What it is
Decade-scale fluctuations in the length of Earth's day come mainly from angular momentum exchanged between the mantle and the core, but which torque couples the two has been unknown. Using the seismically reconstructed differential rotation of the inner core and core flows derived from changes in the magnetic field, the authors find that multidecadal length-of-day changes are driven primarily by a gravitational torque and resisted by electromagnetic and topographic torques, consistent with Earth-like dynamo models.
Why it matters
Electromagnetic and topographic coupling at the core-mantle boundary and a gravitational torque from the inner core had all been proposed, and identifying which dominates turns small changes in the length of day into a probe of the deep Earth. The reconstructed torque histories support a lowermost mantle with near-neutrally buoyant thermochemical piles, a low-viscosity post-perovskite phase and a highly conducting, iron-enriched layer a few kilometres thick at its base, and they suggest an inner core that deforms within only a few years.
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Filed underPulsars and Gravitational Waves Research, Experimental and Theoretical Physics Studies, Geophysics and Gravity Measurements