Equatorially Asymmetric Magnetic Fields and Their Impact on Black Hole Accretion Dynamics

We investigate the impact of equatorial asymmetry in the magnetic field geometry on accretion dynamics around a spinning black hole using axisymmetric general relativistic magnetohydrodynamic simulations. We consider a Fishbone–Moncrief torus orbiting a Kerr black hole with spin parameter $a = 0.9375$, threaded by large-scale magnetic fields that are asymmetric about the equatorial plane. The degree of equatorial asymmetry in the magnetic field is parametrized by an angle, with values of $30^\circ$, $45^\circ$, and $60^\circ$. We examine how this equatorially asymmetric initial magnetic field configuration influences the magnetic field structure, accretion flow morphology, and angular momentum transport across a range of initial plasma beta values ($β= 0.007, 0.005, 0.001$). We find that such deformation in the magnetic field leads to noticeable changes in the inner disk structure, asymmetric outflow patterns in the poloidal plane, and time-dependent variations in accretion rates. These effects are generally more pronounced at lower beta values, where magnetic pressure dominates; in particular, the $30^\circ$ case at $β= 0.001$ exhibits strong and persistent asymmetric inflows and outflows. Our results demonstrate that equatorially asymmetric magnetic field configurations can significantly influence the structure and variability of relativistic accretion flows. These findings motivate future extensions to full three-dimensional studies, where black hole magnetosphere can be explored in a more general setting.

💡 Research Summary

This paper presents a systematic study of how equatorial asymmetry in the large‑scale magnetic field influences the dynamics of a magnetized accretion flow onto a rapidly spinning Kerr black hole (dimensionless spin a = 0.9375). Using the ideal GRMHD code BHAC, the authors perform a suite of axisymmetric (2‑D) simulations in modified Kerr–Schild coordinates with a high‑resolution grid (1024 × 512) that extends from just inside the event horizon (0.9 r_H) out to 2500 r_g. The accretion torus is a relativistic Fishbone–Moncrief configuration, initially in hydrostatic equilibrium, with inner edge at 4 r_g and pressure maximum at 12 r_g, and an adiabatic index γ = 4/3.

The key novelty lies in the initialization of the magnetic field. Instead of the usual equatorial‑symmetric vector potential, the authors prescribe a toroidal vector potential A_φ that is shifted by a polar offset z₀, thereby breaking the north–south symmetry. Three deformation angles are explored: 30°, 45°, and 60°. The functional form

A_φ(r,θ) = f₀ exp