Axial inner core rotation and oscillation

The rigid centre of our planet, the inner core, is not rotating at a uniform rate. This is inferred from the travel time of seismic waves traveling through the inner core. As a fun historical note, Edmund Halley (of comet fame) was the first to suggest that the Earth comprised an inner core and that it was rotating at a different rate than the mantle (See Fig 1, below). This suggestion appears in a Philosophical Transaction of the Royal Society paper in 1692. Halley's suggestion was motivated by observations of the Earth's magnetic field, which was then thought to be purely produced by permanent magnetization. Halley's idea of a differentially rotating "inner core" with its own magnetization could explain the observed changes of the magnetic field.

Dynamically, we expect that fluctuations in longitudinal (east-west) flow deep indide the Earth's fluid core should entrain fluctuations in the rate of inner core rotation by electromagnetic coupling. Oscillations in inner core rotation can also result from normal modes, in particular from gravitational coupling with the mantle (an example from my JSEDI 2026 study is shown in Fig 2 below). Understanding the cause of fluctuations in inner core rotation places useful constraints on quantities such as the inner core viscosity, mass anomalies in the mantle (responsible for gravitational coupling with the inner core) and the conductance at the base of the mantle.

Figure 1: Edmund Halley's model to explain the Earth's magnetic field changes. Color patches represent permanent magnetization.

Figure 2: The angular velocity structure of the mantle (orange arrows), fluid core (blue arrows) and inner core (red arrows) in (a) a generic case of the CMG mode, (b) the rigid core regime and (c) the locked inner core regime. Figure taken from Dumberry, JSEDI, 2026.

Some of my papers on this topic