This paper proposes a possible means for ion acceleration in the topside auroral ionosphere. Specifically, it postulates that the interaction of lower hybrid waves with a small-amplitude (1--2%) density cavity leads to an energy transfer to shorter-wavelength modes. Such a mechanism may help explain the transverse acceleration of positive ions observed in this region. Both analytic and computer simulation results on the growth of these slow phase velocity waves will be presented. The resulting acceleration of test particle ions is consistent with the theory of quasi-linear diffusion of the test particle distribution function. The results of the test particle simulations indicate that this effect may be strong enough to account for the observed ion acceleration. This model is consistent with the results from the Marie and TOPAZ II and III sounding rockets which indicate a strong correlation between transverse ion acceleration, depletions in the background density, and lower hybrid waves. ¿ American Geophysical Union 1996