Influence of solar wind on Jupiter's magnetosphere deduced from currents in the equatorial plane

Galileo is the sixth spacecraft to have visited Jupiter's magnetosphere. Among them the six spacecraft have sampled a fairly large expanse of magnetospheric local times and latitudes under varying solar wind conditions. An adequate (though not optimum) database of field and particles now exists that makes it possible to map the current circuits at a global scale. We have used the magnetic field observations from all six of the spacecraft to compute the electric current density in the equatorial plane of Jupiter's magnetosphere by making certain appropriate assumptions. We show that in the middle magnetosphere the azimuthal currents are much stronger on the nightside (∼ 144 MA between the radial distances of 10 and 50 R J ) than they are on the dayside (∼ 88 MA in the same distance range). From current continuity considerations we conclude that the nightside partial ring current is fed and emptied by field‐aligned currents in the dusk and the dawn sectors, respectively. These currents are similar (but opposite in polarity) to Region 2 field‐aligned currents observed in the Earth's magnetosphere. Because the presence of Region 1 or 2 sense field‐aligned currents in a magnetosphere indicates the presence of solar wind driven convection in a magnetosphere, a surprising conclusion of the present analysis is that the solar wind influence reaches deep into the heart of Jupiter's magnetosphere. Other findings of this study are that (1) the equatorial field strength is remarkably constant over all local times, (2) the equatorial source of the outward field‐aligned currents required for the generation of aurorae is located between the radial distances of 10 and 30 R J with a peak near 20 R J , and (3) the Jovian magnetosphere displays a magnetic field configuration intermediate to a Parker spiral and a magnetosphere driven by solar wind.