Energetic electron beams and trapped electrons at Io

We present results from a continuing study of the magnetic field-aligned en ergetic electron beams and accompanying trapped electron distributions disc overed during the Galileo satellite's passage through Io's cold, dense, low -speed wake on December 7, 1995 [Williams et al., 1996]. A companion paper by Thorne et al. [this issue] presents an analysis and explanation of the e volution of the electron pitch angle distributions measured on approach to Io and describes the resulting energetic electron flow paths and adiabatica lly "forbidden" regions expected to exist around Io. In Io's wake, only bid irectional, field-aligned electron beams are seen; no ion beams are observe d. At energies >similar to 15 keV the measured beams represent an energy fl ow of similar to 0.03 erg cm(-2) s(-1), and if they penetrate Jupiter's atm osphere, they can provide an energy deposition of similar to 15 erg cm(-2) s(-1) at the foot of the Io flux tube. This is sufficient to stimulate obse rvable aurora in Jupiter's atmosphere. Extrapolating the measured spectra t o lower energies yields much higher values (e.g., similar to 10(4) ergs cm( -2) s(-1) at 0.25 keV). The angular width of the measured trapped-like elec tron distributions is independent of energy and varies across Io's wake in a manner consistent with the measured magnetic field variation. We conclude that these electrons are trapped in Io's magnetic field configuration. The narrowness of the beams and the simultaneous existence of an apparently un accelerated trapped electron population provide evidence that the source re gion for the beams is close to Jupiter. A deconvolution of the detector res ponse to the beams gives a beam angular half width of similar to 6 degrees, placing the formation of the beams at an altitude of similar to 0.6-0.7 R- j. The slight energy dependence of the beam width provides a rough upper li mit estimate of <similar to 1.8 (10)(8) cm(-3) for Io's neutral SO, atmosph ere in the flyby region. No proposed acceleration mechanism operating close to Io (neither double layers nor Fermi acceleration via propagating Alfven waves) is able to reproduce measured beam characteristics.