The EUV spectrum of the Sun: SOHO CDS NIS irradiances from 1998 until 2010

Abstract

We present extreme-ultraviolet (EUV) irradiances of the Sun taken during the 1998-2010 period from the Solar and Heliospheric Observatory (SoHO) Coronal Diagnostic Spectrometer (CDS) Normal Incidence Spectrograph (NIS). They were obtained from NIS full-Sun radiance observations, and represent the first set of EUV spectral observations spanning a solar cycle. We compare the CDS line irradiances with those obtained from rocket measurements, one that flew in May 1997 and one in April 2008, together with the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) Solar EUV Experiment (SEE) EUV Grating Spectrograph (EGS) and various historical records. Excellent agreement (to within a relative 20%) is found in most cases, with a few notable exceptions. Lines formed in the transition region show very small changes with the solar cycle, with the exception of the helium lines. The irradiances of lines formed around 1 MK already change during the cycle by a factor ~5; for hotter lines (2.5 MK) the variability reaches factors of the order of 40. For lines formed around 1-3 MK, and to a less extent, the helium lines, we find a good linear correlation between CDS irradiances and the 10.7 cm radio flux, although each line has a different coefficient. No correlation is found for the transition-region lines. Significant discrepancies between the observed irradiances and those modelled is found. This confirms the importance in obtaining EUV spectral measurements of the solar irradiance.