Source region of the smooth high-frequency nightside Uranus kilometer radiation: A ray-tracing study

Assuming the cyclotron maser instability is the source mechanism of the smooth high-frequency, nightside component of Uranian kilometric radiation, a plasma density model is developed for the nightside at r<4.2 r_u based on radio wave observations of the Planetary Radio Astronomy instrument on board the Voyager 2 spacecraft. Using this plasma model and the Q3 magnetic field model (Connerney et al., 1987), a systematic ray tracing of right-hand extraordinary (RX) emission is performmed from a grid of nightside sources along magnetic field lines at frequencies just above the RX cutoff. By requiring that the rays satisfy several observational constraints, we are able to specify the source region with more accuracy than previously prescribed. Our results indicate a source region extended in longitude compared to those previously published. In addition, the emission at higher frequencies that exhibits a periodic ``bite-out'' appears to be observed by the satellite at wave normal angles in the range 90°<Ψ<140°. The emission observed near closest approach has a difficult source region and may have different radiation lobe characteristics. These results are consistent with the cyclotron maser instability for moderately low densities in the source region (f_p/f_ce<0.25, where f_p and f_ce are the plasma and gyrofrequency, respectively).