Solar wind–driven flows in the Jovian magnetosphere

Abstract

Ion anisotropies in the sub‐MeV/nucleon energy region have been measured during the inbound pass of Ulysses through the Jovian magnetosphere. Azimuthal flows in the direction opposite to corotation were detected at several different times, each lasting approximately hours, in the boundary layer and the outer magnetosphere. Similar flows were also observed in parts of the middle magnetosphere whenever Ulysses was far away from the plasma sheet. Such flows were not detected when the Voyager spacecraft traversed the dayside magnetosphere. This could be explained by the fact that Ulysses found the dayside outer magnetosphere in a greatly extended state, compared with the Voyager encounters. In addition, Ulysses also traversed the dayside middle magnetosphere at higher magnetic latitudes than the Voyager spacecraft. The plasma composition during periods of anticorotational flow was more like that measured during solar energetic particle events rather than that measured during the plasma sheet crossings, implying an external source, i.e., the solar wind. From the ion composition and energy spectra we show that 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” interactions, and impulsive penetration of plasmoids were ruled out on the basis of magnetic field measurements and charged particle distribution functions around the time of the outermost magnetopause crossing. Adapting recently formulated models of the situation in the terrestrial magnetosphere to Jupiter, we suggest that the anticorotational flows and solar 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 magnetosphere. In this model, anticorotational flows in the middle magnetosphere could also be caused by solar wind plasma entering the high‐latitude regions directly from the reconnection site.