arXiv subjects

Maarit J. Kapyla

Publications and source records attributed to Maarit J. Kapyla.

3 recordsLinked to original sources

arXiv2021

Small-Scale Dynamo in Supernova-Driven Interstellar Turbulence

Magnetic fields grow quickly even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium of galaxies. Many studies have focused on idealized turbulent driving of the SSD. Here we simulate more realistic supernova-driven turbulence to determine whether it can drive an SSD. Magnetic field growth occurring in our models appears inconsistent with simple tangling of magnetic fields, but consistent with SSD action, reproducing and confirming models by Balsara et al. (2004) that did not include physical resistivity $η$. We vary $η$, as well as the numerical resolution and supernova rate, $\dotσ$, to delineate the regime in which an SSD occurs. For a given $\dotσ$ we find convergence for SSD growth rate with resolution of a parsec. For $\dotσ\simeq\dotσ_{\rm sn}$, with $\dotσ_{\rm sn}$ the solar neighbourhood rate, the critical resistivity below which an SSD occurs is $0.005>η_{\rm crit}>0.001\,\rm kpc^{-1}\,\rm km s^{-1}$, and this increases with the supernova rate. Across the modelled range of 0.5--4 pc resolution we find that for $η<η_{\rm crit}$, the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range $0.2\dotσ_{\rm sn}\leq \dotσ\leq8\dotσ_{\rm sn}$ growth rate increases with $\dotσ$. SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.

astro-ph.GA↗

arXiv2015

Magnetically controlled stellar differential rotation near the transition from solar to anti-solar profiles

Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and anti-solar (AS) otherwise. Hydrodynamic simulations show a transition from SL to AS rotation as the influence of rotation on convection is reduced, but the opposite transition occurs at a different point in the parameter space. The system is bistable, i.e., SL and AS rotation profiles can both be stable. We study the effect of a dynamo-generated magnetic field on the large-scale flows, particularly on the possibility of bistable behavior of differential rotation. We solve the hydromagnetic equations numerically in a rotating spherical shell for a set of different radiative conductivities controlling the relative importance of convection. In agreement with earlier findings, our models display SL rotation profiles when the rotational influence on convection is strong and a transition to AS when the rotational influence decreases. We find that dynamo-generated magnetic fields help to produce SL differential rotation compared to the hydrodynamic simulations. We do not observe any bistable states of differential rotation. In the AS cases we get coherent single-cell meridional circulation, whereas in SL cases we get multi-cellular patterns. In both cases, we obtain poleward circulation near the surface with a magnitude close to that observed in the Sun. Moreover, both differential rotation and meridional circulation have significant magnetic cycle-related variations that are similar in strength to those of the Sun. Purely hydrodynamic simulations of differential rotation and meridional circulation are shown to be of limited relevance as magnetic fields, self-consistently generated by dynamo action, significantly affect the flows.

astro-ph.SR↗

arXiv2014

Quenching and anisotropy of hydromagnetic turbulent transport

Hydromagnetic turbulence affects the evolution of large-scale magnetic fields through mean-field effects like turbulent diffusion and the $α$ effect. For stronger fields, these effects are usually suppressed or quenched, and additional anisotropies are introduced. Using different variants of the test-field method, we determine the quenching of the turbulent transport coefficients for the forced Roberts flow, isotropically forced non-helical turbulence, and rotating thermal convection. We see significant quenching only when the mean magnetic field is larger than the equipartition value of the turbulence. Expressing the magnetic field in terms of the equipartition value of the {\it quenched} flows, we obtain for the quenching exponents of the turbulent magnetic diffusivity about 1.3, 1.1, and 1.3 for Roberts flow, forced turbulence, and convection, respectively. However, when the magnetic field is expressed in terms of the equipartition value of the unquenched flows these quenching exponents become about 4, 1.5, and 2.3, respectively. For the $α$ effect, the exponent is about 1.3 for the Roberts flow and 2 for convection in the first case, but 4 and 3, respectively, in the second. In convection, the quenching of turbulent pumping follows the same power law as turbulent diffusion, while for the coefficient describing the $\bf Ω\times \bf J$ effect nearly the same quenching exponent is obtained as for $α$. For forced turbulence, turbulent diffusion proportional to the second derivative along the mean magnetic field is quenched much less, especially for larger values of the magnetic Reynolds number. However, we find that in corresponding axisymmetric mean-field dynamos with dominant toroidal field the quenched diffusion coefficients are the same for the poloidal and toroidal field constituents.

astro-ph.SR↗