Local time asymmetry of energetic ion anisotropies in the Jovian magnetosphere

Since December 1995 the Galileo spacecraft is in orbit around Jupiter. Up to now the spacecraft performed 25 orbits through the Jovian system within nearly 4 years providing an excellent data coverage in local time specifically for distances r<40RJ. We present first-order anisotropies measured in energetic ion distributions from the Energetic Particles Detector (EPD) onboard Galileo. In this paper, we concentrate on measurements in the inner Jovian magnetosphere at distances between 6 and 40RJ. Results from three different ion species show a pronounced local time asymmetry in the ion distributions at these distances especially between the dawn–prenoon and the dusk–premidnight sector of the magnetosphere. The predominantly first-order anisotropies in the co-rotation direction show larger amplitudes with radial outward components in the dawn sector whereas at dusk smaller anisotropies with small radial inward components are observed. Under the reasonable assumption that the anisotropies are primarily due to flow, we conclude that the flow pattern is asymmetric in local time. The flow in the dusk sector between 10 and 40RJ is reduced compared to the dawn region.