Jovicentric latitude effect on the bKOM radio emission observed by Ulysses/URAP

¹ Max-Planck-Institut für Aeronomie, 37189 Katlenburg-Lindau, Germany
² ARPEGES, Observatoire de Paris, 92195 Meudon, France
³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Corresponding author: C. H. Barrow, barrow@linmpi.mpg.de

Received: 29 August 2000
Accepted: 3 November 2000

Abstract

During 1994 and into 1996, Ulysses was at distances of 5 AU or more from Jupiter and travelling from south to north of the ecliptic plane. At these distances, the jovian broadband radio emission (bKOM) was only received occasionally by the Unified Radio and Plasma Experiment (URAP) on board the Ulysses spacecraft. The signals were generally weak and much care is needed to find and to identify the events. It is found that the visibility and polarization of the bKOM appears to depend upon the jovicentric latitude of Ulysses (D_Uly) at the time of observation. All but two of the bKOM events observed when Ulysses was at northern jovicentric latitudes were predominantly right-hand (RH) polarized while events recorded when the spacecraft was at southerly jovicentric latitudes were all predominantly LH polarized, the change taking place somewhere between , close to jovimagnetic latitude. Compared with previous observations of the bKOM, made by spacecraft considerably closer to Jupiter, the present occurrence probabilities were lower for the LH polarized events although the distribution was similar. For the RH polarized events, however, the distribution was different, the so-called main peak being absent or, perhaps, displaced towards a larger central meridian longitude. It is shown that, in a two-dimensional model, if cyclotron maser emission in a dipole magnetic field is assumed for Jupiter, the detection of bKOM at a given frequency by a spacecraft at a specified location, determines a unique value of β for an assumed value of L and a given field model. This is not true for a three-dimensional model. The present results raise questions on the application of the cyclotron maser theory to bKOM emission.