Smooth radio emission and a new emission at Neptune



The Planetary Radio Astronomy (PRA) experiment [Warwick et al., 1977] on the spacecraft Voyager 2 observed three types of smooth emission: (1) Numerous recurrent episodes are modeled by filled emission cones from midlatitude conjugate sources. (2) An “equatorial” feature seen soon after closest approach includes electron cyclotron harmonic emission above the upper hybrid resonance, as well as smooth recurrent emission; its strange appearance is a result of rapid change in Voyager's magnetic latitude. (3) Unique “high-latitude” emission is seen near closest approach during Voyager's single excursion to high north (+ ) magnetic latitude when fc, the electron cyclotron frequency at the spacecraft, lies in the observable range. The stronger component covers a broad band of frequencies above 2fc; its sensitivity to magnetic field identifies it as extraordinary (X) mode. The weaker component extends smoothly through f = fc and is identified as ordinary (O) mode. At each frequency f the observed sense of polarization reverses when f = 2fc.