The location of the Jovian bow shock and magnetopause: Galileo initial results

The Galileo spacecraft observed multiple bow shock crossings and a single magnetopause crossing on its inbound approach to Jupiter in November 1995. We compare these observations to those obtained on previous missions, taking account of variable solar wind pressure and the disk-like shape of the magnetosphere. We find that the subsolar magnetopause location varies with solar wind dynamic pressure to the −0.22 power in contrast to the terrestrial −0.167 power. This difference is well known, and has been attributed to the presence of hot plasma and centrifugal stretching in the Jovian magnetodisk that lessens the pressure gradients in the outer magnetosphere. Our analysis also reveals for the first time that the magnetopause boundary of magnetosphere exhibits polar flattening, as has been previously inferred from the stretched magnetic field lines seen within the magnetosphere. The bow shock is not so asymmetric in shape. Finally,, we find that Galileo observed expanded boundaries early on its inbound pass, perhaps in part due to hotspot activity at Io in the fall of 1995 prior to the Galileo encounter. Observations within the magnetosphere are consistent with externally driven motion of the boundaries.