Plasma flow in the Jovian magnetosphere and related magnetic effects: Ulysses observations

Abstract

The plasma flow in Jupiter's magnetosphere observed during the Ulysses flyby is compared with previous Pioneer, Voyager and ground-based observations. These data show that near-rigid corotation is enforced in the inner magnetosphere, but that the azimuthal flow plateaus at 150-300 kms^-1 in the middle magnetosphere plasma sheet beyond ~20 R_J. Such flows extend through the prenoon plasma sheet to ~45 R_J in the compressed magnetosphere observed by the Voyagers and to ~70 R_J in the expanded system observed by Ulysses. Higher speeds of ~500 kms^-1 occur in the postmidnight plasma sheet at 75-125 R_J in Voyager data, while preliminary Ulysses evidence is presented for anticorotation in the dusk plasma sheet beyond ~50 R_J. In the outer magnetosphere the dawn and dusk flank flows are antisunward at several hundred kilometers per second, while in the prenoon sector the flow appears to depend magnetospheric state, being corotational at 250-600 kms^-1 when compressed and anti-corotational (and radially in) at ~250 kms^-1 when expanded. These observations are compared with theories proposed by Hill and Vasyliunas, augmented to include the effects of solar wind coupling. This model accounts qualitatively for many features, but not for the anticorotation flows observed. We also compare the flows with the bending of the magnetic field out of meridian planes. Given the subcorotation-anticorotation nature of the observed flow, a pervasive ``lagging'' configuration is expected. This accords with in situ field data, except for the dusk outer magnetosphere observed by Ulysses, where a ``leading'' configuration was observed in the presence of subcorotating (downtail) flow. We conclude that field bending due to the tail-magnetopause current system dominates that due to ionospheric coupling on the dusk flank.