Magnetohydrodynamic convection in accretion discs

Abstract

ABSTRACT Convection has been discussed in the field of accretion discs for several decades, both as a means of angular momentum transport and also because of its role in controlling discs’ vertical structure via heat transport. If the gas is sufficiently ionized and threaded by a weak magnetic field, convection might interact in non-trivial ways with the magnetorotational instability (MRI). Recently, vertically stratified local simulations of the MRI have reported considerable variation in the angular momentum transport, as measured by the stress to thermal pressure ratio α, when convection is thought to be present. Although MRI turbulence can act as a heat source for convection, it is not clear how the two instabilities will interact dynamically. Here, we investigate their interplay in controlled numerical experiments and isolate the generic features of their interactions. We perform vertically stratified, 3D magnetohydrodynamic shearing box simulations with a perfect gas equation of state with the conservative, finite-volume code pluto. We find two characteristic outcomes of the interaction between the two instabilities: (a) straight MRI and (b) MRI/convective cycles, with the latter exhibiting alternating phases of convection-dominated and MRI-dominated flow. During the latter phase, we find that α is enhanced by nearly an order of magnitude, reaching peak values of ∼0.08. In addition, we find that convection in the non-linear phase takes the form of large-scale and oscillatory convective cells. Convection can also help the MRI persist to lower Rm than it would otherwise do. Finally, we discuss how our results help interpret simulations of dwarf novae.