What really makes an accretion disc MAD

ABSTRACT Magnetically arrested accretion discs (MADs) around black holes (BHs) have the potential to stimulate production of powerful jets and account for recent ultra-high-resolution observations BH environments. Their main properties are usually attributed accumulation dynamically significant net magnetic (vertical) flux throughout region, which is then regulated by interchange instabilities. Here, we propose instead that it mainly a important toroidal field – result dynamo action triggered but still relatively weak vertical defines regulates MADs. We suggest rapid convection-like instabilities, involving tubes operating concurrently with magnetorotational instability (MRI), can regulate structure disc escape flux. generalize convective stability criteria equations include effects strong show flows could be driven towards two distinct marginally stable states, one associate confirm plausibility our theoretical model comparing its quantitative predictions simulations both MAD SANE (standard normal evolution; strongly magnetized not ‘arrested’) discs, set help distinguish MADs from other states. Contrary previous claims in literature, argue MRI suppressed probably responsible existence field.