The Io sodium cloud: comparison between observations and numerical models

We have refined a method to derive from Io's sodium cloud spectra precise estimates of the production rate and of the ejection velocity distribution at the exobase. This method is based on a detailed comparison between the observed spectra and synthetic spectra numerically simulated with a model derived from the Smyth and Combi (1988, Astrophys. J. Suppl. 66, 397-411) formalism. The innovative aspect of our approach consists in building up polienergetic spectra as linear combination of normalized single velocity spectra. In this way we simulate the dispersion of the sodium atom ejection velocities at the exobase and derive for each velocity component a production rate. Smyth and Combi (1988, Astrophys. J. 328, 888-918) proposed a similar approach to study the velocity distribution of sodium atoms at the exobase. However, they did not work with spectra but with bidimensional images of the Io's cloud. Moreover they did not try to derive from the model-observation comparison an estimate of the production rate. We have applied the polienergetic model spectra approach to analyze a sequence of 59 observations of the sodium cloud spanning a period of 4 years from February 1990 to April 1994. The echelle spectra have been reduced to separate the fast sodium components and then compared to polienergetic synthetic spectra. We report the behavior of the production rate in the timespan covered by our observations and the analysis of the velocity components at the exobase. We find a relationship between both the production rate and the velocity distribution and lambda_III.