Erratum: The printed version and the earlier electronic version of this figure incorrectly gave the dates as being in 1999; the correct year for both parts is 1997, as shown here.
In 1997 we initiated a synoptic imaging program targeting the Io plasma torus at [S II] 6731Å, using a custom-built imager on the McMath-Pierce west auxiliary telescope on Kitt Peak. This instrument, and data from the 1997 observing season (260 images during 20 nights spanning 40 days) were presented earlier [Woodward et al. 1999]; here we add data from the 1999 season (206 images during 11 nights spanning 32 days) and present preliminary results of our analysis of long-term variability in torus brightness and morphology. In particular, we simultaneously examine changes in: the brightnesses of the ribbon and cold torus (to search for System III and System IV variability); the scale height of the ribbon (to estimate ion temperature variations); and the positions of the ansae (to search for System III and convection electric field variability--example shown below). This work was supported by NASA grants NAG5-6787 and NAS5-30131.
Erratum: The printed version and the earlier electronic version of this figure incorrectly gave the dates as being in 1999; the correct year for both parts is 1997, as shown here.