Magnetospheric electron densities inferred from upper-hybrid band emissions

The magnetospheric electron density N-e can often be obtained to within a few percent from passive radio-wave dynamic spectra when the electron plasma frequency f(pe) (proportional to N-e(1/2)) is greater than the electron cyclotron frequency f(ce). 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 N-e 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 f(uh) = (f(pe)(2) + f(ce)(2))(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.