Mode-Beating Model of ac Helicity Injection

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Abstract
It is shown here that Bevir and Grey's ac helicity-injection current-generation scheme does not depend on Taylor's relaxation principle. Instead, ac helicity injection is identified as the beating of a compressional Alfvén mode with a resistive diffusion mode to produce a surface current. This has the interesting consequence that the mode frequencies can be much higher than Taylor relaxation would allow. Although Taylor's principle is not needed to generate the surface current, it is required for the current to diffuse into the plasma because it is also shown that the current will not diffuse in classically.