John F. Hawley helped explain angular-momentum transport in accretion disks through the magnetorotational instability. With Steven A. Balbus, he showed in 1991 that even a weak magnetic field can destabilize differentially rotating disks whose angular velocity decreases outward.[ref]
A Powerful Local Shear Instability in Weakly Magnetized Disks. I. Linear Analysis by Steven A. Balbus and John F. Hawley. Abstract and sections 2.1–2.5: local instability and magnetic-tension interpretation
In the spring model of magnetorotational instability, magnetic tension couples displaced fluid elements and transfers angular momentum. This can amplify their separation despite stability under the corresponding unmagnetized rotational test.
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