Interstellar radiation as a Maxwell field: improved numerical scheme and application to the spectral energy density

22 pages plus 15 figuresThe recent model of the interstellar radiation field as a Maxwell field is improved by considering separately the different frequencies at the stage of the fitting. Using this improved procedure: (i) It is checked in detail that the model does predict extremely high values of the spectral energy density (SED) on the axis of a galaxy, that however decrease very rapidly when ρ, the distance to the axis, is increased from zero. (ii) The difference between the SED values (with ρ = 1 kpc or 8 kpc), as predicted either by this model or by a recent radiation transfer model, is reduced significantly. (iii) The slower decrease of the SED with increasing altitude z, as compared with the radiation transfer model, is confirmed. We also calculate the evolutions of the SED at large ρ. We interpret these evolutions by determining asymptotic expansions of the SED at large z, and also ones at large ρ