In 1961, L. Slichter pointed out that some of the Earth's seismic
modes might consist of translational oscillations of the solid inner
core about its equilibrium position at the centre of the Earth. This
motion, the "Slichter mode", is principally controlled by the density
jump between the inner and outer core, and the Archimedean force of
the fluid core. Its period is thought to lie within 4-8 hours and is
split into 3 components by the Earth's rotation.