Detection of [O ii] lambda 2471 from the Io Plasma Torus

Abstract

We report the first detection of the O II 2471-A forbidden line from the Io plasma torus. Observations of the east and west torus ansae at 5.7 R(J) on 1992 March 15 and May 1, respectively, using the Goddard High Resolution Spectrograph of the HST have resulted in about 5-sigma detections of the line on both occasions, with line brightnesses of 30 +/- 5 R and 28 +/- 5 R for the east and west ansae respectively in a 2 x 2 arcsec FOV. The lines are Doppler-shifted relative to the rest wavelength (vacuum) of 2471.04 A, with a separation of 1.30 +/- 0.25 A between the east and west lines, implying a torus rotation velocity of 79 +/- 15 km/s, consistent with that expected at a radial distance of 5.7 R(J). Assuming a torus electron temperature and density distribution consistent with Voyager I in situ measurements and homogeneous mixing of O(+) relative to electron density along the line of sight, the observed line brightnesses imply an average O(+) mixing ratio of 0.26 +/- 0.05, in agreement with a value derived previously by Moos et al. (1991), but lower than that derived from Voyager observations. There is no significant difference in average brightness between the east (dawn) and west (dusk) ansa observations.