Origins of the first-order anisotropy of ∼ 1 MeV protons in the Jovian magnetosphere during the Ulysses flyby: flux gradients and plasma flows

Abstract

For zeroth-order distributions of energetic ions which are not too far from isotropy, the field-perpendicular first-order anisotropy results principally from the E × B plasma flow transverse to the magnetic field, and from spatial gradients in the ion flux. A technique is presented which separates these sources of anisotropy by examining their dependence on the ion energy. This technique is applied to data obtained by the Anisotropy Telescopes instrument on the Ulysses spacecraft during the Jupiter flyby in February 1992, using data from adjacent proton channels centred at 1.0 and 1.75 MeV. The results provide a continuous monitoring of both the field-transverse E × B plasma flow and the spatial gradient of ∼ 1 MeV protons along the flyby trajectory, which traversed the near-equatorial prenoon magnetosphere inbound, and the dusk magnetosphere at moderate southerly latitudes outbound. It is shown that during the Ulysses flyby the flux gradient effect contributed significantly to the first-order anisotropy of ∼ 1 MeV protons in all the magnetospheric regions sampled, and was generally dominant in the middle magnetosphere plasma-current sheet (where the e-folding scale length was ∼ 1 R J or less), and on field lines inferred to be mapping thereto. Both inbound and outbound, the results on plasma velocities may be summarized as indicating antisunward flow at ∼ 200 km s − in the outer magnetosphere, a region ∼ 30 R J wide in the equatorial plane inside the magnetopause. The plasma inside this in the middle magnetosphere plasma sheet flowed sunwards at similar speeds. The latter statement applies over a ∼ 30 R J wide region of the equatorial plasma-current sheet from the outer edge at 70–80 R J inwards to ∼ 45 R J, the latter being the minimum equatorial distance of observations of these field lines due to the off-equatorial nature of the spacecraft trajectory. This pattern suggests that the flow observed is driven principally by input of solar wind momentum at the magnetopause rather than by planetary rotation, though the momentum transfer mechanism remains uncertain. This result further suggests that solar wind-driven flows may play a more significant role in Jovian magnetospheric dynamics than previously supposed, particularly when the system is in an expanded state.