENOMENA;
- NOVAE;
- CATACLYSMIC VARIABLES