Figure 3: [S II] 1256Å emission near Io observed with STIS on two different dates, showing possible longitudinal torus asymmetry. Each plot is a profile of emission in the 2" × 25" field of view, as a function of distance from Io. Each dot is the average of emission in a 0.1° radial bin in a 60° wedge around the long direction. The vertical dotted lines mark Io's position. The dashed lines show the underlying torus emission, predicted by a model based on Voyager data; the solid line is the same model scaled and offset so as to give the best fit to emission more than 10 Io radii from Io. These scaling factors (Table 1) undergo a significant increase as Io passes a System III longitude of ~80°.