magnetorotational instability (MRI)
Fr.: instabilité magnétorotationnelle
An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport. To put it simply, the → frozen magnetic field line acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.