Quiet-sun variability observed with SUMER and CDS

Brightness variations of solar features are investigated using time series of images and spectra of quiet-Sun regions at disc centre obtained with the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument onboard the SOHO spacecraft. Ultraviolet emission lines sampling temperatures of the chromosphere, transition region and corona were recorded, with the He I 584.3 Å and O v 629.7 Å lines being recorded simultaneously by both instruments. A comparison shows that both instruments give similar results except that SUMER reveals a factor of three higher absolute and relative variability than CDS. Simple tests suggest that the higher spatial resolution of SUMER compared to CDS, and the broad slit used for the CDS observations, are responsible for this difference. This points to the need for higher spatial resolution for future variability studies. The SUMER results confirm and extend to lower temperatures the trends deduced in an earlier paper from CDS data.