The Extreme‐Ultraviolet Solar Irradiance Spectrum Observed with the Coronal Diagnostic Spectrometer (CDS) onSOHO

Abstract

We present a calibrated solar EUV irradiance spectrum in the ranges 307-380 Å and 515-632 Å. The "Sun as a star" spectrum was recorded by the Coronal Diagnostic Spectrometer (CDS) on SOHO on 1997 May 15 using the normal incidence spectrometer (NIS) with a spectral resolution between 0.3 and 0.6 Å. The relatively high spectral resolution allows the separation of blends and the differentiation of weak emission lines. The full-disk spectrum is compared with simultaneous well-calibrated EUV irradiance measurements from a NASA/LASP rocket payload to validate the preflight calibration of CDS. Significant errors in the preflight calibration were found, and a new calibration has been established for the CDS/NIS system. The present spectrum includes emission lines formed in the temperature range 10⁴ to over 10⁶ K. Line fluxes for the most prominent lines are extracted for the calibration and for the solar irradiance studies. This measurement should represent well solar minimum conditions as the daily 10.7 cm radio flux was 73 (units of 10^-22 W m^-2 Hz^-1). A modest spatial resolution, constrained by the observing mode used, allows for the discrimination between quiet and active sun. The calibrated quiet-Sun irradiance spectrum is compared with previous measurements.