The redshifted network contrast of transition region emission

TL;DR

This study links the redshift observed in transition region emission lines of the quiet Sun to the brightness of pixels, revealing that brighter regions exhibit more redshift, which aligns with loop models involving downflows at footpoints.

Contribution

It introduces a novel analysis of network contrast redshift, showing that pixel brightness correlates with redshift and explaining the net redshift through radiance distribution and loop models.

Findings

Network contrast offset peaks at middle transition region temperatures.

Brighter pixels show more redshift, influencing the overall net redshift.

The effect is consistent with loop models with footpoint downflows.

Abstract

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.