Jupiter Science Results - Plasma and Magnetic Field

A fundamental contribution provided by the plasma (SWOOPS) and magnetic field (FGM/VHM) experiments is the identification of the boundaries between the various magnetospheric regions encountered during the flyby (Fig.3). This "road map" is a useful tool that helps other experiments to place their observations in the correct context. 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 (see table below). 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 suggests that they were moving rapidly outwards at that time. On the outbound leg, multiple magnetopause and bow shock crossings were observed as these boundaries moved inwards then outwards across the spacecraft. This is again indicative of the "elasticity" of the magnetosphere in response to changing solar wind conditions.

Figure 3: Colour-coded spectrogram of the entire 15-day Ulysses flyby from the SWOOPS electron plasma instrument. Electron energy spectra summed over all look directions are displayed using the colour bar shown on the right to code the count rate.

 

 

 

Other results to emerge from magnetic field observations (Fig.4)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.

Figure 4: The magnetic field magnitude measured by the FGM/VHM experiment during (a) the inbound pass and (b) the outbound pass is plotted versus radial distance from the planet. The dashed line superimposed on the data denotes the predicted model field.