What phenomenon limits eddy current flow to a thin outer layer at very high frequencies?

The phenomenon that limits eddy current flow to a thin outer layer at very high frequencies is known as the skin effect. As the frequency of the alternating current increases, the induced eddy currents tend to flow closer to the surface of the conductor rather than penetrating deeply into it. This results in a reduction in the effective cross-sectional area through which the currents can flow, confining them to a limited depth or "skin depth."

The skin depth is defined as the distance into a conductor at which the current density falls to about 37% (1/e) of its value at the surface. At higher frequencies, this depth becomes very small, which is why most of the current is concentrated near the surface. This phenomenon is significant in various applications such as non-destructive testing, where it influences the sensitivity and effectiveness of the eddy current testing method in detecting surface or near-surface defects.

The other options do not specifically describe this phenomenon. High frequency filtration and low frequency filtration are not standard terms associated with the behavior of eddy currents at high frequencies. Thus, focusing on the skin effect accurately describes the limiting condition for eddy current flow at elevated frequencies.