Suppression of small scale dynamo action by an imposed magnetic field

Nonhelical hydromagnetic turbulence with an externally imposed magnetic field is investigated using direct numerical simulations. It is shown that the imposed magnetic field lowers the spectral magnetic energy in the inertial range. This is explained by a suppression of the small scale dynamo. At large scales, however, the spectral magnetic energy increases with increasing imposed field strength for moderately strong fields, and decreases only slightly for even stronger fields. The presence of Alfvén waves is explicitly confirmed by monitoring the evolution of magnetic field and velocity at one point. The frequency $\omega$ agrees with $v_{\mathrm{A}}{k}_1$, where $v_{\mathrm{A}}$ is the Alfvén speed and $k_1$ is the smallest wave number in the box.