The magnetospheric electron density Ne can often be obtained to within a few percent from passive radio-wave dynamic spectra when the electron plasma frequency fpe ($propto$ Ne1/2) is greater than the electron cyclotron frequency fce. This conclusion is based on interleaved active and passive observations from the Radio Plasma Imager (RPI) on the IMAGE satellite in the vicinity of the plasmapause. The Ne determinations are based on the frequency limits of an intense narrowband emission identified as the upper-hybrid band. The lower limit is identified with fpe and the upper limit with the upper-hybrid frequency fuh = (fpe2 + fce2)1/2. These frequency limits and the large amplitude of the emission, typically 20 dB or more above background, suggest strong Z-mode waves, rather than quasi-thermal fluctuations, as the emission source.