Magnetic signatures of Jupiter's bow shock during the Cassini flyby

Abstract

The bow shock crossings detected during the Cassini flyby of Jupiter occur at positions which imply that Jupiter's bow shock was relatively inflated during the early (before 2001, Day 3) and late (after 2001, Day 40) periods of the encounter. A variety of magnetic shock signatures are seen, corresponding to a wide range (15–75 degrees) in the angle θBN between the shock normal and the solar wind field. The time variability of this angle for the “forward” part of the shock (near the nose) is generally consistent with a shock surface similar to that derived by Slavin et al. [1985] from Voyager/Pioneer data. This implies a long‐term stability for the global shape of the Jovian bow shock. Fits of Slavin‐type models to the shock crossing locations indicate that the solar wind pressure varied by an order of magnitude during the early phase of the Cassini flyby (up to 2001, Day 18) and that the bow shock itself was consequently in a highly dynamic state. A preliminary comparison of Cassini magnetometer (MAG) data with solar wind measurements by the Cassini Plasma Spectrometer (CAPS) reveals that the relation between the shock stand‐off distance and the solar wind pressure during the Cassini flyby was similar to that during the Voyager/Pioneer encounters. This indicates that the intrinsic “compressibility” of the Jovian magnetosphere is also probably stable over long timescales. The change in magnetic field direction between solar wind and magnetosheath indicates that the Jovian bow shock is fast magnetosonic, as expected for the high solar wind Mach numbers (>10) at the location of Jupiter.