SOLAR WIND-DRIVEN FLOWS IN THE JOVIAN MAGNETOSPHERE

Ion anisotropies in the sub-MeV/nucleon energy region have been measur ed during the inbound pass of Ulysses through the Jovian magnetosphere . Azimuthal flows in the direction opposite to corotation were detecte d at several different times, each lasting approximately hours, in the boundary layer and the outer magnetosphere. Similar flows were also o bserved in parts of the middle magnetosphere whenever Ulysses was far away from the plasma sheet. Such flows were not detected when the Voya ger spacecraft traversed the dayside magnetosphere. This could be expl ained by the fact that Ulysses found the dayside outer magnetosphere i n a greatly extended state, compared with the Voyager encounters. In a ddition, Ulysses also traversed the dayside middle magnetosphere at hi gher magnetic latitudes than the Voyager spacecraft. The plasma compos ition during periods of anticorotational flow was more like that measu red during solar energetic particle events rather than that measured d uring the plasma sheet crossings, implying an external source, i.e., t he solar wind. From the ion composition and energy spectra we show tha t solar wind interaction may be an important factor in determining the plasma flow in many regions of the dayside magnetosphere. Mechanisms such as large-scale magnetic reconnection, ''viscous-like'' interactio ns, and impulsive penetration of plasmoids were ruled out on the basis of magnetic field measurements and charged particle distribution func tions around the time of the outermost magnetopause crossing. Adapting recently formulated models of the situation in the terrestrial magnet osphere to Jupiter, we suggest that the anticorotational flows and sol ar wind-like composition are caused by ''patchy'' reconnection at high latitudes. Plasma from the reconnected flux tube forms a low-latitude boundary layer, from which the solar wind plasma enters the outer mag netosphere. In this model, anticorotational flows in the middle magnet osphere could also be caused by solar wind plasma entering the high-la titude regions directly from the reconnection site.