Reversals of Toroidal Magnetic Field in Local Shearing Box Simulations of Accretion Disk with a Hot Corona

Abstract The presence of a hot corona above the accretion disk can have important consequences for the evolution of magnetic fields and the Shakura–Sunyaev viscosity parameter α in such a strongly coupled system. In this work, we have performed three-dimensional magnetohydrodynamical shearing-box numerical simulations of an accretion disk with a hot corona above the cool disk. Such a two-layer, piecewise 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 disk 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 disk. This is a rather unique class of evolution of the magnetic field that 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 make 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 .