 Outer Lindblad resonance

The $k_R=+1$ orbital resonance of an $m$-fold perturbation of a galactic disk. For near-circular stellar orbits, ${\Omega }_R$ is the radial epicyclic frequency. The harmonic changes radial action and azimuthal angular momentum in the ratio $\Delta J_R/\Delta J_{\phi }=1/m$, so an orbit gaining angular momentum gains random radial motion.