Problems concerning the radio emission from Jupiter observed by ULYSSES after encounter.

Abstract

Radio observations of Jupiter, made by the hi-band receiver of the Ulysses 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 observed by URAP is apparently quite different and several new features have been found. The polarization and spectral characteristics of this after encounter emission are reviewed for 230 planetary rotations (400=<R_J_=<2000). It is found that almost all of the emission is confined to frequencies of about 400kHz and below. Events showing some similarity to both hectometric radio emission (HOM) and broad-band kilometric radiation (bKOM) can be found after encounter although these occur at somewhat lower frequencies than those typical of the Voyager observations and such events are by no means prolific. The polarization is always predominantly left-handed (LH) and often exclusively so, even for events that correspond approximately to the Main Component of the bKOM which, before encounter, was found to be RH polarized in agreement with Voyager observations. There are other occasions when some unidentifiable type of emission persists for an entire jovian rotation or more, within a frequency band from about 400kHz to below 52kHz and into the lo-band section of the receiver. LH polarization also predominates strongly in this radiation. Some, 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 source regions are at high magnetic latitudes (L>=15) or low magnetic latitudes (L=<10). These two possible source locations are discussed with regard to the characteristics of HOM-type events observed after encounter at CMLs close to 290deg and, to a lesser extent, close to 100deg. It is shown, from geometrical considerations, that a CMI emission cone of half-angle about 20deg on L-shell ~7.5, corresponding to an invariant latitude ~69deg, could radiate LH polarized emission from the southern hemisphere towards the spacecraft, at a given frequency, both before and after the Ulysses encounter. RH emission, from similar cones on the same L-shell in the northern hemisphere, could only be received before encounter, however; consistent with observations. The alternative possibility of a larger emission cone angle at higher magnitude latitudes is also considered. In this case there is no unique value for the emission cone angle but a minimum angle of about 50deg can be inferred which is representative of the geometry for all larger cones. It is implicit, however, that in this latter case some LH emission must come over the southern magnetic pole of Jupiter to reach the spacecraft after encounter.