Nonlinear small-scale dynamos at low magnetic Prandtl numbers
by
Axel Brandenburg(Nordita)
→
Europe/Stockholm
122:026
122:026
Description
Saturated small-scale dynamo solutions driven by isotropic non-helical
turbulence are presented at low magnetic Prandtl numbers down to 0.02,
and, with less certainty, down to 0.01.
In that range, most of the energy is dissipated via Joule heat and,
in agreement with earlier results for helical large-scale dynamos,
kinetic energy dissipation is shown to diminish proportional to
the square root of the magnetic Prandtl number.
Nevertheless, the rms magnetic field strength of the small-scale dynamo
is found to depend only weakly on the value of the magnetic Prandtl number and
decreases by about a factor of 2 as its value is decreased from 1 to 0.01.
Restarting a simulation with a down-scaled magnetic field leads to decay,
giving evidence for a subcritical nature of small-scale dynamos at
low magnetic Prandtl numbers.
This is argued to be a consequence of a suppression of the bottleneck
seen in the kinetic energy spectrum in the absence of a dynamo and,
more generally, a suppression of kinetic energy near the dissipation
wavenumber.
