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 (\(\beta\sim50\)) midplane with slow periodic reversals of the mean azimuthal field (dynamo cycles). The second, the ‘low-\(\beta\) state,’ which is the main subject of this paper, is characterized by a strongly magnetized \(\beta\sim 1\) midplane dominated by a coherent azimuthal field with much stronger turbulence and much larger accretion stress (\(\alpha \sim 1\)). The low-\(\beta\) state emerges in simulations initialized with sufficiently strong azimuthal magnetic fields. The mean azimuthal field in the low-\(\beta\) 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-\(\beta\) 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 \(\beta \ll 1\), provided the thermal scale height of the disk is well resolved). The efficient angular momentum transport in the low-\(\beta\) state may resolve long-standing tension between predictions of magnetorotational turbulence (at high \(\beta\)) 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.