In the 1990s, several different research groups independently published details of studies which suggested that the Earth's solid inner coreplugin-autotooltip__plain plugin-autotooltip_bigEarth's inner core
Since the 1930s, the mainstream view of the 'Inner Core' of planet Earth is that it's solid, and is composed primarily of iron, along with small percentages of nickel and some other light elements.
Since the 1980s, it has been known that the core is able to transmit seismic shear waves (transverse seismic waves) - called is rotating slightly faster that the rest of the planet. (example study )
Results indicated that the core is rotating about 0.3 to 1 degree per year faster - so called 'Super Rotation'. The theories were developed from analysis of seismic data collected during earthquakes.
New (2023) research suggests that the core may have slowed down again, and that the variations - speeding and slowing - might be part of a repeating cycle over several decades.
Why the rotation speed varies, and what drives the super-rotation is unknown.
See : Nature Geoscience (2023) 'Multidecadal variation of the Earth’s inner-core rotation'.
Note that it's not known what effects (if any) the speed changes may have on the Earth’s magnetic fieldplugin-autotooltip__plain plugin-autotooltip_bigEarth’s magnetic field
Without the magnetic field, high energy radiation from space would affect the Earth's upper atmosphere much more dramatically – and high-energy radiation reaching the surface would be far higher. The levels of radiation without.