Plasma and Magnetic Field
On 2 February, almost a week before closest approach, Ulysses crossed the Jovian bow shock at a distance of 113 Jupiter radii (1 Rj = 71 398 km) from the planet. The inbound crossing occurred somewhat earlier than expected based on previous observations by the Voyager spacecraft. A possible interpretation is that the solar wind ram pressure was low, allowing the magnetosphere to temporarily inflate, causing the bow shock to stand off further out from the planet. The magnetopause, the outer boundary of the magnetosphere, was first encountered only four hours after the bow shock crossing, at a distance of 110 Rj. The apparent proximity of these two boundaries, which are typically separated by 20 Rj, also suggested that they were moving rapidly outwards at that time.
Other results to emerge from magnetic field observations include the previously unknown configuration of the dusk side field, which is strongly swept back towards the magnetotail, and the realisation that large-scale current systems are very important in determining the configuration and dynamics of the magnetic field.