THE UNCERTAINTY OF THE URANIAN RADIO-SOURCE LOCATION DUE TO THE NONUNIQUENESS OF THE PLANETARY MAGNETIC-FIELD MODEL

Since the Voyager 2 encounter in early 1986, several investigators hav e attempted to localize the source regions of the smooth high-frequenc y radio emission which was observed by the planetary radio astronomy e xperiment at the nightside of Uranus. The various studies (most of the m are based on the offset tilted dipole (OTD) model of the Uranian mag netic field) yielded significantly different source locations around t he southern magnetic pole of Uranus. This may be a consequence of the individual a priori assumptions of the source model. However, the simp licity of the OTD model (Ness et al., 1986) also cannot adequately rep resent the complexity of the magnetic field at the radio source locati ons near the planet. The aim of this study is twofold. (1) We reanalyz e the various source locations given in the literature (most of them a re based on the OTD model) in the frame of the Q3 magnetic field model (Connerney et al., 1987). Our analysis moves some of the previously d etermined source locations from open toward closed field lines; howeve r, the uncertainty due to the nonuniqueness of the Q3 model remains to o large to exclude the possibility that open field lines are the sourc e of smooth Uranian kilometric radiation. (2) We calculate the uncerta inty of the radio source locations imposed by the nonuniqueness of the Q3 and OTD magnetic field models. We construct solutions by using gen eralized inversion techniques (Connerney, 1981) to obtain estimates of those magnetic field parameters (spherical harmonic coefficients up t o degree and order 6) that are constrained by the magnetometer observa tions. The nonuniqueness of the resulting magnetic field models transl ates into an uncertainty about the radio source locations of some 20-d egrees in Uranocentric coordinates at altitudes of about 1.5 Uranian r adii (R(U)). The present results are important for radio source locati ons at all the outer planets whose magnetic field geometries are repre sented by nonunique magnetic field models.