Why does Earth's outer-core rotate in the opposite direction to the inner-core?
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).