F0 dkFck jp dy fb da x 2 exp [tX z y Iok y a

with ¡3 = rSunr///dSD, b = (1 - y2/fi2)1/2, and z± = r± - rE is the tangential altitude of rays emitted by the Sun center.

A further approximation is made by separating the spectral and spatial components of the non-attenuated irradiance I0(k, y, a) = I1(k)I2(y, a) and by introducing a*c, the averaged absorption cross-section of component i on the wavelength range of channel c. From this, we define t* by:

(/o00 dk Fc (k) Ii(k)).( d y exp [_t*(zx + y)] £ daf*^ Tc(r±) =-^--- PV "

nd yfob ^ffia f_3 dy exp [_/oL(Z±+y)£j=o n(z(s))<d*] St daf^

Note that the occultation process is only a few minutes long. This highlights one last hypothesis we implicitly made: we have considered, in the model above, that solar emission undergoes no change during the whole process.

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