  It is shown that a sub-luminal electromagnetic plasma wave, propagating in
phase with a background sub-luminal gravitational wave in a dispersive medium,
can undergo parametric amplification. For this phenomena to occur, the
dispersive characteristics of the two waves must properly match. The response
frequencies of the two waves (medium dependent) must lie within a definite and
restrictive range. The combined dynamics is represented by a Whitaker-Hill
equation, the quintessential model for parametric instabilities. The
exponential growth of the electromagnetic wave is displayed at the resonance;
the plasma wave grows at the expense of the background gravitational wave.
Different physical scenarios, where the phenomenon can be possible, are
discussed.
