Intense electron beams observed at Io with the Galileo spacecraft

On December 7, 1995, the plasma instrumentation (PLS) on board the Galileo spacecraft detected intense low‐energy electron beams which were aligned along the magnetic field during the close approach of Jupiter's moon Io. The energies of these electrons were in the range of several hundred eV. An average field‐aligned current density of ∼0.05 nA/cm2 associated with the electron beams was measured during three consecutive samples of the electron velocity distributions. The total current in this beam is ∼1 × 106 A and in the range of the current magnitudes of 5 × 106 A previously reported at larger distances with the magnetometer on Voyager 1. Possible contributions to these currents from the thermal electrons and ions were not measured by the Galileo PLS. Estimates of the total energy influxes in the Io footprint in the Jovian atmosphere are in the range of 3 × 1010 W and similar to those obtained from observations of the Jovian far‐ultraviolet emissions with the Hubble Space Telescope. These electron beams are embedded in cool, dense thermal plasmas with kT ≃ 4 eV, a temperature which is similar to that observed in the adjacent torus plasmas which are corotating with Jupiter.