Figure 4: System III variation in Fabry-Perot spectra of the plasma torus. Top: [S II] 6731Å scans from 1988, with best-fit sine curve; peak brightness is at ~130° System III longitude. (This variation does not have independant statistical significance.) Bottom: [S III] 9531Å scans from 1982, with best-fit sine curve, before (dots and dotted line) and after (crosses and solid line) removing an independant 10.2 hour periodicity; peak brightess is at ~150° System III longitude.