Abstract:
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.
