Inner Core Viscous Deformation Generates Gravitational Torque Explaining Millisecond Length-of-Day Variations

Zhang and Dumberry modeled the inner core's non-spherical shape and its differential rotation relative to the mantle. Their calculations demonstrate that gravitational coupling dominates over electromagnetic and viscous drag at the core-mantle boundary when deformation occurs on roughly 10-year timescales. This torque balance reproduces the observed decadal oscillations in length of day without requiring unrealistic mantle viscosity values. The work integrates seismic constraints on inner-core anisotropy with satellite gravity and geomagnetic data, revealing a dynamic feedback loop previously underestimated in angular-momentum budgets.