Detection of [O II] λ2471 from the Io plasma torus

We report the first detection of [O II] λ2471 from the Io plasma torus. Observations of the east and west torus ansae at 5.7R_J on 1992 March 15 and May 1, respectively, using the Goddard High Resolution Spectrograph of the Hubble Space Telescope have resulted in ∼5 σ detections of the line on both occasions, with line brightnesses of 30 ± 5R and 28 ± 5R for the east and west ansae respectively in a 2″ × 2″ field of view. The lines are Doppler-shifted relative to the rest wavelength (vacuum) of 2471.04 Å, with a separation of 1.30 ± 0.25 Å between the east and west lines, implying a torus rotation velocity of 79 ± 15 km s^-1, consistent with that expected at a radial distance of 5.7R_J (≲71.6 km s^-1). Assuming a torus electron temperature and density distribution consistent with Voyager 1 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. However, there is a statistically significant brightness variation between the two west ansa observations which cannot be unambiguously interpreted due to the limited data.