Nordita Astrophysics Seminars

The significance of small-scale dynamos in the Sun and stars

by Jörn Warnecke (Max-Planck Institute for Solar System Research)

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

Albano 3: 6228 - Mega (22 seats)

Albano Building 3

Magnetic fields at small scales are prevalent throughout the Universe. Despite their often detailed observation, the mechanisms behind their generation remain incompletely understood. One potential mechanism is the small-scale dynamo (SSD). However, prevailing numerical evidence suggests that an SSD may not exist at extremely low magnetic Prandtl numbers (PrM), such as those found in the Sun and other cool stars. In a study, we conducted high-resolution simulations using the lowest PrM values achieved to date. Contrary to previous findings, we discovered that the SSD is not only feasible for PrM values as low as 0.0031 but also becomes increasingly easier to activate for PrM values below approximately 0.05. We attribute this behavior to the well-known hydrodynamic phenomenon known as the bottleneck effect. Extrapolating our results to solar PrM values suggests that an SSD could indeed exist under such conditions.
In addition to the SSD, a large-scale dynamo (LSD) mechanism operates on the Sun and other stars, contributing, for instance, to the generation of the sunspot cycle observed in the famous Butterfly diagrams. Furthermore, we investigated the interaction between the SSD and LSD through high-resolution simulations of stellar convection zones. Our findings demonstrate that the small-scale magnetic fields generated by the SSD and LSD can significantly impact, for example, the flow generation in these simulations. Overall, our research highlights the crucial role of the SSD in modeling and understanding the dynamics of solar-like stars.