Reversals of toroidal magnetic field in local shearing box simulations of accretion disc with a hot corona

Abstract

Presence of a hot corona above the accretion disc can have important consequences for the evolution of magnetic fields and the Shakura-Sunyaev (SS) viscosity parameter $α$ in such a strongly coupled system. In this work, we have performed three-dimensional magnetohydrodynamical shearing-box numerical simulations of accretion disc with a hot corona above the cool disc. Such a two-layer, piece-wise isothermal system is vertically stratified under linear gravity and initial conditions here include a strong azimuthal magnetic field with a ratio between the thermal and magnetic pressures being of order unity in the disc region. Instabilities in this magnetized system lead to the generation of turbulence, which, in turn, governs the further evolution of magnetic fields in a self-sustaining manner. Remarkably, the mean toroidal magnetic field undergoes a complete reversal in time by changing its sign, and it is predominantly confined within the disc. This is a rather unique class of evolution of the magnetic field which has not been reported earlier. Solutions of mean magnetic fields here are thus qualitatively different from the vertically migrating dynamo waves that are commonly seen in previous works which model a single layer of an isothermal gas. Effective $α$ is found to have values between 0.01 and 0.03. We have also made a comparison between models with Smagorinsky and explicit schemes for the kinematic viscosity ($ν$). In some cases with an explicit $ν$ we find a burst-like temporal behavior in $α$.