Jovicentric latitude effect on the HOM radio emission observed by Ulysses/URAP at 5 AU from Jupiter

Abstract. During 1994 and into 1996, Ulysses was at dis-tances of 5 AU or more from Jupiter and travelling from south to north of the ecliptic plane between jovicentric latitudes -36° to 20°. Observations by the Unified Radio and Plasma Experiment (URAP) on board the Ulysses spacecraft during this period have been searched for jovian hectometric (HOM) radio events. At these distances, the HOM was only received occasionally. The signals were generally weak and much care was needed to find and to identify the events. All of the HOM events were observed when Ulysses was at jovicentric latitudes between -12.2° < D_Uly < 14.7°, relatively close to the plane of the jovicentric equator. Both senses of polarization were observed with left-hand (LH) predominant. The events occurred when the jovimagnetic latitude D_f was between -8.5° and 14.2° and suggest that the HOM was only detectable within a beam some 23° wide, centred on about 3° jovimagnetic latitude. This is roughly consistent with previous work by Alexander et al. (1979) and by Ladreiter and Leblanc (1989, 1991), based upon observations made by Voyager and other spacecraft when these were relatively close to Jupiter. The results are consistent with an emission process due to the Cyclotron-Maser instability, as suggested by a number of scientists in the past.

Key words. Magnetospheric physics (planetary magnetospheres) – Radio science (radio astronomy)