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

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 with BLs exhibiting opposite behaviors in spin modulation. We perform a detailed analysis of the effects 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 of the formation of angular momentum belts and pressure bumps. The implications for the spin evolution and quasiperiodic oscillations observed in compact objects are also briefly discussed. Our calculations advance the understanding of magnetohydrodynamical accretion processes and lay a foundation for observational studies and numerical simulations.