  Aims: We study the VUV emission of the quiet Sun and the net redshift of
transition region lines in the SUMER spectral range. We aim at establishing a
link with atmospheric processes and interpreting the observed downflow as the
most evident part of the prevailing global coronal mass transport. Methods: We
rank and arrange all pixels of a monochromatic raster scan by radiance and
define equally-sized bins of bright, faint, and medium-bright pixels. Comparing
the bright pixels with the faint pixels, we determine the spectrally-resolved
network contrast for 19 emission lines. We then compare the contrast centroids
of these lines with the position of the line itself. We establish a
relationship between the observed redshift of the network contrast with the
line formation temperature. Results: We find that the network contrast is
offset in wavelength compared to the emission line itself. This offset, if
interpreted as redshift, peaks at middle transition region temperatures and is
10 times higher than the previously reported net redshift of transition region
emission lines. We demonstrate that the brighter pixels are more redshifted,
causing both a significant shift of the network contrast profile and the
well-known net redshift. We show that this effect can be reconstructed from the
radiance distribution. This result is compatible with loop models, which assume
downflows near both footpoints.
