Energetic ion and electron observations at Jupiter's dayside magnetopause: implications for magnetopause location and boundary coupling processes

Abstract

An analysis is presented of the fluxes and anisotropies of energetic (MeV) ions and electrons measured by instruments of the COSPIN package during the first inbound traversal of the Jovian dayside magnetospheric boundary region by the Ulysses spacecraft in February 1992. The aim of the study is to elucidate the nature of the multiple structures observed and reported previously in magnetic field, plasma and wave data. The combined data suggest the presence of a layer of depleted magnetosheath density which lay outside but adjacent to the magnetopause, and a boundary layer which lay within. The width of each of these structures is inferred to be ∼4 R J, assuming an outward boundary expansion speed of ∼100 km s −1. The magnetic field rotated slowly (over ∼10 min) towards the magnetospheric orientation just outside the depletion layer, and subsequently (after ∼50 min) turned rapidly through a small angle (∼20°) to point almost due south at the magnetopause. The major changes in energetic particle properties between interplanetary and outer magnetospheric values took place at the latter boundary. Energetic ion fluxes decreased sharply by a factor of ∼2 from interplanetary to magnetospheric values, while energetic electron fluxes increased by about an order of magnitude. Detailed analysis of the ion distributions indicates that the magnetopause was locally closed at the time of the crossing, i.e. a tangential discontinuity, as expected from the small magnetic shear present. Outside the magnetopause the electron fluxes had previously increased from background values by about two orders of magnitude over a 2 h period as the magnetosphere approached, indicating an escaping magnetospheric population in the magnetosheath similar to that at Earth. These electrons generally streamed antiparallel to the field away from the subsolar region, opposite to the observed streaming direction of the interplanetary ions in the magnetosheath. Inside the magnetospheric boundary layer the energetic ions and electrons exhibited weaker variable counterstreaming along the field lines, suggesting an intermittent presence of open flux tubes which were connected through the magnetopause first to the south and then to the north of the spacecraft. It is suggested that these changes were associated with switches in the east-west magnetic field in the magnetosheath, a suggestion which is supported by the observed magnetic field perturbations and field-per-pendicular ion flows within the layer. The fact that the spin-averaged electron flux did not show significant changes despite the field-aligned streaming is suggested to be due to a sufficiently rapid cross-field drift of these particles. In the outer magnetosphere proper the ion and electron distributions finally approach isotropy.