Rapid, strongly magnetized accretion in the zero-net-vertical-flux shearing box

Abstract

We show that there exist two qualitatively different 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 $\beta\sim1$ midplane dominated by a coherent azimuthal field with much stronger turbulence and much larger accretion stress $\alpha \sim 1$. The low-$\beta$ state is realized in simulations that begin 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 is true even when simulations are initialized at $\beta\ll1$, 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 find 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 simulations.