Global magnetohydrodynamic simulations of the inner regions of protoplanetary discs. I. Zero-net flux regime

TL;DR

This study uses 3D magnetohydrodynamic simulations to explore magnetic field structures and instabilities at the interface of active and dead zones in protoplanetary discs, revealing mechanisms for accretion and vortex formation.

Contribution

First detailed 3D MHD simulations of the inner protoplanetary disc in the zero-net flux regime, showing magnetic field evolution and vortex development at the disc interface.

Findings

Formation of large-scale magnetic loops in the active region

Development of pressure maxima and Rossby vortices

Magnetic field diffusion in the dead zone

Abstract

The inner regions of protoplanetary discs, which encompass the putative habitable zone, are dynamically complex, featuring a well-ionised, turbulent active inner region and a poorly ionised `dead' outer region. In this first paper, we investigate a base-level model of the magnetohydrodynamic processes around the interface between these two regions, using five three-dimensional global magnetohydrodynamic simulations in the zero-net flux regime. 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, large-scale coherent poloidal magnetic field loops form in the magnetorotational instability active region. These loops lead to the accumulation of tightly wound magnetic flux at the disc-corona temperature transition, driving strong, localised accretion flows in the surface layers of the active region. Second, an axisymmetric pressure maximum, extending across multiple disc scale heights, develops as a result of outward mass transport from the active region. This, in turn, triggers the Rossby wave instability and leads to the development of anticyclonic vortices. Third, the dead zone develops magnetic field with a distinct morphology, likely resulting from the outward diffusion of the large-scale poloidal loops in the active zone. This self-consistently generated field exhibits a vertical structure that can drive accretion in the inner dead zone via a weak magnetic-pressure wind. In the second paper in the series, we extend this work to the vertical-net flux regime, where global magnetic flux transport and magnetically driven outflows become dynamically significant.