Regime transitions of dynamos driven by fingering double-diffusive convection

Abstract: Long-term cooling of planetary interiors is expected to progressively establish thermally stable stratification in the liquid outer core, yet its influence on dynamo action remains incompletely understood. Here we present three-dimensional numerical simulations of fingering double-diffusive convection-driven dynamos to investigate how progressively strengthening thermally stable stratification, modifies compositional convection driven flows and dynamos. Our results show that dynamo evolution is governed by a competition between Lorentz-force regulation and stratification-induced flow reorganisation. When stratification is weak, the system remains in a strong-field regime characterised by a relatively stable, dipole-dominated magnetic field, despite reduced flow intensity. As thermally stable stratification strengthens, systematic changes in flow morphology emerge, including the development of prograde equatorial zonal flows. Once a critical stratification strength is exceeded, flow reorganisation becomes dominant, leading to an abrupt transition to a weak-field dynamo accompanied by a sharp decline in magnetic energy. In the weak-field regime, magnetic field intensity exhibits enhanced temporal variability and continues to decay as stratification further strengthens, eventually resulting in dynamo cessation even when compositional buoyancy persists. These results indicate that thermally stable stratification may play an important role in regulating planetary dynamos and provide new insight into the long-term magnetic evolution of terrestrial planets.