The EUV spectrum of the Sun: Irradiances during 1998–2014

¹ DAMTP, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
e-mail: g.del-zanna@damtp.cam.ac.uk
² INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy

Received: 22 June 2015
Accepted: 14 September 2015

Abstract

We present calibrated EUV spectral irradiances obtained from observations with the Solar and Heliospheric Observatory (SOHO) Coronal Diagnostics Spectrometer (CDS) Normal Incidence Spectrometer (NIS) during the 1998–2014 period, which spans the cycle 23 maximum and minimum and the cycle 24 maximum. We revise the corrections for the burn-in of the strong lines and our previous long-term calibration of the NIS. We find no indications of further overall degradation of the instrument responsivities after 2010. We compare the CDS irradiances with those obtained by the prototype and flight instruments aboard the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE) and the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) Solar EUV Experiment (SEE) EUV Grating Spectrograph (EGS). We find overall excellent agreement (to within a relative 10–20%) with the EVE data (especially during 2010–2012), but point out inconsistencies in some of the prototype and flight EVE irradiances. There is overall agreement with some of the TIMED SEE EGS data. We confirm the small variations in the irradiances of low-temperature lines (except the helium lines) and show that the irradiances in the hot (2–3 MK) lines are significantly lower for the cycle 24 maximum compared to the previous one.