Global flows of energetic ions in Jupiter's equatorial plane: First-order approximation

the opportunity to study global energetic ion distributions in Jupiter's ma gnetosphere. We present directional anisotropies of energetic ion distribut ions measured by the Galileo Energetic Particles Detector (EPD). The EPD me asurements of proton (80-1050 keV), oxygen (26-562 keV/nucteon), and sulfur (16-310 keV/nucleon) distributions cover a wide energy range. Spatially, t he data set includes measurements from 6 to 142 Jovian radii (R-J) and cove rs all local times inside the Jovian magnetosphere. For each species a sing le detector head scans almost the entire sky (approximate to 4 pi sr), prod ucing the three-dimensional angular distributions from which the anisotropi es are derived. Consequently, the resulting anisotropy estimates are both g lobal and robust. Such anisotropies, generally produced by convective flow, ion intensity gradients, and field-aligned components, have long been used to estimate flow velocities and to locate spatial boundaries within magnet ospheres. They can therefore provide vital information on magnetospheric ci rculation and dynamics. We find that the EPD measured anisotropies in the J ovian magnetosphere are dominated by a component in the corotational direct ion punctuated by episodic radial components, both inward and outward. Unde r the assumption that anisotropies are produced predominantly by convective flow, we derive flow velocities of protons, oxygen ions, and sulfur ions. The validity of that approach is supported by the fact that these three ind ependently derived flow velocities agree, to a large extent, in this approx imation. Thus, for the first time, we are able to derive the global flow pa ttern in a magnetosphere of an outer planet. In a comparison between the fi rst-order EPD flow velocities and those predicted by a magnetohydrodynamic (MHD) simulation of the Jovian magnetosphere, we find that qualitatively th e directions appear similar, although no firm evidence of steady outflow of ions has been observed at distances covered by Galileo. A first rough comp arison indicates that the measured first-order flow velocities are higher b y at least a factor of 1.5 than the MHD simulation results.