Planet. Space Sci.,47, 205 - 224, 1999
Abstract:
The ExB velocity vectors which correspond to the field-perpendicular
flow of the low-energy plasma have been independently derived for the
Ulysses flyby of Jupiter using energetic ion and thermal electron
data. In this paper these measurements are compared. It is shown that
reasonable quantitive agreement exists during the prenoon inbound
pass, though with some significant differences. Both data sets
indicate the presence of slow field-perpendicular flow in the dayside
outer magnetosphere (~80-110 Rj), which are directed azimuthally
opposite to corotating with the planet and radially inwards, with
magnitudes of ~100 km s-1 in each component. Any variations in this
flow during the outer magnetosphere traversal are not resolved within
the ~ +/-100km s-1 uncertainties in the individual 6 min-averaged
data. Similar flows, but of somewhat smaller magnitude, are also found
in both sets in the higher-latitude region of the inbound middle
magnetosphere flanking the plasma sheet (~45-70 Rj). It is inferred
that these fields lines map equatorially into the outer magnetosphere
at larger distances. Flows within the middle magnetosphere plasma
sheets in this region are on average in the sense of planetary
rotation, but the average azimuthal velocity determined from the ATs
data (~20 km s-1, essentially consistent with zero) is significantly
smaller than that determined from the SWOOPS data (~100 km s-1). A
systematic effect thus seems to be present within the current sheet,
possibly associated with additional (usually small) terms in the
expression for the energetic ion anisotropy which have not been taken
into account in the analysis procedure. Analysis in the inbound data
overall, however, indicates no consistent velocity offset between
these data sets to within a few tens of km s-1, and a unit gradient
between them within a factor of ~1.5. Due to the large uncertainties
in individual @35 min velocities values, however, and the possible
current sheets effects mentioned above, the cross-correlation
coefficient between the data set is low, with an overall value of 0.23
for the principal azimuthal component observed in magnetospheric
region over a 4-day interval on the inbound pass. The probability of
this degree of correletaion appearing by chance, however is only
about one in 500. On the outbound pass, we find that the velocity
estimates determined from the two sets do not agree, even
qualitatively. We believe that this is due to a complicated and
anisotropic background in the electron data which we have been unable
to fully remove.
For preprints of this paper, please contact us.