Solar extreme ultraviolet variability of the quiet Sun

Abstract

The last solar minimum has been unusually quiet compared to the previous minima (since space-based radiometric measurements are available). The Sun's magnetic flux was substantially lower during this minimum. Some studies also show that the total solar irradiance during the minimum after cycle 23 may have dropped below the values known from the two minima prior to that. For chromospheric and coronal radiation, the situation is less clear-cut. The Sun's 10.7 cm flux shows a decrease of ∼4% during the solar minimum in 2008 compared to the previous minimum, but Ca II K does not. Here we consider additional wavelengths in the extreme ultraviolet (EUV), specifically transitions of He I at 584.3 Å and Ov at 629.7 Å, of which the CDS spectrometer aboard SOHO has been taking regular scans along the solar central meridian since 1996. We analysed this unique dataset to verify if and how the radiance distribution undergoes measurable variations between cycle minima. To achieve this aim we determined the radiance distribution of quiet areas around the Sun centre. Concentrating on the last two solar minima, we found out that there is very little variation in the radiance distribution of the chromospheric spectral line He I between these minima. The same analysis shows a modest, although significant, 4% variation in the radiance distribution of the TR spectral line Ov. These results are comparable to those obtained by earlier studies employing other spectral features, and they confirm that chromospheric indices display a small variation, whereas in the transition region a more significant reduction of the brighter features is visible.