Spatially localized bursts of intense lower hybrid wave radiation are often encountered in ionospheric observations. These electric field enhancements have been correlated with depletions in plasma density. This laboratory investigation was undertaken to provide the kind of insight into the nature of the interaction which is difficult to obtain from ionospheric observations alone. The experiments were performed in the LArge Plasma Device (LAPD) at UCLA. The dependence of the interaction on wave frequency, depletion shape and size and density gradient is investigated in three spatial dimensions and in time. This letter concentrates on the dependence of the phenomenon on the density gradient. The main result is that the lower hybrid waves are observed to have the largest electric field in the regions of steepest density gradient. ¿ 2000 American Geophysical Union