  Temporal periodic modulation of the interatomic s-wave scattering length in
Bose-Einstein condensates is shown to excite subharmonic patterns of atom
density through a parametric resonance. The dominating wavelength of the
spatial structures is shown to be primarily selected by the excitation
frequency but also affected by the depth of the spatial modulation via a
nonlinear resonance. These phenomena represent macroscopic quantum analogues of
the Faraday waves excited in vertically shaken liquids.
