Past exam of the advanced fluid dynamics course of the University of Oxford / 2024 / 1 / e

Work locally at $R_0$ with $\Omega =\Omega (R_0)$, and defineHere $\kappa$ is the radial epicyclic frequency. Combining the radial imbalance, the Coriolis force and the magnetic spring gives the spring model of magnetorotational instability:For a normal mode proportional to $e^{-i\omega t}$, the coefficient matrix isSetting its determinant to zero yields the ideal magnetorotational dispersion relation,Its two values of ${\omega }^2$ areso they are real. For $a>0$, their product is negative exactly when $a+D<0$, giving an exponentially growing normal mode. Conversely, $a+D>0$ also makes their sum positive and therefore gives two positive squared frequencies. A fixed nonzero magnetic mode is exponentially unstable for $k^2{v}_A^2+D<0$, oscillatory for $k^2{v}_A^2+D>0$, and marginal at equality. Stability against every allowed nonzero wavenumber in the unbounded ideal model requires $D\ge 0$; finite-domain restrictions can remove the unstable band.

For $\Phi =-GM/R$, the equilibrium relation gives ${\Omega }^2=GM/R^3$, so $D=-3{\Omega }^2$ and ${\kappa }^2={\Omega }^2$. Hence the Keplerian disk has the magnetorotational instability bandWithout the magnetic field, the polynomial becomes ${\omega }^2({\omega }^2-{\kappa }^2)=0$. The Rayleigh centrifugal stability criterion instead requires ${\kappa }^2>0$, equivalently $d(R^4{\Omega }^2)/dR>0$. A Keplerian disk satisfies that hydrodynamic criterion. Magnetic coupling therefore destabilizes a disk that has a restoring hydrodynamic epicyclic oscillation; the field couples angular-momentum exchange between elements, rather than acting only as an isolated restoring spring.

The plotted maximum is the maximum growth rate of ideal magnetorotational instability, $3\Omega /4$ at $k^2{v}_A^2=15{\Omega }^2/16$.