Illustration of horizontal energy flow for the bottom region z = 10 of the spot model at CO line wavelength λ = λ_line. The graphic shows the flow lines of x and y components of the flux vector F. The length of an arrow is proportional to the field vector magnitude ∥ (F_x,F_y) ∥ following the direction of (F_x,F_y), the field vectors shown have been selected randomly. The 3D radiative transport equation is solved for n_ϑ = n_φ = 64 solid angle points. The normalized x and y coordinates are shown on the x and y axes, respectively.