The interaction between the solar wind and a rotating planet causes the field lines in the planetary magnetotail to twist into a helix. Using a simplified magnetotail model, we examine hydromagnetic waves propagating down the magnetopause for such a field configuration and derive the dispersion relation of the waves. It turns out that only under certain special circumstances can the hydromagnetic waves be stable. In a thin magnetopause boundary layer, the helical wave is found to be always stable and its wave frequency depends weakly on the plasma and the field within the layer. The current system of the boundary layer is found to be modulated by the wave and the modulation is proportional to the velocity perturbation of the plasma. The wave influence on the spiral angle is examined briefly for some special cases for which we find the variation of the angle increases monotonically with increasing radial distance.