Box simulations of rotating magnetoconvection

Effects of penetration and turbulent pumping

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Corresponding author: U. Ziegler, uziegler@aip.de

Received: 3 December 2002
Accepted: 31 January 2003

Abstract

Various effects of penetration in rotating magnetoconvection are studied by means of three-dimensional numerical simulations employing the code NIRVANA. A local, 2-layer model is applied dividing the computational domain (which is a rectangular box placed tangentially on a sphere at latitude ) in an unstable polytropic region on top of a stable polytropic region. Different realizations of convection are examined parameterized by Taylor numbers and magnetic field strengths . We find a rather distinctive behavior of the penetration depth Δ on the system parameters . In non-rotating convection Δ is a monotonically decreasing function of which is due to magnetic quenching effects. Also, penetration is subject to rotational quenching, i.e. Δ is reduced for increasing rotation rate. In the intermediate regime of , the effects of rotation and magnetic field on Δ do not simply add (see Fig. [see full text]). We find, nevertheless, a very strong reduction of the penetration depth of overshooting turbulence by both rotation and magnetism. Penetrative convection is closely associated with the mixing of a passive scalar quantity advected with the flow. In the long term, the tracer material penetrates significantly deeper into the stable layer than suggested by Δ which is due to the cumulative effect of isolated, fast-moving plumes. In case of a weak magnetic field, penetrative convection also serves to ensure a downward transport of magnetic flux by turbulent pumping with an average rate measured in units of the sound speed at the top z-boundary. For larger magnetic fields the pumping effect is quenched and even changes sign in the convection zone. This effect is suggested as being due to the effect of “turbulent buoyancy” which in density-stratified media transports a given magnetic field upwards if it is not too strong (Kichatinov & Rüdiger [CITE]).