Understanding Io's space environment interaction: Recent energetic electron measurements from Galileo

We present energetic (> 20 keV) electron measurements from the Energetic Pa rticle Detector on the Galileo spacecraft during recent (orbits 124 and 127 ) near encounters with Jupiter's satellite lo in the upstream (with respect to the plasma flow) and anti-Jupiter flank regions. No magnetic field-alig ned electron beams were observed comparable to those observed during Galile o's JO encounter within lo's plasma wake; only a hint of a weak field-align ed intensity enhancement was observed during the 127 flank encounter. Motiv ated by these new findings, the JO wake observations are reexamined by comb ining high-energy (> 20 keV) and recently published low-energy (< 10 keV) e lectron beam components. Comparisons with observations of Earth auroral pro cesses suggest that the lo electron beams are created at low Jovian altitud es in a downward (with respect to Jupiter) electric current region. We thus suggest that the observed electron beams are not responsible for the spot- like auroral emissions from Jupiter at the magnetic foot point of Io, but r ather constitute part of the return current of the electric circuit that co nnects To to Jupiter. During 127 Galileo entered a region depleted of energ etic electrons that well matches modeled expectations of lo shadowing arisi ng from combined electric and magnetic drifts, Also, within the shadow regi on Galileo moved onto field lines that are connected directly to Io or its atmosphere. Detailed angle distributions reveal the magnetic field strength s (BIO) where the particles were absorbed at lo along lo's flanks. B-Io val ues are greater than the values at Galileo's location.