Impacts of small-scale dynamo on rotating columnar convection in stellar convection zones

Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

^⋆ Corresponding author: bekki@mps.mpg.de

Received: 20 August 2025
Accepted: 23 September 2025

Abstract

Context. Understanding the complex interactions between convection, magnetic fields, and rotation is key to modeling the internal dynamics of the Sun and stars. Under rotational influence, compressible convection forms prograde-propagating convective columns near the equator. The interaction between such rotating columnar convection and the small-scale dynamo (SSD) remains largely unexplored.

Aims. We investigate the influence of the SSD on the properties of rotating convection in the equatorial regions of solar and stellar convection zones.

Methods. A series of rotating compressible magnetoconvection simulations were performed using a local f-plane box model at the equator. The flux-based Coriolis number, Co_*, was varied systematically. To isolate the effects of the SSD, we compared results from hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations.

Results. The SSD affects both convective heat and angular momentum transport. In MHD cases, convective velocity decreases more rapidly with increasing Co_* than in HD cases. This reduction is compensated for by enhanced entropy fluctuations, maintaining the overall heat transport efficiency. Furthermore, a weakly subadiabatic layer is maintained near the base of the convection zone even under strong rotational influence when the SSD is present. These behaviors reflect a change in the dominant force balance: the SSD introduces a magnetostrophic balance at small scales, while geostrophic balance persists at larger scales. The inclusion of the SSD also reduces the dominant horizontal scale of columnar convective modes by enhancing the effective rotational influence. Regarding angular momentum transport, the SSD generates Maxwell stresses that counteract the Reynolds stresses, thereby quenching the generation of mean shear flows.

Conclusions. Small-scale magnetic fields interact nonlinearly with columnar convection and induce substantial modifications in the dynamics of rotating convection. These effects should be accounted for in models of solar and stellar convection.