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Title: Why do observed disks transport angular momentum faster than simulations?
Plain: The magnetorotational instability (magnetic stirring of orbiting gas) explains how disks move angular momentum outward, but simulations give about ten times less transport than outbursts of real disks imply.
Precise: Outburst decay times of fully ionized disks in dwarf novae and X-ray binaries imply Shakura-Sunyaev $\alpha \sim 0.1 \text{ to } 0.4$, while MHD simulations of magnetorotational turbulence without net vertical magnetic flux give $\alpha \sim 0.01 \text{ to } 0.03$. Determine which physics closes the gap: net vertical flux, convection, magnetized winds removing angular momentum, or spiral shocks. Answer: a mechanism with $\alpha$ as a function of ionization state and net flux that reproduces outburst light curves.
Settled by: Global simulations with self-consistent thermodynamics reproducing measured dwarf-nova outburst decay times.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 2007