Abstract
A widely accepted mechanism for the spontaneous appearance of large‐scale magnetic fields in magnetofluids is the turbulent inverse cascade of magnetic helicity. The evidence for the effect is largely computational, and has been acquired by using spectral‐method codes and imposing three‐dimensional (3D) rectangular periodic boundary conditions. We report here similar investigations of the phenomenon that result when a uniform external dc magnetic field is present. With no imposed dc magnetic field, the 1981 results of Meneguzzi et al are recovered, but by adding an externally‐imposed dc magnetic field, we find that it is possible to suppress entirely the inverse magnetic helicity cascade phenomenon. This is a somewhat puzzling result, and we attribute it not to any fundamental change in the physical processes involved as much as to the inconsistency of 3D rectangular periodic boundary conditions (which permit no net current through the basic computational box). These boundary conditions seem inadequate as...
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