Magnetospheric electron densities inferred from upper‐hybrid band emissions

Abstract

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 (∝ 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 f pe 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.