Magnetohydrodynamics in the Inner Regions of Protoplanetary Discs
type: CreativeWork
description: 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…
abstract: 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…
author: Roberts, Matthew
language: eng
keywords: protoplanetary discs, magnetohydrodynamics, accretion, instabilities, astrophysics
license: https://www.repository.cam.ac.uk/bitstreams/b2a763df-9d1d-412a-971e-dbabbb54487c/download
license: http://purl.org/NET/rdflicense/allrightsreserved
date published: 2025-08-17
date modified: 2026-07-22T22:24:13Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.123776