Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations

Abstract

We consider the effect of a subadiabatic layer at the base of the convection zone on convection itself and the associated large-scale dynamos in spherical wedge geometry. We use a heat conduction prescription based on the Kramers opacity law which allows the depth of the convection zone to dynamically adapt to changes in the physical characteristics such as rotation rate and magnetic fields. We find that the convective heat transport is strongly concentrated toward the equatorial and polar regions in the cases without a substantial radiative layer below the convection zone. The presence of a stable layer below the convection zone significantly reduces the anisotropy of radial enthalpy transport. Furthermore, the dynamo solutions are sensitive to subtle changes in the convection zone structure. We find that the kinetic helicity changes sign in the deeper parts of the convection zone at high latitudes in all runs. This region expands progressively toward the equator in runs with a thicker stably stratified layer.

Overshooting in simulations of compressible convection

“…The superadiabatic temperature gradient from the runs in Set K are shown in Figure 10. The value of ∇ − ∇ ad in the RZ is close to that of the hydrostatic solution with ∇T = const in a polytropic atmosphere with adiabatic index n = 13/4, that is ∇ (hs) rad − ∇ ad = −17/85 ≈ −0.165 (see also Käpylä et al 2019b). The transition from nearly adiabatic to the radiative gradient becomes increasingly sharper as F n decreases (e.g.…”

“…Assuming that an extended stable layer forms at the bottom of the domain, its stratification is close to the hydrostatic solution (see, e.g. Käpylä et al 2019b). This, however, can only be confirmed a posteriori as the depth of the convective layer is not pre-determined in the cases where the Kramers opacity law is used.…”

“…Here these studies are revisited by a setup where the heat conductivity is self-consistently computed using the Kramers opacity law. This setup allows the depth of the convection zone to dynamically adapt to changes in the thermodynamic state of the system (Käpylä et al 2019b) and to produce a smooth transition between convective and radiative layers (Brandenburg et al 2000;Käpylä et al 2017). Furthermore, a significantly broader range of imposed flux values F n are covered than in any of the previous studies.…”

Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection

“…More specifically, we use the Pencil Code to study the sensitivity of hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations to changes in the luminosity, to adopting subsets of typical BCs used in the literature, and to varying the centrifugal force. Our simulation setup is similar to that used in Käpylä et al (2019) with a few variations that will be explained in detail. We solve a set of fully compressible hydromagnetics equations…”

“…Here we use a = 1 and b = −7/2, corresponding to the Kramers opacity law for free-free and bound-free transitions (Weiss et al 2004). This formulation has previously been used in local (Brandenburg et al 2000, Käpylä et al 2017b) and semi-global (Käpylä et al 2019) simulations of convection. We refer to the heat conductivity introduced in Equation (12) as K Kramers .…”