Energetic electron beams and trapped electrons at Io

Abstract

We present results from a continuing study of the magnetic field‐aligned energetic electron beams and accompanying trapped electron distributions discovered 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 evolution of the electron pitch angle distributions measured on approach to Io and describes the resulting energetic electron flow paths and adiabatically “forbidden” regions expected to exist around Io. In Io's wake, only bidirectional, field‐aligned electron beams are seen; no ion beams are observed. At energies >∼15 keV the measured beams represent an energy flow of ∼0.03 erg cm−2 s−1, and if they penetrate Jupiter's atmosphere, they can provide an energy deposition of ∼15 erg cm−2 s−1 at the foot of the Io flux tube. This is sufficient to stimulate observable aurora in Jupiter's atmosphere. Extrapolating the measured spectra to lower energies yields much higher values (e.g., ∼104 ergs cm−2 s−1 at 0.25 keV). The angular width of the measured trapped‐like electron 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 unaccelerated trapped electron population provide evidence that the source region for the beams is close to Jupiter. A deconvolution of the detector response to the beams gives a beam angular half width of ∼6°, placing the formation of the beams at an altitude of ∼0.6–0.7 RJ. The slight energy dependence of the beam width provides a rough upper limit estimate of <∼1.8 (10)8 cm−3 for Io's neutral SO2 atmosphere 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.