Radiance variations of the quiet Sun at far-ultraviolet wavelengths

Abstract

We have measured the radiance of quiet-Sun areas at the centre of the solar disk using the vacuum-ultraviolet telescope-spectrograph SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on the Solar and Heliospheric Observatory (SOHO). The radiances of selected emission lines have been measured from March 1996 to the present. The lines that have been observed regularly since the beginning of the SOHO mission are He i 584 Å, Mg x 609 Å and 624 Å, Ne viii 770 Å, N v 1238 Å, and the H i Lyman continuum at 880 Å. We investigate the variability of these emission lines during the solar minimum and the ascending phase of the present solar activity cycle. The transition region and coronal lines show an increasing trend of up to 100% since the sunspot minimum. The results are important for models of solar VUV variability on the basis of radiance contrast ratios of solar disk regions. Our spatially resolved images allow a separation of the network and cell areas of the quiet-Sun. Both regimes show similar variations.