PROBLEMS CONCERNING THE RADIO-EMISSION FROM JUPITER OBSERVED BY ULYSSES AFTER ENCOUNTER

Radio observations of Jupiter, made by the hi-band receiver of the Uly sses Unified Radio and Plasma (URAP) experiment before encounter, are generally consistent with observations of Jupiter made by the Voyager Planetary Radio Astronomy (PRA) experiment. After encounter, however, presumably due to the effects of beaming towards the extreme southerly latitude of the spacecraft, the character of the radio emission obser ved by URAP is apparently quite different and several new features hav e been found. The polarization and spectral characteristics of this af ter encounter emission are reviewed for 230 planetary rotations (400 l ess than or equal to R(J) less than or equal to 2000). It is found tha t almost all of the emission is confined to frequencies of about 400 k Hz and below. Events showing some similarity to both hectometric radio emission (HOM) and broad-band kilometric radiation (bKOM) can be foun d after encounter although these occur at somewhat lower frequencies t han those typical of the Voyager observations and such events are by n o means prolific. The polarization is always predominantly left-handed (LH) and often exclusively so, even for events that correspond approx imately to the Main Component of the bKOM which, before encounter, was found to be RH polarized in agreement with Voyager observations. Ther e are other occasions when some unidentifiable type of emission persis ts for an entire jovian rotation or more, within a frequency band from about 400 kHz to below 52 kHz and into the lo-band section of the rec eiver. LH polarization also predominates strongly in this radiation. S ome, if not all, of the after encounter emission is presumably due to the cyclotron maser instability (CMI) which has been evoked to explain the generation of HOM although opinions differ as to whether the sour ce regions are at high magnetic latitudes (L greater than or equal to 15) or low magnetic latitudes (L less than or equal to 10). These two possible source locations are discussed with regard to the characteris tics of HOM-type events observed after encounter at CMLs close to 290 degrees and, to a lesser extent, close to 100 degrees. It is shown, fr om geometrical considerations, that a CMI emission cone of half-angle about 20 degrees on L-shell similar to 7.5, corresponding to an invari ant latitude similar to 69 degrees, could radiate LH polarized emissio n from the southern hemisphere towards the spacecraft, at a given freq uency, both before and after the Ulysses encounter. RH emission, from similar cones on the same L-shell in the northern hemisphere, could on ly be received before encounter, however; consistent with observations . The alternative possibility of a larger emission cone angle at highe r magnitude latitudes is also considered. In this case there is no uni que value for the emission cone angle but a minimum angle of about 50 degrees can be inferred which is representative of the geometry for al l larger cones. It is implicit, however, that in this latter case some LH emission must come over the southern magnetic pole of Jupiter to r each the spacecraft after encounter.