Excitation of the Sodium D Line Emission Observed in the Vicinity of Io

Abstract

The D2/D1 ratio observed for the sodium emission around Io appears too large for that emission to be only resonant scattering of the solar flux. An optically thick source close to the surface of Io, with an intensity comparable to that observed, could be a significant excitation source. This extra flux, in combination with the solar flux, would cause the D2/D1, ratio to he considerably greater than the value of 1.8 expected for the solar flux alone. A non-thermal mechanism is probably required to raise sufficient sodium to form the observed cloud as fast as required by the suspected time variations. The torus would then extend well beyond the boundaries calculated for a torus populated by thermal escape.