PROBLEMS CONCERNING THE RADIO-EMISSION
FROM JUPITER OBSERVED BY ULYSSES AFTER
ENCOUNTER
BARROW CH
LECACHEUX A
ASTRONOMY AND ASTROPHYSICS
v.301, n.3, SEP, 95, p.903-913
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 se veral new features have been found. The polarization and spectral
characteristics of this after encounter emission are reviewed for 230 planetary rotations (400
less than or equal to R(J) less than or equal to 2000). It is found that almost all of the
emission is confined to frequencies of about 400 kHz 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 o bservations 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 400 kHz to below 52 kHz 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 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 characteristics of HOM-type events observed after
encounte r at CMLs close to 290 degrees and, to a lesser extent, close to 100 degrees. It is
shown, from geometrical considerations, that a CMI emission cone of half-angle about 20
degrees on L-shell similar to 7.5, corresponding to an invariant latitude similar to 69 degrees,
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 50 degrees 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 sp acecraft after encounter.