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A magnetically arrested disk (MAD) is a 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.