Is it true that Earth's outercore rotates in the opposite direction to Earth's inner-core? Is there a plausible explanation for why this phenomenon occurs and what is the available evidence supporting this notion?
Is it true that Earth's outercore rotates in the opposite direction to Earth's inner-core?
No. You misread a badly written for-the-masses science article. The article summary is at best rather confusing and misleading. It invites a reader to misinterpret what's going on.
So what's really going on? It's simple, conceptually. The mantle, outer core, and inner core all rotate in more or less the same direction and at more or less the same rate. However, because outer core is liquid, there exists a possibility for the mantle, outer core, and inner core to exhibit slightly different rotation rates (and perhaps even rotation axes). That's apparently what is happening inside the Earth. The mantle and crust make one rotation per sidereal day. The inner core currently rotates a tiny, tiny bit faster than that, while the outer core rotates a tiny, tiny bit slower than that.
This means that when viewed from the perspective of an Earth-fixed observer, the outer core currently appears to be rotating a tiny, tiny bit westward and the inner core currently appears to be rotating a tiny, tiny bit eastward. It's a matter of perhaps confusing perspective. When viewed from an inertial perspective, the mantle, outer core, and inner core are all rotating more or less synchronously.
When you read one of those science-for-the-masses type articles, it's a good idea to jump to the underlying journal article. In this case, you should try reading Livermore, Hollerbach, and Jackson, "Electromagnetically driven westward drift and inner-core superrotation in Earth’s core", PNAS 110:40 (2013).
The answer is that there are more forces than just the momentum of the earth at play. The electromagnetic force generated by the core is pushing on itself, in fact:
"The fact that Earth's internal magnetic field changes slowly, over a timescale of decades, means that the electromagnetic force responsible for pushing the inner and outer cores will itself change over time. This may explain fluctuations in the predominantly eastwards rotation of the inner core, a phenomenon reported for the last 50 years by Tkalčić et al. in a recent study published in Nature Geoscience."
You see, these changes in the internal portions in the Earth's magnetic field cause a shear. The Earth's magnetic poles are not stationary, they wander. There is clear evidence of this from the paleomagnetic record. This is caused by the viscous convection of the iron nickel outer core. Their numerical models showed that the electromagnetic torque : from P.W. Livermore et al. 2013
On the outer core has a westward direction, while the inner core's torque scales accordingly in the opposite direction.
from P.W. Livermore et al. 2013
Finally a cross section (image above) of the asymmetric flow (off dipole) clearly shows opposite direction in the outer core and inner core (blue vs red on the color bar).
So, in summary:
- The electromagnetic field is modeled as a dipole, but in actuality has ~10% component that is off the dipole.
- The wandering of this dipole (and by consequence, off dipole) magnetic field causes a shear in opposite directions at the inner and outer core.
- This inner and outer core shear scales similarly at the edges (at the inner core outer boundary and at the core mantle boundary) but in opposite directions (so they are of the same magnitude).
- This torque, causes the flow of the inner core and the outer core to spin in opposite directions, as shown by the last figure.
This is a complex article, let me know if there is anything I could explain better. I think I've got this right.
The earth's core may rotate at a faster rate, not in the opposite direction, this would be consistent with heavier elements sinking to form the core during planetary differentiation. Conservation of angular momentum could cause it to rotate faster. See this question in Physics SE.