Variation of EUV solar irradiances along the cycle Vincenzo Andretta 1, Giulio Del Zanna 2, Seth Wieman 3 1 INAF – Osservatorio Astronomico di Capodimonte, Naples, Italy – 2 University of Cambridge – DAMTP, Cambridge, UK – 3 Space Sciences Center, Dep. Phys. and Astr., University of Southern California, Los Angeles, California, USA – Abstract We discuss the variations of the EUV solar irradiances along the solar cycle, presenting SOHO CDS and SDO EVE results. We explain them using radiance observations obtained since 1998 with SOHO CDS. We use the CDS irradiances to understand the variations observed in the SOHO SEM A band (SEM 1) during the previous solar maximum. Based on the CDS-derived spectral irradiances we find, for instance, that the He II 304 A line contributes less than previously estimated to the SEM 1 band. We also discuss how EUV/UV irradiances can be modelled using radiance observations and CHIANTI atomic data. References  Andretta & Del Zanna, A&A 563, A26 (2014)  Del Zanna, Bromage, Landi, and Landini, A&A 379, 708 (2001)  Del Zanna, Andretta, Chamberlin, Woods, and Thompson, A&A 518, A49 (2010)  Del Zanna & Andretta, A&A 528, A139 (2011)  Wieman, Didkovsky, and Judge, Solar Phys. 289, 2907 (2014 Acknowledgements This work has been partially supported by ASI, via ASI-INAF contracts I/013/12/0, I/43/10/0, I/05/07/0, I/023/09/0, I/035/05/0. G.D.Z. acknowledges support from SFTC (UK), from the SOLID EU FP7 network and the STCE (Belgium). Two workshops on the inter-calibration and degradation of instruments working in the EUV range have been organized by the Solar Terrestrial Centre of Excellence (STCE) at the Royal Observatory of Belgium, on April 2013 and on June 2014, in Brussels. Figure 2. Monochromatic, radiometrically calibrated images of the Sun in a few representative lines observed by CDS/NIS during solar cycle 23. Bottom to top: He I 584 Å, O V 630 Å, Mg X 625 Å, Si XII 521 Å. The variability of the radio flux at 10.7 cm ( W m -2 Hz -1 ) is shown in the bottom panel. Figure 1. Solar EUV spectral irradiance (mW m -2 nm -1 ) from CDS at two epochs representative of the maximum ( ) and minimum ( ) of solar cycle 23. Wavelengths are in nm. Figure 8. Comparison of SEM 1 irradiances during cycle 23 and irradiances in the same band obtained from CDS spectral irradiances at the same dates. Figure 4. Areas of active regions (ARs) as seen in different EUV lines, and the contrast between the mean AR and quiet Sun (QS) radiance as function of time during cycle 23. Figure 3. Radiance histograms for a set of representative EUV lines as functions of time: He I 584 Å, O V 630 Å, Mg X 625 Å, Si XI 303 Å, Si XII 521 Å. For comparison, the radio flux F10.7 ( W m -2 Hz -1 ) is also shown. Figure 6. EVE Ver. 4 Daily Average Comparison: Å band from MEGS, ESP ch9, and SOHO/SEM. Figure 7. SOHO SEM 1 passband (green), compared with a CHIANTI synthetic spectra obtained from the SOHO CDS irradiance spectrum for 30 October 2001 and different abundance mixtures. The observed SEM 1 irradiance for that date is also given. Introduction The highly variable solar irradiance in the extreme ultra-violet (EUV) spectral range is of considerable importance in several respects, for instance in the study of the Sun as a star or for its effects on the Earth's atmosphere. The absolute irradiance measurements needed for this kind of studies are however very difficult to attain. 2. SOHO/CDS irradiances Observations with the SOHO Coronal Diagnostics Spectrometer (CDS) covering practically an entire cycle of solar activity (cycle 23) are in fact unique in that respect. A crucial step for this using CDS data for irradiance studies was the characterization of the long-term degradation of the spectral responsivities of the CDS Normal Incidence Spectrometer (NIS) by Del Zanna et al. (2010). The follow-up study by Del Zanna & Andretta (2011) discussed for the first time the variability of the solar EUV irradiance along a full solar cycle: Figures 1 and 2. We then analyzed (Andretta & Del Zanna, 2014) the spatial distribution of the solar radiances along a cycle, identifying the main sources of variability at different temperatures of the solar EUV irradiance: Figure 3 and 4. In particular we show that irradiance of transition region (TR) lines varies by a much smaller factor than coronal lines. Figure 5. Correlation between SOHO CDS or SDO/EVE irradiances and the radio F10.7 flux: the case of the transition region O III 599 Å line is shown on the left, the coronal Mg X 625 Å line is shown on the right. Modelling EUV irradiances We are studying the correlations between EUV irradiances as measured by SOHO/CDS and SDO/EVE and solar proxies, to develop proxy-based spectral irradiances, obtained from CHIANTI atomic data and from observations (Figure 5). SOHO CDS vs. SEM 1 irradiances The SOHO SEM (Solar EUV Monitor) provides more than 17 years of continuous data. The SEM data have been calibrated with sounding rockets. The SEM Å band (SEM 1) is dominated, during minimum conditions, by the He II 304 A line, which is observed in second order by CDS/NIS (see Figure 1). A comparison between SEM 1 and SDO/EVE and a calibration of the 2007 sounding rocket prompted a revision of the SEM 1 effective area (Wieman et al. 2014, see Figure 6), which has a significant secondary component around 200 A (cf. Figure 7). We have taken the CDS He II and Si XI irradiances, complemented them with irradiances obtained from DEM modelling and CHIANTI v. 8 (cf. Figure 7), to show that during solar maximum conditions the He II line contributes less than 50%. We have folded the irradiances with the SEM 1 effective areas (taking into account the SEM degradation model), and obtained the predicted SEM 1 count rates shown in Figure 8. Very good agreement is found during minimum conditions, while during the previous solar maximum the observed SEM 1 count rates are still higher. We are investigating this issue.