The turbulent pressure of magnetoconvection for slow and rapid rotation

Abstract

Motivated by recent simulations of sunspot formation, we extend the theory of the pressure difference between magnetized and non-magnetized gas by Dicke to include rotating turbulence. While the (vertical) background field provides a positive-definite magnetic pressure difference between the magnetized and the unmagnetized gas, Reynolds stress and Maxwell stress of turbulence strongly modify this result. With the quasilinear approximation we demonstrate that the influence of the turbulence differs between the high-conductivity and the low-conductivity limits. Sufficiently small magnetic Reynolds numbers lead to magnetic pressure suppression where indeed the pressure excess can even assume negative values. Box simulations of magnetoconvection subject to a vertical magnetic field carried out with the Nirvana code confirm this overall picture. They also demonstrate how a global rotation {\em reduces} the negative magnetic pressure effect. For rapid rotation the total magnetic pressure difference caused by large-scale magnetic fields { and} turbulence even fully disappears for small field strengths. Magnetic fields of moderate strength thus neither reduce nor enhance the turbulence pressure of rapidly rotating convection. Consequences of this phenomenon for the star formation efficiency are shortly discussed.