We present preliminary reconstructions of the EUV from 26 to 34 nm from February 1997 to May 2005, covering most of solar cycle 23. The reconstruction is based on synthetic EUV spectra calculated with the spectral synthesis code Solar Modeling in 3D (SolMod3D). These spectra are weighted by the relative area coverage of the coronal features as identified from EIT images. The calculations are based on one-dimensional atmospheric structures that represent a temporal and spatial mean of the chromosphere, transition region, and corona. The employed segmentation analysis considers coronal holes, the quiet corona, and active regions identified on the solar disk. The reconstructed EUV irradiance shows a good agreement with observations taken with the CELIAS/SEM instrument onboard SOHO. Further improvement of the reconstruction including more solar features as well as the off-limb detection of activity features will be addressed in the near futur

Haberreiter, Margit

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Comparative Magnetic Minima:Characterizing quiet times in the Sun and starsProceedings IAU Symposium No. 286, 2011C. H. Mandrini & D. F. Webb, eds.c© International Astronomical Union 2012doi:10.1017/S174392131200470XTowards the reconstruction of the EUVirradiance for solar Cycle 23Margit HaberreiterPhysikalisch-Meteorologisches Observatorium Davos/World Radiation Center7260 Davos Dorf, Switzerlandemail: margit.haberreiter@pmodwrc.chAbstract. We present preliminary reconstructions of the EUV from 26 to 34 nm from February1997 to May 2005, covering most of solar cycle 23. The reconstruction is based on syntheticEUV spectra calculated with the spectral synthesis code Solar Modeling in 3D (SolMod3D).These spectra are weighted by the relative area coverage of the coronal features as identiﬁedfrom EIT images. The calculations are based on one-dimensional atmospheric structures thatrepresent a temporal and spatial mean of the chromosphere, transition region, and corona. Theemployed segmentation analysis considers coronal holes, the quiet corona, and active regionsidentiﬁed on the solar disk. The reconstructed EUV irradiance shows a good agreement withobservations taken with the CELIAS/SEM instrument onboard SOHO. Further improvementof the reconstruction including more solar features as well as the oﬀ-limb detection of activityfeatures will be addressed in the near future.Keywords. Sun: corona, Sun: UV radiation, line: formation1. IntroductionThe solar EUV irradiance is the main energy input for the upper Earth’s atmospherewith important eﬀects on the ionosphere and thermosphere. The solar energy outputchanges on short time-scales of minutes to hours as well as longer times-scales such asthe 27-day solar rotation cycle or the 11-year solar cycle. There is also indication thatthe EUV irradiance might shows a secular trend (Didkovsky et al. 2010).The EUV radiation incident on the upper Earth’s atmosphere leads to a change in itstemperature and density (see e.g. Solomon et al. 2010). In order to understand the eﬀectsof the changing EUV radiation on the Earth’s atmosphere a continuous data set coveringthe short-term and long-term variations is essential. However, as space instruments arelimited with regard to their temporal and spectral coverage, reliable models are neededto ﬁll the gaps of the observational data sets.Several reconstruction approaches involve the use of proxies to describe the EUV vari-ability. Lean et al. (2011) employ the two and three component NRLSSI model basedon the Mg II and F10.7 index to characterize and forecast the EUV variations. A furtherexample for an empirical model is SOLAR2000 (Tobiska et al. 2000), a model based onan extensive number of irradiance proxies.There is also ongoing work to determine which proxies, or spectral lines, are the bestrepresentatives for the variations of the entire EUV spectrum (see e.g. Kretzschmar et al.2009, Dudok de Wit et al. 2009).Proxy models have been quite successful in describing the EUV variations, however,in order to understand the complete physical processes driving the irradiance variations,it is important to model the variations of the full solar spectra. Warren (2006) utilizesdiﬀerential emission measure distributions derived from spatially and spectrally resolved97at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392131200470XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 11 Jul 2017 at 17:20:50, subject to the Cambridge Core terms of use, available98 M. Haberreitersolar observations and full-disk solar images. The reconstruction presented here followsthe same principle as used by Haberreiter et al. (2005) and Shapiro et al. (2011). Itincludes the calculation of synthetic spectra with a radiative transfer code for variousactivity features on the solar disk. Weighting the spectra by ﬁlling factors derived fromthe relative area coverage of these activity features or from proxy data then yields thetime dependent irradiance spectrum.In the following section the spectral synthesis code Solar Modeling in 3D (SolMod3D)is introduced. Then, in Section 3 the reconstruction approach is described brieﬂy. Finally,in Section 4 our results are compared with the EUV irradiance observations carried outwith the CELIAS/SEM instrument (Hovestadt et al. 1995) onboard the SOHO mission.2. Spectral SynthesisThe spectral synthesis of the EUV is carried out with the SolMod3D code. It is a state-of-the-art radiative transfer code in full non-local thermodynamic equilibrium (NLTE).SolMod3D allows for the spherical line-of-sight integration which is very important forthe correct calculation of the coronal emission. The code has already been successfullyemployed for the calculation of the quiet Sun EUV spectrum (Haberreiter 2011) and forsolar limb studies (Thuillier et al. 2011).The ﬁnal goal is to reconstruct the EUV from 10 to 100 nm. Here, however, we focuson the wavelength range from 26 to 34 nm as observed with the SOHO/SEM instrumentonboard SOHO. The calculations of the spectra are based on semi-empirical structuresthat represent a temporal and spatial mean of the photosphere, chromosphere, transitionregion and corona. For the photosphere, chromosphere, and transition region the fullNLTE radiative transfer is solved based on the latest semi-empirical atmospheric struc-tures by Fontenla et al. (2009). The coronal lines are calculated as optically thin lines.Currently we employ four coronal structures; three represent diﬀerent features of thequiet Sun, i.e. the quiet corona, coronal network and active coronal network as describedin Haberreiter (2011). For the bright coronal regions a fourth structure is used. Details ofthis structure will be presented in a future publication. The coronal holes are currentlyrepresented with a contrast of 0.5 with respect to the quiet corona.It is important to note that these atmosphere structures constitute a temporal and spa-tial mean. As such, these structures fail to reproduce the very rich short-term variationsof the solar atmosphere, in particular the corona. However, as we are mainly interestedin the daily EUV variations, 1D structures are considered to be a suitable representationof the solar atmosphere. The advantage of the 1D atmosphere structures is that it allowsus to take into account an extensive atomic data set (14,000 atomic levels and 170,000spectral lines), essential for a realistic calculation of the EUV spectrum.3. ReconstructionFirst, intensity spectra are calculated for diﬀerent features on the solar disk as describedabove. These spectra are then weighted by ﬁlling factors derived from the analysis of solarimages. For the study presented in this paper we use the 3-component analysis of imagestaken with the Extreme UV Imaging Telescope (Delaboudinie`re et al. 1995, EIT) onboardSOHO carried out by Barra et al. (2009). The segmentation analysis is based on the so-called fuzzy clustering , a robust and fast technique that allows to monitor and trackactive regions on the solar disk. The analysis provides the disk-integrated relative areacoverage of coronal holes, the quiet corona, and active coronal regions detected on thesolar disk. However, extended active regions located beyond the solar limb are not yetat https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392131200470XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 11 Jul 2017 at 17:20:50, subject to the Cambridge Core terms of use, availableReconstruction of the EUV irradiance 99Figure 1. Preliminary results of the reconstruction of the EUV photon ﬂux at 1 AU for thewavelength range from 26 to 34 nm (black squares) compared with the SOHO/SEM observationsfor the same wavelength range (grey crosses).taken into account. To compensate the missing radiation, we assume here an additional20% increase of the irradiance due to the ﬂux coming from the extended corona. Thedetailed feature analysis of the oﬀ-limb features will be included in future work.4. ResultsFig. 1 shows the reconstruction of the EUV from 26 to 34 nm from Feb 14, 1997 toMay 1, 2005 compated with the SOHO/SEM observations covering the same wavelengthrange. The average quiet corona is represented as 75% of the quiet corona, 22% of thecoronal network, and 3% of the active coronal network. This combination has alreadybeen used by Haberreiter (2011) for modeling the EUV for solar minimum conditions.For the coronal holes we use a contrast of 0.5 with respect to the quiet Sun intensity. Wetested the eﬀect of the contrast of coronal holes and conclude that the contrast of thecoronal holes shows a minor eﬀect on the overall EUV variability. The variations in theEUV are mainly driven by the varying area coverage of the bright active regions.As the segmentation analysis of Barra et al. (2009) includes only on-disk features, themissing radiation from the extended corona has to be compensated by assuming that 20%of the on-disk radiation account for the radiation of the extended corona. The oﬀ-limbcontribution will be studied in detail in the near future. A reﬁnement of the features inthe segmentation analysis will also be addressed in upcoming studies. Finally, for longerwavelength ranges we also need to account for the detailed center-to-limb variations ofthe contribution from the chromospheric features. Here, the goal is to implement PSPTimages (see e.g. Ermolli et al. 2007). Given the limitations of these preliminary resultsthe agreement with the SOHO/SEM observations is very promising.at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392131200470XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 11 Jul 2017 at 17:20:50, subject to the Cambridge Core terms of use, available100 M. Haberreiter5. ConclusionWe have presented a preliminary version of the reconstruction of the EUV variationsfrom Feb 14, 1997 to May 1, 2005 for the wavelength range between 26 and 34 nm andcompared it with observations taken with the CELIAS/SEM instrument. This work isbased on the calculation of spectra in the EUV with the SolMod3D code and the ﬁllingfactors for three coronal components. The good agreement of the reconstruction with theobserved irradiance variations shows that our approach is suitable for the study of theEUV variability. Nevertheless, further analysis is required, in particular the study of theoﬀ-limb contribution to the EUV irradiance and the consideration of additional coronalactivity features.AcknowledgementsWe thank Veronique Delouille and Cis Verbeeck for kindly providing the segmentationanalysis of the EIT data. SOHO is a project of international cooperation between ESAand NASA. MH acknowledges support by the Holcim Foundation for the Advancementof Scientiﬁc Research.ReferencesBarra, V., Delouille, V., Kretzschmar, M., & Hochedez, J. 2009, A&A, 505, 361Delaboudinie`re, J.-P., Artzner, G. E., Brunaud, J., et al. 1995, Solar Phys., 162, 291Didkovsky, L. V., Judge, D. L., Wieman, S. R., & McMullin, D. 2010, in: S. R. Cranmer,J. T. Hoeksema, & J. L. Kohl (eds.), SOHO-23: Understanding a Peculiar Solar Minimum,Astronomical Society of the Paciﬁc Conference Series, Vol. 428, p. 73Dudok de Wit, T., Kretzschmar, M., Lilensten, J., & Woods, T. 2009, GRL, 36, 10107Ermolli, I., Criscuoli, S., Centrone, M., Giorgi, F., & Penza, V. 2007, A&A, 465, 305Fontenla, J. M., Curdt, W., Haberreiter, M., Harder, J., & Tian, H. 2009, ApJ, 707, 482Haberreiter, M. 2011, Solar Phys., 274, 473Haberreiter, M., Krivova, N. A., Schmutz, W., & Wenzler, T. 2005, Adv. Sp. Res, 35, 365Hovestadt, D., Hilchenbach, M., Bu¨rgi, A., et al. 1995, Solar Phys., 162, 441Kretzschmar, M., Dudok de Wit, T., Lilensten, J., et al. 2009, Acta Geophysica, 57, 42Lean, J. L., Woods, T. N., Eparvier, F. G., et al. 2011, JGR, 116, 1102Shapiro, A. I., Schmutz, W., Rozanov, E., Schoell, M., Haberreiter, M., Shapiro, A. V., & Nyeki,S. 2011, A&A, 529, A67Solomon, S. C., Woods, T. N., Didkovsky, L. V., Emmert, J. T., & Qian, L. 2010, GRL, 37,16103Thuillier, G., Claudel, J., Djafer, D., et al. 2011, Solar Phys., 268, 125Tobiska, W. K., Woods, T., Eparvier, F., et al. 2000, Journal of Atmospheric and Solar-Terr.Phys., 62, 1233Warren, H. P. 2006, Ad. Sp. Res, 37, 359DiscussionJeffrey Linsky: I would like to draw your attention to a recently published paperby Juan Fontenla in which he updated the atmospheric models of the solar atmosphere(Fontenla et al. 2011). In particular the formation of the Lyman continuum has beenimproved. These latest set of solar atmosphere models has been successfully employedto reproduce the Lyman continuum of stars with various eﬀective temperatures (Linskyet al. 2012).Margit Haberreiter: We thank Jeﬀ Linsky for this comment and will follow up onthe recommended publications.at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392131200470XDownloaded from https:/www.cambridge.org/core. 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Towards the reconstruction of the EUV irradiance for solar Cycle 23

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