C. Gissinger, A. Iskakov, S. Fauve, E. Dormy

Euro.Phys.Lett. 82, 29001(2008)

We study the effect of different boundary conditions on the kinematic dynamo threshold of von Kármán type swirling flows in a cylindrical geometry. Using an analytical test flow, we model different boundary conditions: insulating walls all over the flow, effect of sodium at rest on the cylinder side boundary, effect of sodium behind the impellers, effect of impellers or side wall made of a high-magnetic-permeability material. We find that using high-magnetic-permeability boundary conditions decreases the dynamo threshold, the minimum being achieved when they are implemented all over the flow

Christophe Gissinger, Emmanuel Dormy, Stephan Fauve.

Phys. Rev. Lett. 101, 144502 (2008)

We present a numerical study of the magnetic field generated by an axisymmetrically forced flow in a spherical domain. We show that, even in the absence of nonaxisymmetric velocity fluctuations, a mean magnetic field with a dominant axisymmetric dipolar component can be generated via a secondary bifurcation from an equatorial dipole. We understand the dynamical behaviors that result from the interaction of equatorial and axial dipolar modes using simple model equations for their amplitudes derived from symmetry arguments.

Christophe Gissinger, Sébastien Fromang, Emmanuel Dormy.

M.N.R.A.S, 394 (2008)

We present kinematic simulations of a galactic dynamo model based on the large scale differential rotation and the small scale helical fluctuations due to supernova explosions. We report for the first time direct numerical simulations of the full galactic dynamo using an unparameterized global approach. We argue that the scale of helicity injection is large enough to be directly resolved rather than parameterized. While the actual superbubble characteristics can only be approached, we show that numerical simulations yield magnetic structures which are close both to the observations and to the previous parameterized mean field models. In particular, the quadrupolar symmetry and the spiraling properties of the field are reproduced. Moreover, our simulations show that the presence of a vertical inflow plays an essential role to increase the magnetic growth rate. This observation could indicate an important role of the downward flow (possibly linked with galactic fountains) in sustaining galactic magnetic fields.

Henrik Latter, Gordon Ogilvie

Icarus (2008), 195, 725

This paper examines the onset of the viscous overstability in dense particulate rings. First, we formulate a dense gas kinetic theory that is applicable to the saturnian system. Second, we put this model to work computing the equilibrium properties of dense planetary rings, which we subsequently compare with the results of N-body simulations. Finally, we present the linear stability analyses of these equilibrium states, and derive criteria for the onset of viscous overstability in the self-gravitating and non-self-gravitating cases. The latter results compare favourably with the simulations of Salo et al. (2001)