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

We present energetic (>20 keV) electron measurements from the Energetic Particle Detector on the Galileo spacecraft during recent (orbits 124 and 127) near encounters with Jupiter's satellite Io in the upstream (with respect to the plasma flow) and anti-Jupiter flank regions. No magnetic field-aligned electron beams were observed comparable to those observed during Galileo's J0 encounter within Io's plasma wake; only a hint of a weak field-aligned intensity enhancement was observed during the 127 flank encounter. Motivated by these new findings, the J0 wake observations are reexamined by combining high-energy (>20 keV) and recently published low-energy (<10 keV) electron beam components. Comparisons with observations of Earth auroral processes suggest that the Io electron beams are created at low Jovian altitudes 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 rather constitute part of the return current of the electric circuit that connects Io to Jupiter. During 127 Galileo entered a region depleted of energetic electrons that well matches modeled expectations of Io shadowing arising from combined electric and magnetic drifts. Also, within the shadow region Galileo moved onto field lines that are connected directly to Io or its atmosphere. Detailed angle distributions reveal the magnetic field strengths (B_Io) where the particles were absorbed at Io along Io's flanks. B_Io values are greater than the values at Galileo's location.