The ambient background plasma is shown to have an important stabilizing influence on the generation of electrostatic, purely growing, finite ion gyroradius, flute instabilities driven by ion loss cone distribution functions associated with shaped barium injection across the ambient magnetic field in the ionosphere. This stabilization implies that such short scale size instabilities and the striations they produce will only be present if the background plasma density is low enough. Consistent with the experimental observations of the striations, it is shown that for the parameters of the Buaro barium release experiment the background plasma density is insufficient to completely stabilize the finite ion gyroradius modes; however, background ion density does reduce the growth rate from the level predicted with a theory which does not include the background plasma dynamics.