Cycle-related solar vacuum ultraviolet (VUV) variability

Abstract

Various studies has shown that the Sun was in an extremely inactive state during the prolonged minimum of the cycle 23 compared to other recent minima. Following these studies, in this thesis work, it was tried to have a closer look at the variability of the quiet Sun network and internetwork radiance between the last two solar minima. The aim of this dissertation is to find out whether there is a change in the radiance properties of the upper chromosphere and transition region (at some specific wavelengths of the VUV spectrum), over the last solar cycle. We carried out a statistical study of the radiance variability of the chromospheric He I 58.4 nm (formation temperature T ~ 20,000 K) and transition region O V 63.0 nm (T ~ 250,000 K) spectral lines in the quiet Sun close to disc-center, over a period of 15 years from 1996 to 2011, with the main focus on comparing the last two solar minima. We used observations from the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO), which provides daily observations of the Sun in various wavelengths and has a record long enough to cover the last two solar minima. It was found, that the radiance distribution of the transition region O V line, has undergone remarkable changes during the solar minimum 23 compared to the previous minimum. Such variations were not seen in the radiance distribution of the chromospheric He I line. These results suggest that different heating mechanisms may trigger radiation from the chromosphere and the transition region at the quiet Sun. While the latter seems to be more magnetic in nature, the former is less affected by the solar magnetic activity.