  The origin of solar transition region redshifts is not completely understood.
Current research is addressing this issue by investigating three-dimensional
magneto-hydrodynamic models that extend from the photosphere to the corona. By
studying the average properties of emission line profiles synthesized from the
simulation runs and comparing them to observations with present-day
instrumentation, we investigate the origin of mass flows in the solar
transition region and corona. Doppler shifts were determined from the emission
line profiles of various extreme-ultraviolet emission lines formed in the range
of $T=10^4-10^6$ K. Plasma velocities and mass flows were investigated for
their contribution to the observed Doppler shifts in the model. In particular,
the temporal evolution of plasma flows along the magnetic field lines was
analyzed. Comparing observed vs. modeled Doppler shifts shows a good
correlation in the temperature range $\log(T$/[K])=4.5-5.7, which is the basis
of our search for the origin of the line shifts. The vertical velocity obtained
when weighting the velocity by the density squared is shown to be almost
identical to the corresponding Doppler shift. Therefore, a direct comparison
between Doppler shifts and the model parameters is allowed. A simple
interpretation of Doppler shifts in terms of mass flux leads to overestimating
the mass flux. Upflows in the model appear in the form of cool pockets of gas
that heat up slowly as they rise. Their low temperature means that these
pockets are not observed as blueshifts in the transition region and coronal
lines. For a set of magnetic field lines, two different flow phases could be
identified. The coronal part of the field line is intermittently connected to
subjacent layers of either strong or weak heating, leading either to mass flows
into the loop or to the draining of the loop.
