Gary A. Glatzmaier of the Institute of Geophysics & Planetary Physics at Los Alamos National Laboratory is one of the pioneers in modeling the earth's core. He provides this response:
"As reported in their July 18 article in Nature, Xiaodong Song and Paul G. Richards of Lamont-Doherty Earth Observatory find evidence that the inner core rotates slightly faster than the surface and outer layers of the earth. They were led to look for this effect by a prediction of it published last year by myself and Paul H. Roberts of the University of California at Los Angeles.
"In our geodynamo model, the magnetic field is generated by convection in the fluid outer core and by local shears in the fluid flow that develop just above the solid inner core because of its rotation rate. The magnetic field permeates both the outer and inner cores, however, so the inner and outer cores are magnetically coupled to each other. The inner core in our simulation is thus forced to rotate at some average of the rate of the eastward fluid flows (jet streams) in the outer core that carry along the magnetic field. This co-rotation rate (between the inner core and the jet streams in the other core) settles at whatever value will result in a zero net torque on the inner core. If the rotation rate of the inner core changed for some reason, there would develop a huge net magnetic torque that would quickly restore it to the co-rotation rate.
"This effect that Roberts and I found in our simulation is analogous to the way in which a synchronous electric motor works, with the inner core acting like the rotor. The jet streams in the fluid outer core result from thermal and compositional buoyancy forces, Coriolis forces, pressure gradients and magnetic Lorentz forces. Hence, the super-rotation of the inner core is really neither a pure cause nor a pure effect of the magnetic field. The situation is much more intricate: the inner core rotation, the fluid flow, the magnetic field, and the thermal and compositional distributions all have complicated nonlinear feedbacks on one another, which is why solving this problem required running a three-dimensional model on a supercomputer.
"Of course, all these statements depend on how well the computer model simulates the real earth. The magnetic field produced in the Glatzmaier-Roberts model, measured at the surface, is quite similar in strength, structure and time dependence to the earth's actual field. Note that less than 1 percent of the total magnetic energy of the simulated field exists outside the core. The magnetic energy in the inner core is only about 10 percent of the magnetic energy in the outer core (where the field is generated), but it is about 100 times greater than the kinetic energy resulting from the rotation of the inner core relative to the rest of the earth. Therefore, there should be plenty of magnetic energy available to keep the inner core rotating in step with the average fluid flow just above the inner core; this flow is eastward, which is why the inner core is rotating faster than the earth's mantle and crust outside the core.
"Viscous forces in the fluid core are extremely small relative to the magnetic forces. As a result, even though the earth's overall rotation is slowing because of the gravitational pull of the moon, viscous forces in the fluid probably have very little effect on the rotation rate of the inner core. On the other hand, the precession of the earth's rotation axis resulting from the sun and moon (the precession of the equinoxes) may produce additional forces that drive the fluid flow in the core and thereby influence the generation of the magnetic field and the rotation of the inner core."
Philip N. Froelich, the director of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, offers more of an outsider's perspective:
"The seismic evidence presented by Song and Richards strongly indicates that the solid inner core is moving with respect to the outer crust of the earth. This evidence is based on the observation that the axis of fastest sound (seismic) velocity through the solid inner core--which is angled about 10 degrees from the earth's spin axis--is moving eastward at about one degree a year. Seismic waves move faster through the core in the north-south direction than in the equatorial direction probably because of the preferential orientation of iron crystals in the inner core; they all seem lined up in one direction. It is not known, however, why this direction is offset by 10 degrees from the present rotation axis. (Perhaps most of the inner core's iron crystallized when the earth's spin axis was slightly different.)