RAPID, STRONGLY MAGNETIZED ACCRETION IN THE ZERO-NET-VERTICAL-FLUX SHEARING BOX

We show that there exist two qualitatively distinct turbulent states of the zero-net-vertical-flux shearing box. The first, which has been studied in detail previously, is characterized by a weakly magnetized (β ∼ 50) midplane with slow periodic reversals of the mean azimuthal field (dynamo cycles). The second, the ‘low-β state,’ which is the main subject of this paper, is characterized by a strongly magnetized β ∼ 1 midplane dominated by a coherent azimuthal field with much stronger turbulence and much larger accretion stress (α ∼ 1). The low-β state emerges in simulations initialized with sufficiently strong azimuthal magnetic fields. The mean azimuthal field in the low-β state is quasi steady (no cycles) and is sustained by a dynamo mechanism that compensates for the continued loss of magnetic flux through the vertical boundaries; we attribute the dynamo to the combination of differential rotation and the Parker instability, although many of its details remain unclear. Vertical force balance in the low-β state is dominated by the mean magnetic pressure except at the midplane, where thermal pressure support is always important (this holds true even when simulations are initialized at β ≪ 1, provided the thermal scale height of the disk is well resolved). The efficient angular momentum transport in the low-β state may resolve long-standing tension between predictions of magnetorotational turbulence (at high β) and observations; likewise, the bifurcation in accretion states we identify may be important for understanding the state transitions observed in dwarf novae, X-ray binaries, and changing-look AGN. We discuss directions for future work, including the implications of our results for global accretion disk models and simulations.