As one may see from changes of optical depth with time the more dence stream has more high optical depth and with time, the picture keeps the same, i.e. more dense stream has higher optical depth. The different colors refer to the different moments of time. Two features is due to two density enhancements. The spectral features behave just in the same way. Let us now change location of these dense spots.
Colors are just the same as on the previous plots.
So it accelerate in a differnt way then spectral features themself. Therefore, it is obvious that the trace of minimum on the gray-scale diagrams of DACs, do not really reflect the motion of parcels of material. As one may see for all sets of parameters the less dense component become dominant with time. Next picture represents the nonuniform density distribution for ONE spot-stream. One may clearly see that minimum of emission is drifting along the absorbtion line profile.