Concentrating initially on the higher-quality 1997 data, we examined 260 images spanning 20 nights, looking for longitudinal asymmetries. The clearest example (though not the most dramatic) is the night of 1997/09/19 (figure 1). In images "torus06" through "torus08" the lower half of the eliptical arc of the torus (which corresponds to the part of the torus on the opposite side of Jupiter from us, and therefore the part approaching the ansa) is brighter than the upper half. In "torus09" they are equal, and in subsequent images the upper half is brighter until the torus arc becomes closed. This is consistent with a longitudinal enhancement starting at 70°-80° and ending at 160°-180° rotating through the field of view. We found asymmetries at similar longitudes on other nights in 1997, including 1997/10/11, three nights before the STIS data showing the dramatic brightening were acquired. Examination of our 1999 and 2000-01 data is ongoing; it is complicated by the lower quality of these data, and the fact that the torus is less "open" at the longitude of interest because of different viewing geometry.
To quantify this asymmetry, we took vertical slices through the east side of the torus in images taken at about 0° CML and compared the intensity of the two sections of the torus arc thus intersected; four such images are shown in figure 2. The two images from September 1997 show an enhancement of about 2, which is typical of all our images for that month. On October 11, 1997, however, the contrast is 6.2, and a comparable contrast is present on October 15. Little of the 1999 data have been examined yet, but thus far no significant contrast has been seen.
We then examined that part of our STIS data which covers the right longitude range, and for which the geometry reasonably separates torus emission from iogenic; the two such visits are shown in figure 3. Here we have modelled the emission from a nominal symmetric torus and determined what scaling factor will best fit the data (far from Io). On 1997/10/14, the model fits the data as is (scaling factor of ~1) initially, but affter ~70° longitude it leaps to 2 and then 2.7, consistent with the groundbased data. (It is still short of the groundbased factor of 6, perhaps because of the contribution of Io and/or a longitudinal temperature change, or perhaps because we are sampling the ribbon in these data, as opposed to the cold torus in the groundbased.) On 2000/02/20, the model fits the data after 70°, but the torus is undetectable beforehand--as if the enhancement exists, but in an overall reduced torus. (Indeed, returning to figure 2,it is clear that on 1997/10/14 the increased contrast is due at least partially to a decrease in intensity outside of 70°-180° System III.)
Finally, we revisited our Fabry-Perot scanning data from the 1980's (figure 4). At the time, we were puzzled to find that when plotted in System III, our data peaked at 130°-150°, which was contrary to conventional wisdom. Now, however, we note that it is consistent with our more recent data.