Angular momentum transport in thin magnetically arrested discs

In accretion discs 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-scalemagnetic fields via magnetic braking, and large-scale magnetic flux can build-up on to the black hole and within the disc leading to a magnetically arrested disc (MAD). Such an 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 approximate to 0.1) MAD simulation, where it has a more dramatic effect on the dynamics of the disc than for thicker discs. Large amounts of flux are pushed off the black hole into the disc, leading to temporary decreases in stress, then this flux is reprocessed as the stress increases again. Throughout this process, 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.