= John F. Hawley
{c}
<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.<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]{ref}
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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