Sunlit Io atmospheric [O I] 6300 Å emission and the plasma torus

Abstract

A large database of sunlit Io [O I] 6300 Å emission, acquired over the period 1990–1999, with extensive coverage of Io orbital phase angle ϕ and System III longitude λIII, exhibits significant long‐term and short‐term variations in [O I] 6300 Å emission intensities. The long‐term average intensity shows a clear dependence on λIII, which establishes conclusively that the emission is produced by the interaction between Io's atmosphere and the plasma torus. Two prominent average intensity maxima, 70° to 90° wide, are centered at λIII ≈ 130° and λIII ≈ 295°. A comparison of data from October 1998 with a three‐dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggests a basis for study of the torus interaction with Io's atmosphere. The observed short‐term, erratic [O I] 6300 Å intensity variations fluctuate ∼20–50% on a timescale of tens of minutes with less frequent fluctuations of a factor of ∼2. The most likely candidate to produce these fluctuations is a time‐variable energy flux of field‐aligned nonthermal electrons identified recently in Galileo plasma science data. If true, the short‐term [O I] intensity fluctuations may be related to variable field‐aligned currents driven by inward and outward torus plasma transport and/or transient high‐latitude, field‐aligned potential drops. A correlation between the intensity and emission line width indicates molecular dissociation may contribute significantly to the [O I] 6300 Å emission. The nonthermal electron energy flux may produce O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. The [O I] 6300 Å emission database establishes Io as a valuable probe of the torus, responding to local conditions at Io's position.