Galileo at Jupiter: Changing states of the magnetosphere and first looks at Io and Ganymede

Abstract

Several investigations based on data from the magnetometer on the Galileo Orbiter are described. From Galileo's initial inbound pass, we learn that the magnetosphere can experience large changes in its magnetic configuration. Between 50 and 30 R j, the radial magnetic forces on the plasma were larger in 1995 than in 1973 when Pioneer 10 passed through the same region of the magnetosphere, implying that either external or internal plasma forces were also larger. Although there are several ways to interpret the change of the magnetic configuration, we suggest that the variations are governed principally by the solar wind dynamic pressure and that the dayside magnetosphere as far in as ∼30 R j may be more strongly affected by the solar wind than has previously been recognized. Minor effects of higher mass loading may also be present. Data from the flyby of Io show a large magnetic perturbation; we argue that it is plausible that Io has an intrinsic magnetic field and that also contributions from plasma perturbations are significant. We find unexpected evidence that molecular ions are being picked up over a large spatial region in the vicinity of the moon. During the pass by Ganymede we observed a large magnetic perturbation consistent with an intrinsic dipole field. The multiple flybys of Ganymede scheduled for later portions of Galileo's mission will allow us to test our understanding of the magnetic signature.