Free-surface horizontal waves generated by low-frequency alternating magnetic fields

New types of electromagnetic parametric instability have been observed at the periphery of the free surface of a liquid metal pool in the presence of a low-frequency magnetic field. An experimental set-up is used to observe the motion of a mercury layer on a substrate located in a solenoidal coil supplied with low-frequency alternating electric currents. The Lorentz body forces produced are mainly oscillatory and generate motion in the liquid. Various regular and irregular free-surface patterns are observed. Two-dimensional stability analysis shows that axisymmetric waves are directly forced by the electromagnetic forces while azimuthal waves are the result of instability. The experimental stability diagram exhibits 'tongues' as already observed for parametric resonance instability. For high magnetic field strengths, the free-surface patterns become highly unstructured. High-amplitude 'fingers' as well as solitary waves are observed. Measurements of the deformation observed on photographs are compared with rough theoretical estimates.