Io's sodium emission cloud

Strong evidence that Io's sodium emission is due to resonant scattering is given by our observations which show a monotonic increase of emission intensity with residual solar intensity. In addition we detected no emission during three eclipse observations of Io. We propose a resonant scattering model with two spacial components comprising an optically thick atmosphere extending 10³ km above Io's surface surrounded by an optically thin cloud which forms a partial torus around Jupiter. In this model a flux of 10⁷ cm^?2 sec^?1 sodium atoms are sputtered from Io's surface by heavy energetic ions which are accelerated in a plasma sheath around Io. The atoms sputtered from the surface collide with atoms in Io's atmosphere so the equipartition of kinetic energy is established. The total sodium abundance is about 3 × 10¹³ cm^?2. During Io's day, sodium and other atmospheric constituents are ionized, giving rise to the ionosphere observed by Pioneer 10. Atoms escape by means of Jeans escape from the critical level, which is at the top of the atmosphere and the base of the cloud. We have observed sodium emission 6arcsec (6 Io diameters) above and below Io's orbital plane and 23arcsec toward Jupiter in Io's orbital plane. No emission was detected at maximum elongation 180° from Io. We interpret these results to mean that atoms escaping from Io form a partial torus whose thickness is about 12 arcsec and whose length is at least one-fifth of Io's orbital circumference.