6 research outputs found
Magnetic helicity fluxes in an alpha-squared dynamo embedded in a halo
We present the results of simulations of forced turbulence in a slab where
the mean kinetic helicity has a maximum near the mid-plane, generating
gradients of magnetic helicity of both large and small-scale fields. We also
study systems that have poorly conducting buffer zones away from the midplane
in order to assess the effects of boundaries. The dynamical alpha quenching
phenomenology requires that the magnetic helicity in the small-scale fields
approaches a nearly static, gauge independent state. To stress-test this steady
state condition we choose a system with a uniform sign of kinetic helicity, so
that the total magnetic helicity can reach a steady state value only through
fluxes through the boundary, which are themselves suppressed by the velocity
boundary conditions. Even with such a set up, the small-scale magnetic helicity
is found to reach a steady state. In agreement with earlier work, the magnetic
helicity fluxes of small-scale fields are found to be turbulently diffusive. By
comparing results with and without halos, we show that artificial constraints
on magnetic helicity at the boundary do not have a significant impact on the
evolution of the magnetic helicity, except that "softer" (halo) boundary
conditions give a lower energy of the saturated mean magnetic field.Comment: 12 pages, 5 figures, submitted to GAF
The mean electro-motive force, current- and cross-helicity under the influence of rotation, magnetic field and shear
The mean electromotive force (MEMF) in a rotating stratified
magnetohydrodynamical turbulence is studied.Our study rests on the mean-field
magnetohydrodynamics framework and approximation. We compute the effects
of the large-scale magnetic fields (LSMF), global rotation and large-scale
shear flow on the different parts of the MEMF (such as - effect,
turbulent diffusion, turbulent transport, etc.) in an explicit form. The
influence of the helical magnetic fluctuations which stem from the small-scale
dynamo is taken into account, as well. In the paper, we derive the equation
governing the current helicity evolution. It is shown that the joint effect of
the differential rotation and magnetic fluctuations in the stratified media can
be responsible for the generation, maintenance and redistribution of the
current helicity. The implication of the obtained results to astrophysical
dynamos is considered as well.Comment: 27 pages, 8 figures, submitted to GAF
Chandrasekhar-Kendall functions in astrophysical dynamos
Some of the contributions of Chandrasekhar to the field of
magnetohydrodynamics are highlighted. Particular emphasis is placed on the
Chandrasekhar-Kendall functions that allow a decomposition of a vector field
into right- and left-handed contributions. Magnetic energy spectra of both
contributions are shown for a new set of helically forced simulations at
resolutions higher than what has been available so far. For a forcing function
with positive helicity, these simulations show a forward cascade of the
right-handed contributions to the magnetic field and nonlocal inverse transfer
for the left-handed contributions. The speed of inverse transfer is shown to
decrease with increasing value of the magnetic Reynolds number.Comment: 10 pages, 5 figures, proceedings of the Chandrasekhar Centenary
Conference, to be published in PRAMANA - Journal of Physic
MHD simulations of small and large scale dynamos
Isotropic homogeneous hydromagnetic turbulence is studied using numerical simulations at resolutions of up to 1024^3 meshpoints. It is argued that, in contrast to the kinematic regime, the nonlinear regime is characterized by a spectral magnetic power that is decreasing with increasing wavenumber, regardless of whether or not the turbulence has helicity. This means that the root-mean-square field strength converges to a limiting value at large magnetic Reynolds numbers. The total (magnetic and kinetic) energy spectrum tends to be somewhat shallower than k^{-5/3}, in agreement with the findings of other groups. In the presence of helicity, an inverse cascade develops, provided the scale separation between the size of the computational box and the scale of the energy carrying eddies exceeds a ratio of at least two. Finally, the constraints imposed by magnetic helicity conservation on mean-field theory are reviewed and new results of simulations are presented