Inner Core Rotation Rate from Small-Scale Heterogeneity and Time-Varying Travel Times

Seismic observations have provided critical evidence for inner core differential rotation.

“Seismic observations have provided critical evidence for IC differential rotation (Creager, 1997;Song & Richards, 1996;Vidale et al., 2000), and different models have been proposed.”

Inner-core rotation estimates are now converging from early estimates of several times larger.

“These estimates are now converging from early estimates of several times larger (e.g., Creager, 1997;Song & Richards, 1996).”

Using the fine-scale gradient leads to a higher estimate of the rotation rate exceeding 0.2°/yr for super-rotation, close to results based on relatively large spatial structures.

“This will lead to a higher estimate of the rotation rate exceeding 0.2°/yr for super-rotation, close to the results based on relatively large spatial structures (Creager, 1997;Song, 2000;Tkalčić, 2024;Tkalčić et al., 2013;Zhang et al., 2005).”

The paper resolves a fine-scale gradient, whereas earlier works typically adopted lateral gradients averaged over much larger scales of several tens of degrees.

“A key improvement is that we resolve a fine-scale gradient, whereas earlier works typically adopted lateral gradients averaged over much larger scales of several tens of degrees (e.g., Creager, 1997;Zhang et al., 2005).”

Seismological observations confirmed differential rotation of the inner core relative to the mantle, with varying rotation rates in the eastward (prograde) direction, settling to the range from 0.3 to 1.1 years per degree faster than the mantle.

“These observations confirmed the theoretical predictions of differential rotation of the inner core relative to the mantle, with varying rotation rates in the eastward (prograde) direction, settling to the range from 0.3 to 1.1 years per degree faster than the mantle.”

The linearity in the observed velocity gradient in the volume of the inner core along the South Sandwich Islands region to Alaska (SSI-Alaska) path is a robust approximation.

“the linearity in the observed velocity gradient in the volume of the inner core along the South Sandwich Islands region to Alaska (SSI-Alaska) path (Creager, 1997) is shown to be a robust approximation (Tkalčić, Bodin, et al., 2013).”