Abstract
AbstractThe Λ effect describes a rotation‐induced nondiffusive contribution to the Reynolds stress. It is commonly held responsible for maintaining the observed differential rotation of the Sun and other late‐type stars. Here, the sensitivity of the Λ effect to small‐scale magnetic fields and compressibility is studied by means of forced turbulence simulations either with anisotropic forcing in fully periodic cubes or in density‐stratified domains with isotropic forcing. Effects of small‐scale magnetic fields are studied in cases where the magnetic fields are self‐consistently generated by a small‐scale dynamo. The results show that small‐scale magnetic fields lead to a quenching of the Λ effect which is milder than in cases where also a large‐scale field is present. The effect of compressibility on the Λ effect is negligible in the range of Mach numbers from 0.015 to 0.8. Density stratification induces a marked anisotropy in the turbulence and a vertical Λ effect if the forcing scale is roughly two times larger than the density scale height.
Highlights
Solar and stellar differential rotation is thought to arise due to the interaction of density-stratified convective turbulence and global rotation of the star (e.g., Rüdiger 1980; Rüdiger 1989; Rüdiger et al 2013)
The coefficients pertaining to the Λ effect were extracted by fitting the latitudinal profiles of the off-diagonal Reynolds stresses with the same procedure as in Käpylä (2019)
The effects of small-scale magnetic fields, compressibility and background density stratification on the Λ effect were studied with numerical simulations of forced turbulence with an isothermal equation of state
Summary
Solar and stellar differential rotation is thought to arise due to the interaction of density-stratified convective turbulence and global rotation of the star (e.g., Rüdiger 1980; Rüdiger 1989; Rüdiger et al 2013). Such an effort is undertaken with a controlled set of simulations where the minimal ingredients (rotation and anisotropic turbulence) for the Λ effect are included while the Mach number is varied Another aspect that has not received much attention is the contribution of density stratification to the anisotropy of turbulence (see, Brandenburg et al 2012) and the resulting Λ effect in rotating cases. Isolating these effects in convection is not possible because the forcing due to the convective instability is in itself highly anisotropic.
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