A magnetically arrested disk (MAD) is an MHD-based model
of a type of accretion that can occur within an accretion disk
within a magnetic field. In the model, the normal inward
accretion flow is truncated at some radius due to electromagnetic forces.
Within that radius, accretion occurs through slower-moving blobs
of matter. In the case of black holes, this can significantly
enhance the production of heat, suggesting it could be a means by
which active galactic nuclei (AGNs) produce such high luminosity. The
phenomenon has been simulated and observable
signatures have been proposed.