Global magnetohydrodynamic simulations of the inner regions of protoplanetary discs. II. Vertical-net-flux regime

Abstract

The inner regions of protoplanetary discs, which encompass the putative habitable zone, are dynamically complex, featuring a relatively well-ionised, turbulent active zone located interior to a poorly ionised 'dead' zone. In this second paper, we investigate a model of the magnetohydrodynamic processes around the interface between these two regions, using five three-dimensional global magnetohydrodynamic simulations of discs threaded by a large-scale poloidal-net-flux magnetic field. We employ physically motivated profiles for Ohmic resistivity and ambipolar diffusion, alongside a simplified thermodynamic model comprising a cool disc and hot corona. Our results show that, first, the interface acts as a one-way barrier to inward transport of large-scale magnetic flux from the dead zone. This leads to magnetic flux depletion throughout most of the active zone, whereby it either advects inwards to the inner numerical boundary or accumulates just inside the interface. Second, two sources of strong variability emerge from the interface due to the difficulty of maintaining a constant, vertically integrated electrical current across distinct and evolving magnetic-field states. Third, despite the weak magnetothermal wind in the dead zone, a pressure maximum forms at the interface, leading to Rossby-wave-induced vortices. Fourth, unlike the model of Iwasaki et al. (2024), there is no 'transition zone' devoid of magnetic flux and magnetic winds. Instead, multiple outflow zones span all disc radii reflecting the radially varying launch conditions, with an inner turbulent wind impinging upon an outer, more laminar one. Fifth, a heated corona prevents the 'puffing up' of poloidal-net-flux, active disc regions.