Specifies the magnitude of a bottom surface isotropic fluorescence source.
fluorescence value
where value is a number greater or equal to 0.0,
constant for all wavelengths.
Must be used together with source solar file. The units of the fluorescence should
obviously be the same as for the solar source in source solar file.
For wavelength dependent fluorescence use fluorescence_file. The default fluorescence is 0.0.
Currently only works with the cdisort solver.