Magnetohydrodynamics in the Inner Regions of Protoplanetary Discs
The inner regions of protoplanetary discs, which encompass the putative habitable zone, comprise a highly ionised, turbulent active region and a poorly ionised outer dead zone. The complex magnetohydrodynamic processes at this dead–active zone interface significantly impact the disc's evolution – influencing global magnetic flux transport, strong accretion variability, interactions between turbulent and laminar flows, and the formation of pressure maxima and vortices – yet remain poorly understood. This thesis develops a more complete understanding of how magnetically-mediated processes around the dead–active zone interface shape the structure and evolution of the inner regions of protoplanetary discs, using global three-dimensional non-ideal magnetohydrodynamic simulations performed with IDEFIX. In the first part of this thesis, we examine the zero-net flux regime. We show that large-scale coherent poloidal magnetic field loops form in the active region and accumulate at the disc–corona temperature transition, concentrating accretion in the surface layers. Concurrently, outward mass transport from the active region establishes a pressure maximum that triggers the Rossby wave instability, producing vortices whose coherence is weakened by turbulence at the interface. Meanwhile, the dead zone develops a magnetic field of a distinct morphology – likely diffused outward from the active zone – consistent with that required for weak magnetic-wind-driven accretion. In the second part of this thesis, we investigate the vertical-net flux regime, revealing three key results. First, magnetic flux is continually depleted from the active region, altering its long-term dynamics. Second, two novel sources of short-timescale accretion variability, localised at the dead–active zone interface, are identified: a complex reconfiguration of the local structure, driven by the topological rearrangement of poloidal electric-current circuits; and strong, intermittent magnetic flux expulsion events that drive repeated transitions between strong- and weak-field states in the outermost active zone. Third, despite the (initial) weak magnetothermal wind in the dead zone, a robust pressure maximum once again forms at the interface, leading to Rossby wave-induced vortex formation. The final part of this thesis departs from global magnetohydrodynamics and focuses on the stability of these vortices. Motivated by their small pre-merger aspect ratios, we conduct high-resolution unstratified hydrodynamic simulations to study the elliptical instability in Rossby wave-induced vortices. We demonstrate that the cores of these self-consistently formed vortices become unstable to the fastest-growing branch of the rotating elliptical instability, leading to a competition between the merger and the small-aspect-ratio vortex-destruction timescales.
protoplanetary discs, magnetohydrodynamics, accretion, instabilities, astrophysics
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