[Previous] | [Session 35] | [Next]
R.J. Oliversen (NASA/GSFC), F. Scherb (U Wisc), W.H. Smyth (AER), M.E. Freed (Ratheon ITSS), R.C. Woodward, Jr. (U Wisc), M.L. Marconi (Fresh Pond Res Inst), K.D. Retherford (JHU), O.L. Lupie (CSC)
Extensive Io [O I] 6300Å\ observations, covering the period 1990-1999, exhibit significant long-term and short-term intensity variations. The long-term average intensity shows a clear dependence on system III longitude, which establishes conclusively that the emission is produced by the interaction between Io's atmosphere and the plasma torus. Two prominent average intensity maxima, 70-90 degrees wide, are centered at system III longitudes ~130 and ~295. A comparison of data from October 1998 with a three-dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggest a basis for study of the torus interaction with Io's atmosphere. The observed short-term [O I] 6300Å\ intensity variations are separated from the underlying variations caused by large-scale structure of the plasma torus (e.g., electron ribbon location with local time, magnetic latitude, and system III longitudinal asymmetries), with typical fluctuations of 25-50% on timescales of tens of minutes and less frequent fluctuations of a factor of 2. The most likely candidate to produce these fluctuations is a time-variable energy flux of field-aligned nonthermal electrons identified recently in Galileo PLS data. This connection raises the possibility that time scales of [O I] 6300Å\ emission fluctuations may be related to scale sizes of inward and outward torus plasma transport cells and/or high magnetic latitude created double-layer electron beams. A positive correlation discovered between the intensity and the emission line width may indicate that molecular dissociation also contributes to the [O I] 6300Å\ emission. The nonthermal electron energy flux may explain this correlation through production of O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. This work is supported by NASA's Planetary Astronomy program.