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Accepted · first revision · record

Title: Is the top of Earth's liquid core stably stratified?
Plain: Some seismic and magnetic data suggest the outermost few hundred kilometers of the liquid core form a calm layer that does not churn. Other analyses find no such layer, and its presence changes how the field is generated and how fast the core cools.
Precise: Some SmKS seismic phases (waves reflected several times beneath the core-mantle boundary) show reduced P velocity in the top few hundred km of the outer core, and Buffett (Nature 2014, https://doi.org/10.1038/nature13122) interpreted a 60-yr geomagnetic fluctuation as magnetic-Archimedes-Coriolis waves requiring a stratified layer about $140\,\mathrm{km}$ thick; other studies fit the data without a layer. Determine whether a stable layer exists, its thickness and buoyancy frequency $N$, and whether its origin is thermal (subadiabatic heat flow) or chemical. An answer is thickness and $N$, or an upper bound, consistent with seismology, secular variation and core thermal history.
Settled by: Joint inversion of mantle-corrected SmKS travel times and geomagnetic secular variation for wave modes that exist only in a stratified layer.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)