Convective velocity suppression via the enhancement of subadiabatic layer: Role of the effective Prandtl number

TL;DR

This study demonstrates that increasing the effective Prandtl number in solar convection simulations suppresses convective velocities by enhancing the subadiabatic layer, aligning models more closely with observed solar dynamics.

Contribution

It introduces a numerical confirmation of convective velocity suppression through increased Prandtl number, highlighting the role of magnetic fields and subadiabatic layer formation.

Findings

Higher Prandtl number reduces convective amplitude.

Subadiabatic layer is extended and intensified.

Convective velocities are suppressed in the lower convection zone.

Abstract

It has recently been recognized that the convective velocities achieved in the current solar convection simulations might be over-estimated. The newly-revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this paper, we propose and numerically confirm a possible suppression mechanism of convective velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global convective amplitude in the high-Prandtl thermal convection is thus reduced especially in the lower part of the convection zone via the change in the mean entropy profile which becomes more subadiabatic near the base and less superadiabatic in the bulk.