Sharanya Sur, Christoph Federrath, Dominik R. G. Schleicher, Robi Banerjee, Ralf S. Klessen
We study the influence of initial conditions on the magnetic field
amplification during the collapse of a magnetised gas cloud. We focus on the
dependence of the growth and saturation level of the dynamo generated field on
the turbulent properties of the collapsing cloud. In particular, we explore the
effect of varying the initial strength and injection scale of turbulence and
the initial uniform rotation of the collapsing magnetised cloud. In order to
follow the evolution of the magnetic field in both the kinematic and the
nonlinear regime, we choose an initial field strength of $\simeq 1\,\mkG$ with
the magnetic to kinetic energy ratio, $E_{\rm m}/E_{\rm k} \sim 10^{-4}$. Both
gravitational compression and the small-scale dynamo initially amplify the
magnetic field. Further into the evolution, the dynamo-generated magnetic field
saturates but the total magnetic field continues to grow because of
compression. The saturation of the small-scale dynamo is marked by a change in
the slope of $B/\rho^{2/3}$ and by a shift in the peak of the magnetic energy
spectrum from small scales to larger scales. For the range of initial Mach
numbers explored in this study, the dynamo growth rate increases as the Mach
number increases from $v_{\rm rms}/c_{\rm s}\sim 0.2$ to 0.4 and then starts
decreasing from $v_{\rm rms}/c_{\rm s}\sim 1.0$. We obtain saturation values of
$E_{\rm m}/E_{\rm k} = 0.2 - 0.3$ for these runs. Simulations with different
initial injection scales of turbulence also show saturation at similar levels.
For runs with different initial rotation of the cloud, the magnetic energy
saturates at $E_{\rm m}/E_{\rm k}\sim 0.2 - 0.4$ of the equipartition value.
(Abridged)
View original:
http://arxiv.org/abs/1202.3206
No comments:
Post a Comment