  In accretion disks with large-scale ordered magnetic fields, the
magnetorotational instability (MRI) is marginally suppressed, so other
processes may drive angular momentum transport leading to accretion. Accretion
could then be driven by large-scale magnetic fields via magnetic braking, but
large-scale magnetic flux can build-up onto the black hole and within the disk
leading to a magnetically-arrested disk (MAD). Such a MAD state is unstable to
the magnetic Rayleigh-Taylor (RT) instability, which itself leads to vigorous
turbulence and the emergence of low-density highly-magnetized bubbles. This
instability was studied in a thin (ratio of half-height H to radius R, $H/R
\approx 0.1$) MAD simulation, where it has a more dramatic effect on the
dynamics of the disk than for thicker disks. We find that the low-density
bubbles created by the magnetic RT instability decrease the stress (leading to
angular momentum transport) in the disk rather than increasing magnetic
torques. Indeed, we find that the dominant component of the stress is due to
turbulent magnetic fields, despite the suppression of the axisymmetric MRI and
the dominant presence of large-scale magnetic fields. This suggests that the
magnetic RT instability plays a significant role in driving angular momentum
transport in MADs.
