Nonlinear wavepackets in Kelvin-Helmhotz instability in hydromagnetics

The instability of small but finite amplitude waves propagating at the interface of two layers of highly conducting incompressible fluids in relative motion in presence of external uniform magnetic field is studied. Using the method of multiple scales nonlinear evolution equations are derived for both linearly stable and marginally stable cases. It is found that in the linearly stable case both the modes are modulationally unstable. The nonlinear cut-off wavenumbers are determined. (orig.)