THE IO SODIUM CLOUD - COMPARISON BETWEEN OBSERVATIONS AND NUMERICAL-MODELS

We have refined a method to derive from In's sodium cloud spectra prec ise estimates of the production rate and of the ejection velocity dist ribution at the exobase, This method is based on a detailed comparison between the observed spectra and synthetic spectra numerically simula ted 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 dispe rsion 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 di d not work with spectra but with bidimensional images of the Io's clou d, Moreover they did not try to derive from the model-observation comp arison an estimate of the production rate, We have applied the poliene rgetic model spectra approach to analyze a sequence of 59 observations of the sodium cloud spanning a period of 4 years from February 1990 t o April 1994. The echelle spectra have been reduced to separate the fa st sodium components and then compared to polienergetic synthetic spec tra, We report the behavior of the production rate in the timespan cov ered by our observations and the analysis of the velocity components a t the exobase. We find a relationship between both the production rate and the velocity distribution and lambda(m). (C) 1998 Academic Press.