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We suggest that the hysteretic cycle of black hole state transitions arisesfrom two established properties of accretion disks: the increase in turbulentstress in disks threaded by a net magnetic field and the ability of thick (butnot thin) disks to advect such a field radially. During quiescence, magneticfield loops are generated by the magnetorotational instability at the interfacebetween the inner hot flow and outer thin disk. Vertical flux is advected intoand accumulates stochastically within the inner flow, where it stimulates theturbulence so that $\alpha \sim 1$. The transition to a geometrically thininner disk occurs when $L \sim \alpha^2 L_{\rm Edd} \sim L_{\rm Edd}$, and thefirst "thin" disk to form is itself moderately thick, strongly magnetized, andable to advect field inward. These properties favor episodic jet production. Asthe accretion rate declines magnetic flux escapes, $\alpha$ decreases to$\alpha \sim 0.01 - 0.1$, and a hot inner flow is not re-established until $L\ll L_{\rm Edd}$. We discuss possible observational consequences of ourscenario.

A mechanism for hysteresis in black hole binary state transitions