Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers

Abstract

Magnetic fields on small scales are ubiquitous in the universe. Though they can often be observed in detail, their generation mechanisms are not fully understood. One possibility is the so-called small-scale dynamo (SSD). Prevailing numerical evidence, however, appears to indicate that an SSD is unlikely to exist at very low magnetic Prandtl numbers ($Pr_M$) such as are present in the Sun and other cool stars. We have performed high-resolution simulations of isothermal forced turbulence employing the lowest $Pr_M$ values so far achieved. Contrary to earlier findings, the SSD turns out to be not only possible for $Pr_M$ down to 0.0031, but even becomes increasingly easier to excite for $Pr_M$ below $\simeq\,$0.05. We relate this behaviour to the known hydrodynamic phenomenon referred to as the bottleneck effect. Extrapolating our results to solar values of $Pr_M$ indicates that an SSD would be possible under such conditions.