IMAGE RPI measurements of electron density in the plasmatrough Jerry Goldstein, James L Green, Robert F Benson, Bodo W Reinisch, Patricia H Reiff The plasmapause and plasmatrough interface of the magnetosphere occurs where the corotation electric field gives way to solar-wind-driven convection. Number density levels in the plasmatrough are typically depleted ($1-10 cm^-3, relative to the higher densities 10^2 - 10^4 cm^-3 found in the outer plasmasphere. Understanding this interface is key to understanding the dynamics of the plasmasphere and the inner magnetosphere. The IMAGE Radio Plasma Imager (RPI) data provides us a rich opportunity to study in detail the dynamics of the plasma density structure of the plasmapause/trough region with unprecedented accuracy. As a remote sounder, RPI produces echoes in which a density distribution can be obtained. In addition, the very long antennas (500 m tip-to-tip X and Y, and 20 m tip-to-tip Z axis) on IMAGE are more than four or five times the local Debye length allowing RPI to observe the plasma line down to a frequency of 3 kHz (RPI's lowest frequency limit) during its passive observations. The measurement of the plasma line and the local resonances obtained during sounding enable us to study the dynamics of the plasma density structure in this important region of the magnetosphere. In this paper, we show a variety of examples in which these type of measurements are used to study the dependence of the electron density Ne in the plasmapause/trough region with geomagnetic activity at a variety of local times. _______________ Presented at the Fall American Geophysical Union Meeting, San Francisco, CA., December 15-19, 2000