A new reference spectrum for the EUV irradiance of the quiet Sun: 2. Comparisons with observations and previous models

Abstract

In a companion paper we presented a new reference spectrum for the EUV irradiance of the quiet Sun based primarily on intensities calculated from a quiet Sun emission measure distribution and recent compilations of atomic data. The contributions of optically thick emission lines and continua were included empirically. In this paper we present comparisons between this reference spectrum and independently measured irradiance observations, previous quiet Sun reference spectra, and the predictions of empirical EUV irradiance models. These comparisons indicate relatively good agreement among fluxes of emission lines formed in the solar chromosphere and transition region. The fluxes for coronal emission lines in previous quiet Sun reference spectra, however, do not agree with our calculated fluxes or with a recent irradiance observation taken at a low level of solar activity. Coronal emission lines in the Atmospheric Explorer E (AE‐E) quiet Sun reference spectrum SC21REFW have fluxes that are typically smaller than our calculated fluxes by factors of 2 or more. We also identify inconsistencies in the earlier reference spectrum of Heroux and Hinteregger [1978] (F74113), where the fluxes of many coronal emission lines with wavelengths below 250 Å are inconsistent with the fluxes of coronal emission lines at longer wavelengths. The fluxes of EUV continua in the various reference spectra and irradiance observations also differ significantly.