アイテム詳細

要旨

Magnetic fields on small scales are ubiquitous in the Universe. Although 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 those that are present in the Sun and other cool stars. Here we have performed high-resolution simulations of isothermal forced turbulence using the lowest Pr_M values achieved so far. Contrary to earlier findings, the SSD not only turns out to be possible for Pr_M down to 0.0031 but also becomes increasingly easier to excite for Pr_M below about 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.