Boundary Layers of Circumplanetary Disks around Spinning Planets II. Global Modes with Azimuthal Magnetic Fields

TL;DR

This paper investigates how magnetic fields influence global wave modes in circumplanetary disk boundary layers, revealing effects on angular momentum transfer, magnetic field amplification, and implications for spin evolution and oscillations.

Contribution

It provides a systematic analysis of magnetic effects on boundary layer wave modes, including magnetic resonances, growth rates, and the potential for angular momentum belts.

Findings

Magnetic fields can reverse the sign of angular momentum flux near the boundary.

Stronger magnetic fields tend to lower the terminal spin rates of planets.

Magnetic field amplification occurs within the boundary layer.

Abstract

The accretion of material from disks onto weakly magnetized objects invariably involves its traversal through a material surface, known as the boundary layer (BL). Our prior studies have revealed two distinct global wave modes for circumplanetary disks (CPDs) with BLs exhibit opposite behaviors in spin modulation.We perform a detailed analysis about the effect of magnetic fields on these global modes, highlighting how magnetic resonances and turning points could complicate the wave dynamics. The angular momentum flux becomes positive near the BL with increasing magnetic field strength. We also examine the perturbation profile to demonstrate the amplification of magnetic fields within the BL. The dependence of growth rates on the magnetic field strength, and the spin rate are systematically investigated. We find that stronger magnetic fields tend to result in lower terminal spin rates. We stress the potential possibility for the formation of angular momentum belts and pressure bumps. The implication for the spin evolution and quasi-period oscillations observed in compact objects are also briefly discussed. Our calculations advance the understanding of magnetohydrodynamical (MHD) accretion processes and lays a foundation for observational studies and numerical simulations.