Neptune's radio emissions

Abstract

Neptune's radio emission was discovered by the Planetary Radio Astronomy experiment (PRA) a few days before Voyager 2 closest approach. However, its detection was possible as early as 30 days before encounter and 22 days after encounter. At least two types of emissions, a smooth and a bursty emission were recorded in the frequency range of 20 kHz to 1300 kHz. They differ by their spectral, temporal and polarization characteristics. The smooth emission exhibits both polarizations depending on the frequency of occurrence and on the longitude of Neptune (NLS); the pattern which is repeated before and after encounter is located in the same NLS range. The bursty emission consists of very short duration bursts, narrow-banded, and also strongly polarized. The bursts, as well as the smooth emission, can be described in terms of rotation in a period of 16.10 hours. The fore shock of Neptune's magnetosphere, the impacts of dust at ring-crossing and many different low frequency waves were also detected by the PRA experiment during the period of the encounter. We describe here the polarization response of the instrument in order to determine the true polarization of the incoming waves. At the closest approach the smooth radio source was occulted by the planet. By modeling the intensity profile at the encounter we have been able to conclude that there is at least one radio source in the auroral zone of the northern magnetic hemisphere where a good fit to the observations can be obtained when assuming an emission cone model. The implications on the magnetic field strength will be discussed.