Low-frequency radio emissions at Neptune



The Voyager 2 plasma wave receiver detected weak radio emissions from Neptune's magnetosphere in the frequency range of 3 – 60 kHz. The emissions occurred in bursts lasting for typically 1.5 hours, often occurring twice per planetary rotation. Most of these radio bursts were detected within several degrees of the magnetic equatorial plane. During the passage through the magnetosphere, electrostatic upper hybrid resonance bands were observed close to the magnetic equator in conjunction with intensifications of the radio emissions at frequencies close to and above the upper hybrid bands. Further, near closest approach, the radio emissions were observed to cross the right-hand cutoff frequency with no apparent attenuation. We conclude that the Neptunian radio emissions below about 60 kHz are produced by mode conversion from the upper hybrid waves and propagate in the ordinary mode into beams within about 12° of the magnetic equator. There is also evidence of an extraordinary mode emission at about 60 kHz which is apparently generated by an entirely different source from the escaping continuum radiation.