Small-scale and large-scale dynamos in global convection simulations of solar-like stars

It has been recently shown that a small-scale dynamo (SSD) instability could be possible in solar-like low magnetic Prandtl number Pm plasmas. It has been proposed that the presence of SSD can potentially have a significant impact on the dynamics of the large-scale dynamo (LSD) in the stellar convection zones. Studying these two dynamos, SSD and LSD, together in a global magnetoconvection model requires high-resolution simulations and large amounts of computational resources. Starting from a well-studied global convective dynamo model that produces cyclic magnetic fields, we systematically increased the resolution and lowered the diffusivities to enter the regime of Reynolds numbers that allow for the excitation of SSD on top of the LSD. We studied how the properties of convection, generated differential rotation profiles, and LSD solutions change with the presence of SSD. We performed convective dynamo simulations in a spherical wedge with the Pencil Code. The resolutions of the models were increased in 4 steps by a total factor of 16 to achieve maximal fluid and magnetic Reynolds numbers of over 500. We found that the differential rotation is strongly quenched by the presence of the LSD and SSD. Even though the small-scale magnetic field only mildly decreases increasing Re, the large-scale field strength decreases significantly. We do not find the SSD dynamo significantly quenching the convective flows as claimed recently by other authors; in contrast, the convective flows first grow and then saturate for increasing Re. Furthermore, the angular momentum transport is highly affected by the presence of small-scale magnetic fields, which are mostly generated by LSD. These fields not only change the Reynolds stresses, but also generate dynamically important Maxwell stresses. The LSD evolution in terms of its pattern and field distribution is rather independent of the increase in Rm.
Comment: 17 pages, 18 figures with an Appendix with 3 pages and 3 figures, submitted to A&A