Atmospheric transmissivity- Solar Analyst

[EleniAmpatzi]

Dear all,

I would appreciate your help on the description of the atmospheric transmissivity parameter used in Topo view/Solar Analyst. In the Help section in ArcGIS 9.3 it is defined as the ratio of global irradiance on earth to extraterrestrial solar radiation in the direction of the zenith. Nevertheless in journal articles referring to the use of Solar Analyst, I have seen it defined as the transmissivity of beam irradiance only (this was my understanding too as it is used only for the calculation of the direct irradiance only), through cloud particles, aerosol particles (natural and artificial) and molecules of gases [calculated as ??=kt (1-kd), where kt is the clearness index and kd the diffuse ratio)).
Which of the two is the correct one for the algorithm employed by the Solar Analyst?
Thank you in advance for any replies.

[EleniAmpatzi]

Further to my previous post, in the Webhelp topic of ArcGIS 9.2 (not 9.3) I read:
"transmissivity... is the ratio of solar radiation outside the atmosphere to that reaching the earth's surface (averaged all wavelengths) for the shortest path (in the direction of the zenith)". If what is meant by the author is actually the reverse ratio (global to extraterrestrial), that would equal to the clearness index. However my understanding is that the software requires as input the proportion of extraterrestrial that finds its way as beam irradiance at sea level.
Could someone clarify this?