Radiance variations of vacuum-ultraviolet emission lines of the quiet sun observed with SUMER on SOHO

Abstract

The vacuum-ultraviolet telescope-spectrometer SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on the Solar and Heliospheric Observatory (SOHO) has been used to measure the radiance of quiet Sun areas at the centre of the solar disk. We present the radiance data of selected emission lines that have been observed regularly since the beginning of the SOHO mission. In the spectral lines He i 584 Å, Mg x 609 Å and 624 Å, N v 1238 Å, Fe xii 1242 Å, and Ne viii 770 Å measurements of the radiance of quiet regions at the solar disk centre have been made from March 1996 to the present. We discuss the variability of these emission lines during the solar minimum and the ascending phase of the present solar activity cycle. The observed lines show different variability of the quiet Sun: The optically thin, chromospheric and coronal lines show variations up to 80%, while the optically thick He i 584 Å line shows no variation. The results are important for models that calculate solar VUV variability on the basis of radiance contrast ratios of solar disk regions. Our spatially resolved images allow a separation of brighter and darker areas of the quiet Sun network. Both regimes show similar variations.