Archontis dynamo
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Author: Dr. Vasilis D. Archontis, University of St. Andrews
Dr. Vasilis D. Archontis accepted the invitation on 4 July 2007 (self-imposed deadline: 4 September 2007).
This article will briefly cover: the concept of a laminar dynamo that firstly saturates at equipartition but later on undergoes transitions between turbulent and laminar solutions.
Magnetic fields are ubiquitous in astrophysical bodies throughout the cosmos. They vary in scale and strength from strong magnetic fields in e.g.\ solar active regions to relatively weak magnetic fields in planets and galaxies. These fields are controlled by the motions of plasmas through a transfer of kinetic energy to magnetic energy: the resulting amplification and regeneration of the magnetic field is called dynamo action (Cowling \cite{Cowling1934}; Moffatt \cite{Moffatt1978}; Parker \cite{Parker1979}). The study of dynamo action in the limit of infinitely high magnetic Reynolds number is the subject of fast dynamo theory and is relevant to most astrophysical systems, where the diffusion timescales are typically much larger than the advection timescales.
Considering the importance of the magnetic forces relative to the motion of the fluid, one divides dynamo action into two regimes: the linear kinematic regime in which the flow amplifies the magnetic field exponentially by e.g.\ stretching the magnetic field lines, and the non-linear regime where the magnetic field becomes strong enough to modify the initial flow topology through the Lorentz force, and consequently halts the exponential growth.
Studies of kinematic dynamos have improved our understanding of the exponential amplification of weak magnetic fields by prescribed flows. However, knowledge of the exact mechanisms behind the maintenance of magnetic fields against resistive diffusion is still incomplete for many classes of dynamos.
In the case of non-linear fast dynamos advances have been made through numerical magneto-hydrodynamical (MHD) simulations (e.g., Nordlund et al.\ \cite{Nordlund+ea92}; Podvigina \& Pouquet \cite{Podvigina+Pouquet1994}; Brandenburg et al.\ \cite{Brandenburg+ea95}; Zienicke et al.\ \cite{Zienicke+ea98}; Archontis et al.\ \cite{Archontisb+ea03}; Schekochihin et al.\ \cite{Schekochihin+ea04}; Mininni et al.\ \cite{Mininnib+ea05}; Cameron \& Galloway \cite{Cameron+Galloway2006a}). Although the details of the saturation mechanisms are unresolved it has been suggested that the dynamos saturate either because of a suppression of the stretching ability of the flows and a reduction of the field dissipation or via a balance between vigorous stretching and strong dissipation.
In many astrophysical systems the stretching of the magnetic field lines, which drives the initial exponential amplification of the magnetic energy, is achieved by turbulent flows. An interesting issue is then the equilibrium field strength at which this turbulent dynamo action saturates. Moreover, if the generated magnetic field at saturation has magnetic energy equal to the kinetic energy of the flow (equipartition), then this dynamo is able to generate strong magnetic fields that are comparable with those observed in many astrophysical systems.
| Invited by: | Dr. Søren Bertil F. Dorch, The Niels Bohr Institute and the Royal Library, Copenhagen, Denmark |
