The Periodicity of Three-dimensional Oscillatory Reconnection

Oscillatory reconnection is a dynamic, magnetic relaxation mechanism in which a perturbed null point reverts to equilibrium via time-dependent reconnection. In this paper, we investigate the long-term periodic signal generated by a 3D magnetic null point when it is perturbed by a nonperiodic driver for a variety of driving amplitudes. We solve the 3D nonlinear magnetohydrodynamic equations using a bespoke numerical boundary condition (a sponge region) that damps wave reflections and thus allows the long-term periodic signal at the 3D null point to be investigated. We observe multiple cycles of the 3D oscillatory reconnection mechanism for the first time. We find that the periodicity is both constant and independent of the choice of driving amplitude. Furthermore, the resultant time-dependent current density at the null point, normalized by the driving amplitude, is invariant. We extract a single period for oscillatory reconnection at a 3D null point, opening the future possibility of using this characteristic period as a diagnostic tool to reveal indirectly the fundamental plasma properties of 3D null points.