Dynamo and Jet Interconnections in GRMHD Simulations of Black Hole Accretion Disks

Abstract We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of black hole (BH) accretion disks that investigate how a magnetorotational instability (MRI)-driven dynamo regulates jet formation and evolution. Unlike typical standard and normal evolution (SANE)/magnetically arrested disk (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 such 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 10 orbits. Decomposition of the induction equation shows that the dynamo term is stronger in 3D compared to 2.5D, confirming sustained large-scale field generation. These dynamo-generated fields are advected inward with minimal lag, producing correlated peaks in poloidal and toroidal field strengths from r max to the horizon. Early in the evolution, these peaks imprint onto the jet’s Poynting 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 coherence parameter C BH below ≈0.6. In contrast, the SANE case with similar accretion rate and horizon magnetic flux maintains higher coherence because its initial large-scale field persists, allowing its jet to survive. Our results demonstrate that dynamo-driven field evolution and horizon magnetic-field coherence regulate jet longevity, establishing a direct dynamo–jet connection in GRMHD disks.