Polarization and localization of the Uranian radio sources

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Abstract
We describe a new direction finding method for studying low‐frequency radio emissions when observed from spacecraft. The method is based on the polarization measurement of the received radiation and works if the radio source contains a circularly polarized component. It is especially suitable in the case of the nonthermal, low‐frequency emissions of the planets, which are known to have a large degree of circular polarization. In order to apply this method to the Voyager 2 observations of the Uranian kilometric radiation (UKR), we develop a quantitative model of the polarization response of the planetary radio astronomy experiment (PRA) aboard the Voyager spacecraft. The model parameters, valid in the low‐frequency range where the antennas are electrically short, are obtained by analyzing several Voyager 1 and 2 maneuvers that occurred during the Saturn and Uranus encounters. We find that the PRA orthogonal monopoles can be accurately modeled by two crossed, quasi‐orthogonal dipoles; the plane of the equivalent dipoles is displaced by 23.3° ± 0.5° from the physical antenna plane in the opposite direction to the magnetometer boom. By analyzing an inbound maneuver, when the spacecraft was at 8 RU from the planet, the source of the low‐frequency UKR component (or smooth low‐frequency component (SLF)) is found to lie in the northern (dayside) magnetic hemisphere, in a restricted range of magnetic longitudes. By assuming that the radio source emits at the local electron gyrofrequency, an independent analysis of the intrisic (not due to the antenna geometry) polarization reversal, observed near the magnetic equator crossing, leads to the same source location. After correction for the antenna response, the SLF emission is found to be 100% circularly polarized, the sense of polarization being in accordance with an O mode emission. The direct localisation of the broadband component (or smooth high‐frequency component (SHF)), obtained from an outbound spacecraft maneuver at 12 RU from Uranus, is in agreement with a radio source in the southern (nightside) magnetic hemisphere. However, the analysis of the polarization reversal, due to the slowly change of the source‐antenna geometry when the spacecraft was passing round the planet at 6 RU, gives inconsistent results, suggesting that more than a single source were observed at that time. The observed circular polarization degree is large, and the polarization sense corresponds to an X mode emission. During one of the spacecraft maneuvers, the SLF radio source could be directly mapped with an angular resolution corresponding to a small fraction of one planetary radius. The source location is found to depend on the frequency, the higher frequencies being observed at lower altitudes from the planet, in agreement with the usually assumed morphology for the planetary auroral radio sources. Despite of the high precision of the method, more refined results could not be obtained, probably because of propagation effects close to the radio source.

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