Ulysses observations of noncorotational flows in the outer dayside Jovian magnetosphere

Abstract

Using data from the Anisotropy Telescopes on the Ulysses spacecraft obtained during the February 1992 encounter with Jupiter, we have derived the bulk velocity of ions in the energy range 0.7–1.3 MeV which were detected in Jupiter's outer magnetosphere (at a jovicentric distance of between ∼80 and ∼110 R J ). The derived ion bulk velocity typically has a component of ∼200 km s −1 in the sense opposite to corotation with Jupiter, and a component directed radially inward towards Jupiter, of comparable magnitude. Velocities consistent with appropriate corotation were only observed after Ulysses had entered Jupiter's middle magnetosphere at jovicentric distances of less than 80 R J . The ion energy spectrum at ∼ 1 MeV is well characterized by a power law, with a spectral index of ∼2.2–3 in the outer magnetosphere, but significantly less (∼2) in the solar wind, magnetospheric boundary layer, and magnetosheath plasma. The estimated proton to alpha ratio at ∼ 5 MeV is ∼2–5 in the outer magnetosphere, ∼ 2 in the magnetosheath and boundary layer, and ∼ 1.5 in the solar wind, immediately prior to the Jupiter encounter.