High resolution spectra of Io's neutral potassium and oxygen clouds.

The results of a short series of observations of Io's oxygen and potassium neutral clouds are reported. The long-slit spectra were taken 0.5 R_J_ from the disc of Io, with the slit parallel to the projection of Jupiter's rotation axis. [O I]λ6300 emission was detected at 8.8(+/-1.7)Rayleigh, in good agreement with the only other previously reported measurement of this emission remote from Io (Brown, ApJ 244, 1072, 1981). The spatial distribution of the emission suggests a dependence upon the position of Io with respect to the Io plasma torus (IPT). In addition to the well-known slow potassium cloud, observations of the K I λ7699 emission also showed a fast component. The properties of this component suggest either a charge-exchange mechanism between thermalised torus ions and neutrals in Io's upper atmosphere or removal of ions from Io's atmosphere with subsequent non-charge transfer ion-neutral collisions prior to a final charge exchange. The data provide no evidence of the dissociation of K-bearing molecular ions remote from Io.