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P.E. Geissler (Univ. Arizona), W.H. Smyth (AER, Inc.), A.S. McEwen (Univ. Arizona), W. Ip (N.C.U., Taiwan, ROC), M.J.S. Belton (N.O.A.O.), T.V. Johnson (J.P.L.), A.P. Ingersoll (Cal. Tech.), K. Rages (NASA-Ames), W. Hubbard (Univ. Arizona), Galileo Imaging Team
The Solid State Imaging system on the Galileo spacecraft has detected colorful visible-wavelength emissions from Io's atmosphere while the moon was eclipsed by Jupiter (Geissler et al., Science, 6 August 1999). Diffuse emissions have been seen in 17 distinct observations so far, acquired during 15 eclipses over the course of 11 orbits. Recorded partly to monitor thermal emission from discrete volcanic centers, these images provide our first detailed look at visible aurorae on a solar system satellite.
Io's aurorae are expected to vary periodically with the changing orientation of the jovian magnetic field. Both the locations and the intensities of the emissions might vary because of fluctuations in the abundance of electrons that stimulate the emissions. One such effect is easy to discern in the available data: the faint red glow surrounding the limb is consistently brighter on the pole of Io closest to the jovian magnetic equator (the densest part of the plasma torus). The locations of the equatorial plume glows, while tied to known sites of volcanic activity on the surface, also appear to fluctuate in latitude - in most cases correlating with the positions of the tangent points of jovian magnetic field lines. However, we have not yet been able to find a systematic trend in the disk-integrated brightness of Io as a function of magnetic longitude, nor can we see a pattern to the brightness variations of the plume glows. Several factors complicate the analysis, including the changing volcanic activity on Io,variations in the viewing geometry of the observations, temporal changes in Io's atmosphere with time elapsed after the start of an eclipse, and the effects of moonlight reflected from nearby satellites and sunlight scattered into Jupiter's shadow by aerosols in Jupiter's upper atmosphere. Here we report progress in identifying and accounting for these effects.