Dynamo and Jet interconnections in GRMHD simulations of black hole accretion disks

TL;DR

This work investigates how MRI-driven dynamos regulate jet formation in black-hole accretion disks using global 3D GRMHD simulations with sub-SANE initial conditions of multiple small-scale loops. The authors show robust large-scale dynamo cycles, evidenced by butterfly diagrams and induction-equation decomposition, which generate fields that are efficiently advected inward to the horizon and modulate the jet's electromagnetic output. Jet longevity then depends on the coherence of the horizon magnetic field, with jets shutting down when the signed-to-unsigned flux ratio at the horizon, $\\mathcal{C}_{\rm BH}$, drops below approximately $0.6$, despite comparable flux and accretion rates. These results establish a direct dynamo–jet connection in GRMHD disks and suggest disk dynamos could explain quasi-periodic jet variability observed across BH systems, with implications for interpreting QPOs, flares, and polarization swings in XRBs and AGN. All key math is presented in $...$ format, including $a$, $\\Phi$, $\\dot{M}_{\\rm BH}$, and $\\mathcal{C}_{\rm BH}$, to maintain precise, machine-readable notation.

Abstract

We present global 3D GRMHD simulations of black hole (BH) accretion disks designed to investigate how MRI-driven dynamo action regulates jet formation and evolution. Unlike standard SANE/MAD setups that impose a coherent large-scale poloidal loop, our "sub-SANE" initial conditions use multiple same-polarity small-scale magnetic loops. Rapid reconnection erases magnetic memory and enables large-scale dynamo to emerge early from MRI turbulence. We perform two such sub-SANE simulations at different BH spins ($a = 0.5, 0.9375$) and compare them with conventional SANE runs. The sub-SANE disks show regular large-scale dynamo cycles with periods of about ten orbits. Decomposition of the induction equation shows that the turbulent dynamo term is stronger in 3D compared to 2.5D and balances advection in the saturated state, confirming sustained large-scale field generation. These dynamo-generated fields are advected inward with minimal time lag, producing correlated peaks in both poloidal and toroidal field strengths from $r_{\rm max}$ to the horizon. Early in the evolution, these peaks imprint directly onto the jet's electromagnetic energy flux, indicating that the jet mirrors the dynamo wave. Though jets form at early times, the sub-SANE runs eventually undergo jet shutdown. We show that this occurs when the magnetic field at the horizon loses coherence, as quantified by a decline in the signed-to-unsigned flux ratio $\mathcal{C}_{\rm BH}$ below $\approx 0.6$. In contrast, the SANE reference case with similar accretion rate and horizon magnetic flux maintains high magnetic coherence because its initial large-scale field persists, allowing its jet to survive. Our results show that both dynamo-driven field evolution and horizon magnetic-field coherence critically regulate jet longevity, establishing a direct dynamo-jet connection in GRMHD disks.

Figures (31)

• Figure 1: Block structure for the 3D simulations in the $\phi=0$ plane ($z$-axis contains the spin-axis of the BH). Static mesh refinement of level two is applied in all directions in the disk region, identified by the density contour in the background. The mesh is axisymmetric. However, there is no symmetry assumption involved in the evolution of 3D simulations.
• Figure 2: Initial magnetic field configuration in the $\phi=0$ plane (shown in solid while lines) for both the 2D and 3D SANE simulations set through the vector potential (see Eq. (\ref{['saneic']})), with the colormap showing density, $\rho$.
• Figure 3: As Fig. \ref{['fig:sane_init_condition']} but for 3D sub-SANE simulations (see Eq. (\ref{['subsaneic']})).
• Figure 4: The top panel shows magnetisation, $\sigma=b^2/ \rho$ in the $x-z$ plane at $t=1000 M$ for 3DSS-9375 and 2DS-9375 in the left and right halves respectively. The bottom panel shows Lorentz factor $\Gamma=(1-v^iv_i)^{-1/2}$ at $t=1000 M$ for 3DSS-9375 (left) and $t=1350 M$ for 2DS-9375 (right). The parabolic structure of the jet is more prominent in the SANE case.
• Figure 5: The top and bottom panels show magnetisation $\rm{log}_{10}\sigma$ in the disk ($\rho_\text{cutoff}=[0.6,1])$ and jet ($\Gamma_\text{cutoff}=[1.1,2.5]$) region for the 3DSS-9375 run at $t=1000M$ and $t=3500M$ respectively. The magnetic field lines are shown by the grey lines. We see that the field lines are more coherent in the funnel region at $t=1000M$ and they eventually become tangled at the base of the jet at $t=3500M$. This leads to the shut down of the jet, as seen by the diminished $\sigma$ in the jet.