Ulysses observations of field-perpendicular plasma flows in the Jovian magnetosphere: comparison of ExB velocity vectors derived from energetic ion and thermal electron data

Abstract

The Ex B 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 quantitative agreement exists during the prenoon inbound pass, though with some significant differences. Both data sets indicate the presence of slow field-perpendicular flows in the dayside outer magnetosphere (∼80–110 R J), which are directed azimuthally opposite to corotation 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 ∼±100 km s −1 uncertainties in the individual 35 min-averaged data. Similar flows, but of somewhat smaller magnitude, are also found in both data sets in the higher-latitude region of the inbound middle magnetosphere flanking the plasma sheet (∼45–70 R J). It is inferred that these field lines map equatorially into the outer magnetosphere at larger distances. Flows within the middle magnetosphere plasma sheet 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 of 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 velocity values, however, and the possible current sheet effects mentioned above, the cross-correlation coefficient between the data sets is low, with an overall value of 0.23 for the principal azimuthal component observed in magnetospheric regions over a 4-day interval on the inbound pass. The probability of this degree of correlation appearing by chance, however, is only about one in 500. On the outbound pass, we find that the velocity estimates determined from the two data 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.