Author name code: krivova ADS astronomy entries on 2022-09-14 author:"Krivova, Natalie A." ------------------------------------------------------------------------ Title: Erratum: "Faculae Cancel out on the Surfaces of Active Suns" (2022, ApJL, 934, L23) Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L. Bibcode: 2022ApJ...936L..17N Altcode: No abstract at ADS Title: Faculae Cancel out on the Surfaces of Active Suns Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L. Bibcode: 2022ApJ...934L..23N Altcode: 2022arXiv220706816N Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterization of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance, they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots or faculae. Here, we utilize a surface flux transport model to show that more effective cancellation of diffuse magnetic flux associated with faculae leads to spot area coverages increasing faster with stellar magnetic activity than that by faculae. Our calculations explain the observed dependence between solar spot and facular area coverages and allow its extension to stars that are more active than the Sun. This extension enables anticipating the properties of stellar signal and its more reliable mitigation, leading to a more accurate characterization of exoplanets and their atmospheres. Title: Predictions of Astrometric Jitter for Sun-like Stars. III. Fast Rotators Authors: Sowmya, K.; Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Krivova, N. A.; Solanki, S. K. Bibcode: 2022ApJ...934..146S Altcode: 2022arXiv220607702S A breakthrough in exoplanet detections is foreseen with the unprecedented astrometric measurement capabilities offered by instrumentation aboard the Gaia space observatory. Besides, astrometric discoveries of exoplanets are expected from the planned space mission, Small-JASMINE. In this setting, the present series of papers focuses on estimating the effect of the magnetic activity of G2V-type host stars on the astrometric signal. This effect interferes with the astrometric detections of Earth-mass planets. While the first two papers considered stars rotating at the solar rotation rate, this paper focuses on stars having solar effective temperature and metallicity but rotating faster than the Sun, and consequently more active. By simulating the distribution of active regions on such stars using the Flux Emergence And Transport model, we show that the contribution of magnetic activity to the astrometric measurements becomes increasingly significant with increasing rotation rates. We further show that the jitter for the most variable periodic Kepler stars is high enough to be detected by Gaia. Furthermore, due to a decrease in the facula-to-spot area ratio for more active stars, the magnetic jitter is found to be spot dominated for rapid rotators. Our simulations of the astrometric jitter have the potential to aid the interpretation of data from Gaia and upcoming space astrometry missions. Title: Solar cyclic activity over the last millennium reconstructedfrom annual 14C data (Corrigendum) Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2022A&A...664C...3U Altcode: No abstract at ADS Title: Reconstructions of solar activity and variability from cosmogenic isotope data Authors: Krivova, Natalie; Usoskin, Ilya Bibcode: 2022cosp...44.1046K Altcode: Our knowledge about the activity and variability of the Sun on the millennial time scale relies on cosmogenic isotope data. Modulated by the solar open magnetic field, the fluctuating flux of the energetic cosmic ray particles in the vicinity of Earth leads to the formation of varying amounts of the radionuclides in the atmosphere. These radionuclides are naturally stored in terrestrial archives for millennia and can be measured in laboratories nowadays. In combination with appropriate models, such data then allow us to go back in time and understand how the Sun behaved in the past. We will review the latest progress in understanding and reconstruction of the solar activity over the last millennia. Title: Historical Ca II K observations for irradiance reconstructions Authors: Chatzistergos, Theodosios; Krivova, Natalie; Ermolli, Ilaria; Leng Yeo, Kok; Solanki, Sami; Mandal, Sudip Bibcode: 2022cosp...44.1551C Altcode: Solar irradiance has been measured by various instruments from space since 1978 and exhibits variations on all accessible timescales. Irradiance variations prior to satellite measurements, knowledge of which is important for climate studies, can only be recovered with models. Models have successfully been used to reconstruct solar irradiance variations by attributing the variability on time scales longer than about a day to the evolution of the solar surface magnetic field. However, to account for the different contributions of surface magnetic features, such as sunspots and faculae, models require appropriate input data. Unfortunately, due to the scarcity of suitable facular data prior to the period of direct irradiance measurements, most available irradiance reconstructions rely on sunspot data alone or on other, indirect data of solar magnetism. Historical Ca II K archives carry direct information on facular regions, but they have not been used to their full potential for irradiance reconstructions. The main reasons limiting their use were the difficulty to account for the non-linear response of the photographic plates along with plentiful large-scale artefacts affecting the images. We have extensively studied Ca II K data to overcome most issues affecting them thus paving the way for them to be used for accurate irradiance reconstructions. We will present our latest work on employment of Ca II K data for reconstructing past irradiance variations. Title: Multiwavelength Mitigation of Stellar Activity in Astrometric Planet Detection Authors: Kaplan-Lipkin, Avi; Macintosh, Bruce; Madurowicz, Alexander; Sowmya, Krishnamurthy; Shapiro, Alexander; Krivova, Natalie; Solanki, Sami K. Bibcode: 2022AJ....163..205K Altcode: 2021arXiv211206383K Astrometry has long been a promising technique for exoplanet detection. At the theoretical limits, astrometry would allow for the detection of smaller planets than previously seen by current exoplanet search methods, but stellar activity may make these theoretical limits unreachable. Astrometric jitter of a Sun-like star due to magnetic activity in its photosphere induces apparent variability in the photocenter of order 0.5 mR ⊙. This jitter creates a fundamental astrophysical noise floor preventing detection of lower-mass planets in a single spectral band. By injecting planet orbits into simulated solar data at five different passbands, we investigate mitigation of this fundamental astrometric noise using correlations across passbands. For a true solar analog and a planet at 1 au semimajor axis, the 6σ detection limit set by stellar activity for an ideal telescope at the best single passband is 0.01 Earth masses. We found that pairs of passbands with highly correlated astrometric jitter due to stellar activity, but with less motion in the redder band, enable higher-precision measurements of the common signal from the planet. Using this method improves detectable planet masses at 1 au by up to a factor of 10, corresponding to at best 0.005 Earth masses for a Sun-like star with a perfect telescope. Given these results, we recommend that future astrometry missions consider proceeding with two or more passbands to reduce noise due to stellar activity. Title: Time Lag Between Cosmic-Ray and Solar Variability: Sunspot Numbers and Open Solar Magnetic Flux Authors: Koldobskiy, Sergey A.; Kähkönen, Riikka; Hofer, Bernhard; Krivova, Natalie A.; Kovaltsov, Gennady A.; Usoskin, Ilya G. Bibcode: 2022SoPh..297...38K Altcode: Solar magnetic activity drives the dominant 11-year cyclic variability of different space environmental indices, but they can be delayed with respect to the original variations due to the different physical processes involved. Here, we analyzed the pairwise time lags between three global solar and heliospheric indices: sunspot numbers (SSN), representing the solar surface magnetic activity, the open solar flux (OSF), representing the heliospheric magnetic variability, and the galactic cosmic-ray (GCR) intensity near Earth, using the standard cross-correlation and the more detailed wavelet-coherence methods. All the three indices appear highly coherent at a timescale longer than a few years with persistent high coherence at the timescale of the 11-year solar cycle. The GCR variability is delayed with respect to the inverted SSN by about eight 27-day Bartels rotations on average, but the delay varies greatly with the 22-year cycle, being shorter or longer around positive A + or negative A − solar polarity epochs, respectively. The 22-year cyclicity of the time lag is determined by the global heliospheric drift effects, in agreement with theoretical models. The OSF lags by about one year behind SSN, and is likely determined by a combination of the short lifetime of active regions and a longer (≈3 years) transport time of the surface magnetic field to the poles. GCRs covary nearly in antiphase with the OSF, also depicting a strong 22-year cycle in the delay, confirming that the OSF is a good index of the heliospheric modulation of GCRs. This provides an important observational constraint for solar and heliospheric physics. Title: Final Report for SAG 21: The Effect of Stellar Contamination on Space-based Transmission Spectroscopy Authors: Rackham, Benjamin V.; Espinoza, Néstor; Berdyugina, Svetlana V.; Korhonen, Heidi; MacDonald, Ryan J.; Montet, Benjamin T.; Morris, Brett M.; Oshagh, Mahmoudreza; Shapiro, Alexander I.; Unruh, Yvonne C.; Quintana, Elisa V.; Zellem, Robert T.; Apai, Dániel; Barclay, Thomas; Barstow, Joanna K.; Bruno, Giovanni; Carone, Ludmila; Casewell, Sarah L.; Cegla, Heather M.; Criscuoli, Serena; Fischer, Catherine; Fournier, Damien; Giampapa, Mark S.; Giles, Helen; Iyer, Aishwarya; Kopp, Greg; Kostogryz, Nadiia M.; Krivova, Natalie; Mallonn, Matthias; McGruder, Chima; Molaverdikhani, Karan; Newton, Elisabeth R.; Panja, Mayukh; Peacock, Sarah; Reardon, Kevin; Roettenbacher, Rachael M.; Scandariato, Gaetano; Solanki, Sami; Stassun, Keivan G.; Steiner, Oskar; Stevenson, Kevin B.; Tregloan-Reed, Jeremy; Valio, Adriana; Wedemeyer, Sven; Welbanks, Luis; Yu, Jie; Alam, Munazza K.; Davenport, James R. A.; Deming, Drake; Dong, Chuanfei; Ducrot, Elsa; Fisher, Chloe; Gilbert, Emily; Kostov, Veselin; López-Morales, Mercedes; Line, Mike; Močnik, Teo; Mullally, Susan; Paudel, Rishi R.; Ribas, Ignasi; Valenti, Jeff A. Bibcode: 2022arXiv220109905R Altcode: Study Analysis Group 21 (SAG21) of the Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, and plages. This SAG has brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current needs that can be addressed in this context to make the most of transit studies from current NASA facilities like HST and JWST. The analysis produced 14 findings, which fall into three Science Themes encompassing (1) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities ("The Sun as the Stellar Benchmark"), (2) how stars other than the Sun extend our knowledge of heterogeneities ("Surface Heterogeneities of Other Stars") and (3) how to incorporate information gathered for the Sun and other stars into transit studies ("Mapping Stellar Knowledge to Transit Studies"). Title: Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli, Ilaria; Yeo, Kok Leng; Mandal, Sudip; Solanki, Sami K.; Kopp, Greg; Malherbe, Jean-Marie Bibcode: 2021A&A...656A.104C Altcode: 2021arXiv210905844C Context. Knowledge of solar irradiance variability is critical to Earth's climate models and understanding the solar influence on Earth's climate. Direct solar irradiance measurements have only been available since 1978. Reconstructions of past variability typically rely on sunspot data. However, sunspot records provide only indirect information on the facular and network regions, which are decisive contributors to irradiance variability on timescales of the solar cycle and longer.
Aims: Our ultimate goal is to reconstruct past solar irradiance variations using historical full-disc Ca II K observations to describe the facular contribution independently of sunspot observations. Here, we develop the method and test it extensively by using modern CCD-based (charge-coupled device) Ca II K observations. We also carry out initial tests on two photographic archives.
Methods: We employ carefully reduced and calibrated Ca II K images from 13 datasets, including some of the most prominent series, such as those from the Meudon, Mt Wilson, and Rome observatories. We convert them to unsigned magnetic field maps and then use them as input to the adapted Spectral and Total Irradiance Reconstruction (SATIRE) model to reconstruct total solar irradiance (TSI) variations over the period 1978-2019, for which direct irradiance measurements are available.
Results: The reconstructed irradiance from the analysed Ca II K archives agrees well with direct irradiance measurements and existing reconstructions. The model also returns good results on data taken with different bandpasses and images with low spatial resolution. Historical Ca II K archives suffer from numerous inconsistencies, but we show that these archives can still be used to reconstruct TSI with reasonable accuracy provided the observations are accurately processed and the effects of changes in instrumentation and instrumental parameters are identified and accounted for. The reconstructions are relatively insensitive to the TSI reference record used to fix the single free parameter of the model. Furthermore, even employment of a series, itself reconstructed from Ca II K data, as a reference for further reconstructions returns nearly equally accurate results. This will enable the Ca II K archives without an overlap with direct irradiance measurements to be used to reconstruct past irradiance.
Conclusions: By using the unsigned magnetic maps of the Sun reconstructed from modern high-quality Ca II K observations as input into the SATIRE model, we can reconstruct solar irradiance variations nearly as accurately as from directly recorded magnetograms. Historical Ca II K observations can also be used for past irradiance reconstructions but need additional care, for example identifying and accounting for discontinuities and changes in the quality of the data with time. Title: Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-region Nesting Authors: Sowmya, K.; Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Witzke, V.; Mints, A.; Krivova, N. A.; Solanki, S. K. Bibcode: 2021ApJ...919...94S Altcode: 2021arXiv210701493S Ultra-precise astrometry from the Gaia mission is expected to lead to astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the factors that could hamper such detections is the astrometric jitter caused by the magnetic activity of the planet host stars. In our previous study, we modeled astrometric jitter for the Sun observed equator-on. In this work, we generalize our model and calculate the photocenter jitter as it would be measured by the Gaia and Small-JASMINE missions for stars with solar rotation rate and effective temperature, but with various values of the inclination angle of the stellar rotation axis. In addition, we consider the effect of metallicity and of nesting of active regions (i.e., the tendency of active regions to emerge in the vicinity of each other). We find that, while the jitter of stars observed equator-on does not have any long-term trends and can be easily filtered out, the photocenters of stars observed out of their equatorial planes experience systematic shifts over the course of the activity cycle. Such trends allow the jitter to be detected with continuous measurements, in which case it can interfere with planet detectability. An increase in the metallicity is found to increase the jitter caused by stellar activity. Active-region nesting can further enhance the peak-to-peak amplitude of the photocenter jitter to a level that could be detected by Gaia. Title: The relationship between bipolar magnetic regions and their sunspots Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Jiang, J. Bibcode: 2021A&A...654A..28Y Altcode: 2021arXiv210914313Y Context. The relationship between bipolar magnetic regions (BMRs) and their sunspots is an important property of the solar magnetic field, but it is not well constrained. One consequence is that it is a challenge for surface flux transport models (SFTMs) based on sunspot observations to determine the details of BMR emergence, which they require as input, from such data.
Aims: We aimed to establish the relationship between the amount of magnetic flux in newly emerged BMRs and the area of the enclosed sunspots, and examine the results of its application to an established SFTM.
Methods: Earlier attempts to constrain BMR magnetic flux were hindered by the fact that there is no extensive and reliable record of the magnetic and physical properties of newly emerged BMRs currently available. We made use of the empirical model of the relationship between the disc-integrated facular and network magnetic flux and the total surface coverage by sunspots reported in a recent study. The structure of the model is such that it enabled us to establish, from these disc-integrated quantities, an empirical relationship between the magnetic flux and sunspot area of individual newly emerged BMRs, circumventing the lack of any proper BMR database.
Results: Applying the constraint on BMR magnetic flux derived here to an established SFTM retained its key features, in particular its ability to replicate various independent datasets and the correlation between the model output polar field at the end of each cycle and the observed strength of the following cycle. The SFTM output indicates that facular and network magnetic flux rises with increasing sunspot magnetic flux at a slowing rate such that it appears to gradually saturate. This is analogous to what earlier studies comparing disc-integrated quantities sensitive to the amount of faculae and network present to sunspot indices had reported. The activity dependence of the ratio of facular and network flux to sunspot flux is consistent with the findings of recent studies: although the Sun is faculae-dominated (such that its brightness is mostly positively correlated with activity), it is only marginally so as facular and network brightening and sunspot darkening appear to be closely balanced. Title: On the size distribution of spots within sunspot groups Authors: Mandal, Sudip; Krivova, Natalie A.; Cameron, Robert; Solanki, Sami K. Bibcode: 2021A&A...652A...9M Altcode: 2021arXiv210403534M The size distribution of sunspots provides key information about the generation and emergence processes of the solar magnetic field. Previous studies of size distribution have primarily focused on either the whole group or individual spot areas. In this paper we investigate the organisation of spot areas within sunspot groups. In particular, we analysed the ratio (R) of the area of the biggest spot (Abig_spot) inside a group, to the total area of that group (Agroup). We used sunspot observations from Kislovodsk, Pulkovo, and Debrecen observatories, together covering solar cycles 17-24. We find that at the time when the group area reaches its maximum, the single biggest spot in a group typically occupies about 60% of the group area. For half of all groups, R lies in the range between roughly 50% and 70%. We also find R to change with Agroup, such that R reaches a maximum of about 0.65 for groups with Agroup ≈ 200 μHem and then remains at about 0.6 for larger groups. Our findings imply a scale-invariant emergence pattern, providing an observational constraint on the emergence process. Furthermore, extrapolation of our results to larger sunspot groups may have a bearing on the giant unresolved starspot features found in Doppler images of highly active Sun-like stars. Our results suggest that such giant features are composed of multiple spots, with the largest spot occupying roughly 55-75% of the total group area (i.e., the area of the giant starspots seen in Doppler images). Title: Forward modelling of Kepler-band variability due to faculae and spots Authors: Johnson, Luke J.; Norris, Charlotte M.; Unruh, Yvonne C.; Solanki, Sami K.; Krivova, Natalie; Witzke, Veronika; Shapiro, Alexander I. Bibcode: 2021MNRAS.504.4751J Altcode: 2021arXiv210411544J; 2021MNRAS.tmp.1190J Variability observed in photometric light curves of late-type stars (on time-scales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in light-curve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) light curves of active G2, K0, M0, and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of light-curve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density, and active band latitudes is explored. For our light-curve modelling approach we used ACTRESS, a geometrically accurate model for stellar variability. ACTRESS generates two-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, light curves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied ACTRESS in Monte Carlo simulations, calculating light-curve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of $1{{\ \rm per\ cent}}$, the typical variability of a solar-type star is around 2 parts per thousand. The presence of faculae clearly affects the mean brightness and light-curve shape, but has relatively little influence on the variability. Title: Modeling Stellar Ca II H and K Emission Variations. I. Effect of Inclination on the S-index Authors: Sowmya, K.; Shapiro, A. I.; Witzke, V.; Nèmec, N. -E.; Chatzistergos, T.; Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2021ApJ...914...21S Altcode: 2021arXiv210313893S The emission in the near-ultraviolet Ca II H and K lines is modulated by stellar magnetic activity. Although this emission, quantified via the S-index, has been serving as a prime proxy of stellar magnetic activity for several decades, many aspects of the complex relation between stellar magnetism and Ca II H and K emission are still unclear. The amount of measured Ca II H and K emission is suspected to be affected not only by the stellar intrinsic properties but also by the inclination angle of the stellar rotation axis. Until now, such an inclination effect on the S-index has remained largely unexplored. To fill this gap, we develop a physics-based model to calculate S-index, focusing on the Sun. Using the distributions of solar magnetic features derived from observations together with Ca II H and K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of the solar S-index over four activity cycles. Further, using the distribution of magnetic features over the visible solar disk obtained from surface flux transport simulations, we obtain S-index time series dating back to 1700 and investigate the effect of inclination on S-index variability on both the magnetic activity cycle and the rotational timescales. We find that when going from an equatorial to a pole-on view, the amplitude of S-index variations decreases weakly on the activity cycle timescale and strongly on the rotational timescale (by about 22% and 81%, respectively, for a cycle of intermediate strength). The absolute value of the S-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies. Title: Modelling the evolution of the Sun's open and total magnetic flux Authors: Krivova, N. A.; Solanki, S. K.; Hofer, B.; Wu, C. -J.; Usoskin, I. G.; Cameron, R. Bibcode: 2021A&A...650A..70K Altcode: 2021arXiv210315603K Solar activity in all its varied manifestations is driven by the magnetic field. Two global quantities are particularly important for many purposes, the Sun's total and open magnetic flux, which can be computed from sunspot number records using models. Such sunspot-driven models, however, do not take into account the presence of magnetic flux during grand minima, such as the Maunder minimum. Here we present a major update of a widely used simple model, which now takes into account the observation that the distribution of all magnetic features on the Sun follows a single power law. The exponent of the power law changes over the solar cycle. This allows for the emergence of small-scale magnetic flux even when no sunspots have been present for multiple decades and leads to non-zero total and open magnetic flux also in the deepest grand minima, such as the Maunder minimum, thus overcoming a major shortcoming of the earlier models. The results of the updated model compare well with the available observations and reconstructions of the solar total and open magnetic flux. This opens up the possibility of improved reconstructions of the sunspot number from time series of the cosmogenic isotope production rate. Title: Solar cyclic activity over the last millennium reconstructed from annual 14C data Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N. A.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2021A&A...649A.141U Altcode: 2021arXiv210315112U
Aims: The 11-year solar cycle (Schwabe cycle) is the dominant pattern of solar magnetic activity reflecting the oscillatory dynamo mechanism in the Sun's convection zone. Solar cycles have been directly observed since 1700, while indirect proxy data suggest their existence over a much longer period of time, but generally without resolving individual cycles and their continuity. Here we reconstruct individual solar cycles for the last millennium using recently obtained 14C data and state-of-the-art models.
Methods: Starting with the 14C production rate determined from the so far most precise measurements of radiocarbon content in tree rings, solar activity was reconstructed in the following three physics-based steps: (1) correction of the 14C production rate for the changing geomagnetic field; (2) computation of the open solar magnetic flux; and (3) conversion into sunspot numbers outside of grand minima. All known uncertainties, including both measurement and model uncertainties, were straightforwardly accounted for by a Monte-Carlo method.
Results: Cyclic solar activity is reconstructed for the period 971-1900 (85 individual cycles) along with its uncertainties. This more than doubles the number of solar cycles known from direct solar observations. We found that the lengths and strengths of well-defined cycles outside grand minima are consistent with those obtained from the direct sunspot observations after 1750. The validity of the Waldmeier rule (cycles with fast-rising phase tend to be stronger) is confirmed at a highly significant level. Solar activity is found to be in a deep grand minimum when the activity is mostly below the sunspot formation threshold for about 250 years. Therefore, although considerable cyclic variability in 14C is seen even during grand minima, individual solar cycles can hardly be reliably resolved therein. Three potential solar particle events, ca. 994, 1052, and 1279 AD, are shown to occur around the maximum phases of solar cycles.
Conclusions: A new approximately 1000-year-long solar activity reconstruction, in the form of annual (pseudo) sunspot numbers with the full assessment of all known uncertainties, is presented based on new high-precision Δ14C measurements and state-of-the-art models, more than doubling the number of individually resolved solar cycles. This forms a solid basis for new, more detailed studies of solar variability.

Tabular data of the reconstructed activity are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/649/A141 Title: VizieR Online Data Catalog: 1000-year sunspot series (Usoskin+, 2021) Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2021yCat..36490141U Altcode: A new quantitative reconstruction of annually resolved solar activity, in the form of SNs (at least outside grand minima) with a full uncertainty assessment, is presented for the period 971-1900. For the first time, individual solar cycles are presented for the whole of the last millennium, more than doubling the existing statistics of solar cycles.

Three tabular files are presented.

File 'osf.dat' contains annual reconstructions of the opens solar flux (OSF) [Fo] along with its 1-sigma uncertainties [sF] and the smoothed (22-yr SSA -- see text) values [<Fo>]. This table corresponds to Figure 8 in the text.

File 'osn.dat' contains annual reconstructions of the sunspot number (SN) [SN] along with its 1-sigma uncertainties [sSN] and the smoothed (22-yr SSA -- see text) values [<SN>]. This table corresponds to Figure 11 in the text.

File 'table1.dat' presents a textual tabular version of Table 1 in the text and contains the internal cycle number [n], years of minimum [Ymin] and maximum [Ymax] of each cycle, cycle-averaged sunspot number [<SN>], its 1-sigma uncertainty [sS], cycle length (min-to-min) [T], and quality flag [q].

(3 data files). Title: Amplifying variability of solar-like stars by active longitudes and nesting Authors: Isik, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2021csss.confE.279I Altcode: Many solar-type stars with near-solar rotation periods exhibit much stronger variability than the Sun (Reinhold et al. 2020). Some of these stars even show very regular, sine-like light curves. Motivated by solar activity complexes, we developed a numerical model to quantify the effect of active-region (AR) nesting and active longitudes on stellar brightness variations in the rotational time scale. Modelling ARs with facular and spot components, we simulated light curves covering four years and using the Kepler passband. We found that the combined effect of the degree of nesting and the activity level, both being somewhat higher than on the Sun, can explain the whole range of observed light-curve amplitudes of solar-like stars. While nesting at random longitudes can explain variability amplitudes and light-curve morphology in many cases, active-longitude-type nesting reproduces sine-like light curves and the highest amplitude variability. Title: Modelling Solar Ca II H&K Emission Variations Authors: Krishnamurthy, Sowmya; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina-E.; Chatzistergos, Theodosis; Yeo, Kok Leng; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2021csss.confE.154K Altcode: The emission in the near ultraviolet Ca II H&K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between the coverage of a stellar disk by magnetic features and Ca II H&K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we made use of the distributions of the solar magnetic features derived from the simulations of magnetic flux emergence and surface transport, together with the Ca II H&K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code.We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer's line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity. Title: Predictions of Astrometric Jitter for Sun-like Stars. I. The Model and Its Application to the Sun as Seen from the Ecliptic Authors: Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2021ApJ...908..223S Altcode: 2020arXiv201212312S The advent of Gaia, capable of measuring stellar wobbles caused by orbiting planets, raised interest in the astrometric detection of exoplanets. Another source of such wobbles (often also called jitter) is stellar magnetic activity. A quantitative assessment of the stellar astrometric jitter is important for a more reliable astrometric detection and characterization of exoplanets. We calculate the displacement of the solar photocenter due to the magnetic activity for an almost 16 yr period (1999 February 2-2014 August 1). We also investigate how the displacement depends on the spectral passband chosen for observations, including the wavelength range to be covered by the upcoming Small-JASMINE mission of JAXA. This is done by extending the SATIRE-S model for solar irradiance variability to calculating the displacement of the solar photocenter caused by the magnetic features on the surface of the Sun. We found that the peak-to-peak amplitude of the solar photocenter displacement would reach 0.5 μas if the Sun were located 10 pc away from the observer and observed in the Gaia G filter. This is by far too small to be detected by the Gaia mission. However, the Sun is a relatively inactive star so one can expect significantly larger signals for younger, and, consequently, more active stars. The model developed in this study can be combined with the simulations of emergence and surface transport of magnetic flux which have recently become available to model the astrometric jitter over the broad range of magnetic activities. Title: Reconstructing solar irradiance from Ca II K observations Authors: Chatzistergos, T.; Krivova, N.; Ermolli, I.; Yeo, K. L.; Solanki, S. K.; Puiu, C. C.; Giorgi, F.; Mandal, S. Bibcode: 2020AGUFMA237...10C Altcode: To understand the influence of the Sun on Earth's system, long and accurate measurements of solar irradiance are a prerequisite. The available direct measurements of solar irradiance since 1978 are clearly not sufficient for this purpose. This stimulated development of models used to reconstruct past solar irradiance variations from alternative observations. The main driver of the irradiance variations on time scales of days to millennia is the evolution of the solar surface magnetic field in form of dark sunspots and bright faculae and network. Therefore, models require input data describing the contributions of these various magnetic regions on the Sun at earlier times. Unfortunately, records that can be used to describe the facular and network contributions are barely longer than the direct irradiance measurements. Thus, irradiance reconstructions to earlier periods have to rely on sunspot data alone. Data that have hardly been used for solar irradiance reconstructions until now are full-disc solar observations in the Ca II K line. Such data exist since 1892 from various observatories and include all the needed information describing faculae and the network. However, they are plagued by a bunch of various problems and artefacts, and recovering the non-linear response of the photographic material to the radiation is non-trivial since the required information is usually missing, too. We have developed a method to process Ca II K observations from various sources and demonstrated the higher accuracy achieved by our method compared to other techniques presented in the literature. Here we use the carefully reduced Ca II K observations from multiple archives to reconstruct solar irradiance variations. Title: Erratum: "Amplification of Brightness Variability by Active-region Nesting in Solar-like Stars" (2020, ApJL, 901, L12) Authors: Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2020ApJ...905L..36I Altcode: No abstract at ADS Title: Reconstructing solar irradiance from Ca II K observations Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli, Ilaria; Leng Yeo, Kok; Solanki, Sami K.; Puiu, Cosmin C.; Giorgi, Fabrizio; Mandal, Sudip Bibcode: 2020AGUFM.A237..10C Altcode: To understand the influence of the Sun on Earth's system, long and accurate measurements of solar irradiance are a prerequisite. The available direct measurements of solar irradiance since 1978 are clearly not sufficient for this purpose. This stimulated development of models used to reconstruct past solar irradiance variations from alternative observations. The main driver of the irradiance variations on time scales of days to millennia is the evolution of the solar surface magnetic field in form of dark sunspots and bright faculae and network. Therefore, models require input data describing the contributions of these various magnetic regions on the Sun at earlier times. Unfortunately, records that can be used to describe the facular and network contributions are barely longer than the direct irradiance measurements. Thus, irradiance reconstructions to earlier periods have to rely on sunspot data alone. Data that have hardly been used for solar irradiance reconstructions until now are full-disc solar observations in the Ca II K line. Such data exist since 1892 from various observatories and include all the needed information describing faculae and the network. However, they are plagued by a bunch of various problems and artefacts, and recovering the non-linear response of the photographic material to the radiation is non-trivial since the required information is usually missing, too. We have developed a method to process Ca II K observations from various sources and demonstrated the higher accuracy achieved by our method compared to other techniques presented in the literature. Here we use the carefully reduced Ca II K observations from multiple archives to reconstruct solar irradiance variations. Title: How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations(Corrigendum) Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2020A&A...642C...2Y Altcode: No abstract at ADS Title: The Dimmest State of the Sun Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Rempel, M.; Anusha, L. S.; Shapiro, A. I.; Tagirov, R. V.; Witzke, V. Bibcode: 2020GeoRL..4790243Y Altcode: 2021arXiv210209487Y How the solar electromagnetic energy entering the Earth's atmosphere varied since preindustrial times is an important consideration in the climate change debate. Detrimental to this debate, estimates of the change in total solar irradiance (TSI) since the Maunder minimum, an extended period of weak solar activity preceding the industrial revolution, differ markedly, ranging from a drop of 0.75 W m-2 to a rise of 6.3 W m-2. Consequently, the exact contribution by solar forcing to the rise in global temperatures over the past centuries remains inconclusive. Adopting a novel approach based on state-of-the-art solar imagery and numerical simulations, we establish the TSI level of the Sun when it is in its least-active state to be 2.0 ± 0.7 W m-2 below the 2019 level. This means TSI could not have risen since the Maunder minimum by more than this amount, thus restricting the possible role of solar forcing in global warming. Title: Simulating Variability due to Faculae and Spots on GKM Stars Authors: Johnson, Luke; Unruh, Yvonne; Norris, Charlotte; Solanki, Sami; Krivova, Natalie; Witzke, Veronika; Shapiro, Alexander Bibcode: 2020EPSC...14..844J Altcode: Stellar variability is a dominant noise source in exoplanet surveys and results largely from the presence of photospheric faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We model the lightcurves of active late-type stars as they rotate, using emergent intensity spectra calculated from 3D magnetoconvection simulations of G, K and M-type stellar atmosphere regions at different viewing angles to reproduce centre-to-limb brightness variations. We present mean expected variability levels for several cases and compare with solar and stellar observations. We also investigate the wavelength dependence of variability. Fig. 1: Example of our geometrically accurate lightcurve modelling approach. Top: normalised intensity maps of a limb darkened, solar-type star viewed in the \textit{Kepler} band at rotational phase 0.5 with stellar inclinations 90 deg (left) and 30 deg (right). At 90 deg, the star is viewed equator-on. Middle: Corresponding lightcurves calculated at inclinations 90 deg (black line) and 30 deg (red line). Bottom: HealPix map representing the active stellar surface, cosine-scaled in latitude and flattened in longitude to resemble a solar synoptic map. The quiet photosphere is displayed in orange, facular regions are bright yellow and spot regions are dark blue. The crosses represent the centres of the stellar discs in the top panel. Fig. 2: Example showing simulated lightcurves calculated at different wavelengths. Rotational lightcurves are on the left, transit lightcurves on the right. In the centre, one hemisphere of the simulated stellar surface is shown, with a quarter of the disc shown in each wavelength band. 'Giant' spots and facular regions are used in this example. The transit path is highlighted in grey. Title: Amplification of Brightness Variability by Active-region Nesting in Solar-like Stars Authors: Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2020ApJ...901L..12I Altcode: 2020arXiv200900692I Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behavior of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR properties and differential rotation. We show that a moderate increase of the emergence frequency from the solar value combined with the increase of the degree of nesting can explain the full range of observed amplitudes of variability of Sun-like stars with nearly the solar rotation period. Furthermore, nesting in the form of active longitudes, in which ARs tend to emerge in the vicinity of two longitudes separated by 180°, leads to highly regular, almost sine-like variability patterns, rather similar to those observed in a number of solar-like stars. Title: Sunspot area catalog revisited: Daily cross-calibrated areas since 1874 Authors: Mandal, Sudip; Krivova, Natalie A.; Solanki, Sami K.; Sinha, Nimesh; Banerjee, Dipankar Bibcode: 2020A&A...640A..78M Altcode: 2020arXiv200414618M Context. Long and consistent sunspot area records are important for understanding long-term solar activity and variability. Multiple observatories around the globe have regularly recorded sunspot areas, but such individual records only cover restricted periods of time. Furthermore, there are systematic differences between these records and require cross-calibration before they can reliably be used for further studies.
Aims: We produce a cross-calibrated and homogeneous record of total daily sunspot areas, both projected and corrected, covering the period between 1874 and 2019. In addition, we generated a catalog of calibrated individual group areas for the same period.
Methods: We compared the data from nine archives: Royal Greenwich Observatory (RGO), Kislovodsk, Pulkovo, Debrecen, Kodaikanal, Solar Optical Observing Network (SOON), Rome, Catania, and Yunnan Observatories, covering the period between 1874 and 2019. Cross-comparisons of the individual records were done to produce homogeneous and inter-calibrated records of daily projected and corrected areas. As in earlier studies, the basis of the composite is formed by the data from RGO. After 1976, the only datasets used are those from Kislovodsk, Pulkovo, and Debrecen observatories. This choice was made based on the temporal coverage and the quality of the data. While there are still 776 days missing in the final composite, these remaining gaps could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the additional archives cover these periods.
Results: In contrast to the SOON data used in previous area composites for the post-RGO period, the properties of the data from Kislovodsk and Pulkovo are very similar to those from the RGO series. They also directly overlap the RGO data in time, which makes their cross-calibration with RGO much more reliable. Indeed, comparing our area catalog with previous such composites, we find improvements both in data quality and coverage. We also computed the daily Photometric Sunspot Index, which is widely used, for example, in empirical reconstructions of solar irradiance.

Generated composites are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/A78 and at http://www2.mps.mpg.de/projects/sun-climate/data.html Title: Modelling solar irradiance from ground-based photometric observations Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Giorgi, Fabrizio; Krivova, Natalie A.; Puiu, Cosmin Constantin Bibcode: 2020JSWSC..10...45C Altcode: 2020arXiv200810735C Total solar irradiance (TSI) has been monitored from space since 1978, i.e. for about four solar cycles. The measurements show a prominent variability in phase with the solar cycle, as well as fluctuations on timescales shorter than a few days. However, the measurements were done by multiple and usually relatively short-lived missions. The different absolute calibrations of the individual instruments and the unaccounted for instrumental trends make estimates of the possible long-term trend in the TSI highly uncertain. Furthermore, both the variability and the uncertainty are strongly wavelength-dependent. While the variability in the UV irradiance is clearly in-phase with the solar cycle, the phase of the variability in the visible range has been debated. In this paper, we aim at getting an insight into the long-term trend of TSI since 1996 and the phase of the solar irradiance variations in the visible part of the spectrum. We use independent ground-based full-disc photometric observations in Ca II K and continuum from the Rome and San Fernando observatories to compute the TSI since 1996. We follow the empirical San Fernando approach based on the photometric sum index. We find a weak declining trend in the TSI of {-7.8}-0.8+4.9× 1{0}-3 Wm-2 y-1 between the 1996 and 2008 activity minima, while between 2008 and 2019 the reconstructed TSI shows no trend to a marginally decreasing (but statistically insignificant) trend of {-0.1}-0.02+0.25× 1{0}-3 Wm-2 y-1. The reference TSI series used for the reconstruction does not significantly affect the determined trend. The variation in the blue continuum (409.2 nm) is rather flat, while the variation in the red continuum (607.1 nm) is marginally in anti-phase, although this result is extremely sensitive to the accurate assessment of the quiet Sun level in the images. These results provide further insights into the long-term variation of the TSI. The amplitude of the variations in the visible is below the uncertainties of the processing, which prevents an assessment of the phase of the variations. Title: Analysis of full-disc Ca II K spectroheliograms. III. Plage area composite series covering 1892-2019 Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K.; Banerjee, Dipankar; Barata, Teresa; Belik, Marcel; Gafeira, Ricardo; Garcia, Adriana; Hanaoka, Yoichiro; Hegde, Manjunath; Klimeš, Jan; Korokhin, Viktor V.; Lourenço, Ana; Malherbe, Jean-Marie; Marchenko, Gennady P.; Peixinho, Nuno; Sakurai, Takashi; Tlatov, Andrey G. Bibcode: 2020A&A...639A..88C Altcode: 2020arXiv200501435C Context. Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. The archives of full-disc Ca II K observations that have been performed more or less regularly at various sites since 1892 can serve as an important source of such information.
Aims: We derive the plage area evolution over the last 12 solar cycles by employing data from all Ca II K archives that are publicly available in digital form, including several as-yet-unexplored Ca II K archives.
Methods: We analysed more than 290 000 full-disc Ca II K observations from 43 datasets spanning the period between 1892-2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts affecting many of the images, including some very specific artefacts, such as bright arcs found in Kyoto and Yerkes data. Our employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here.
Results: We produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. The 'backbone' approach was used to bridge the series together. We have also shown that the selection of the backbone series has little effect on the final composite of the plage area. We quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during the activity maxima.
Conclusions: We provide the most complete so far time series of plage areas based on corrected and calibrated historical and modern Ca II K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.

The whole series described in the paper are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A88 Title: How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2020A&A...639A.139Y Altcode: 2020arXiv200614274Y Context. How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars.
Aims: We aim to extend and improve on earlier studies that established that the facular-to-sunspot-area ratio diminishes with total sunspot coverage.
Methods: Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as "facular indices", are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model, taking into account how active regions evolve and the possible non-linear relationship between plage emission, facular magnetic flux, and sunspot area. This model was incorporated into a model of total solar irradiance (TSI) to elucidate the implications for solar and stellar brightness variations.
Results: The reconstruction of the facular indices from the sunspot indices with the model presented here replicates most of the observed variability, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. This explains why even though solar cycle 23 is significantly weaker than cycle 22, TSI rose to comparable levels over both cycles. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition. This happens as sunspot darkening starts to rise more rapidly with activity than facular and network brightening. This bolsters the interpretation of this behaviour of Sun-like stars as the transition from a faculae-dominated to a spot-dominated regime. Title: Reply to the comment of T. Metcalfe and J. van Saders on the Science report "The Sun is less active than other solar-like stars" Authors: Reinhold, T.; Shapiro, A. I.; Solanki, S. K.; Montet, B. T.; Krivova, N. A.; Cameron, R. H.; Amazo-Gómez, E. M. Bibcode: 2020arXiv200704817R Altcode: This is our reply to the comment of T. Metcalfe and J. van Saders on the Science report "The Sun is less active than other solar-like stars" by T. Reinhold, A. I. Shapiro, S. K. Solanki, B. T. Montet, N. A. Krivova, R. H. Cameron, E. M. Amazo-Gomez. We hope that both the comment and our reply lead to fruitful discussions which of the two presented scenarios is more likely. Title: Connecting measurements of solar and stellar brightness variations Authors: Nèmec, N. -E.; Işık, E.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Unruh, Y. Bibcode: 2020A&A...638A..56N Altcode: 2020arXiv200406974N Context. A comparison of solar and stellar brightness variations is hampered by the difference in spectral passbands that are used in observations, and also by the possible difference in the inclination of the solar and stellar rotation axes from the line of sight.
Aims: We calculate the rotational variability of the Sun as it would be measured in passbands used for stellar observations. In particular, we consider the filter systems used by the CoRoT, Kepler, TESS, and Gaia space missions. We also quantify the effect of the inclination of the rotation axis on the solar rotational variability.
Methods: We employed the spectral and total irradiance reconstruction (SATIRE) model to calculate solar brightness variations in different filter systems as observed from the ecliptic plane. We then combined the simulations of the surface distribution of the magnetic features at different inclinations using a surface flux transport model with the SATIRE calculations to compute the dependence of the variability on the inclination.
Results: For an ecliptic-bound observer, the amplitude of the solar rotational variability, as observed in the total solar irradiance (TSI), is 0.68 mmag (averaged over solar cycles 21-24). We obtained corresponding amplitudes in the Kepler (0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag), Gaia GRP (0.62 mmag), and Gaia GBP (0.86 mmag) passbands. Decreasing the inclination of the rotation axis decreases the rotational variability. For a sample of randomly inclined stars, the variability is on average 15% lower in all filter systems we considered. This almost compensates for the difference in amplitudes of the variability in TSI and Kepler passbands, making the amplitudes derived from the TSI records an ideal representation of the solar rotational variability for comparison to Kepler stars with unknown inclinations.
Conclusions: The TSI appears to be a relatively good measure of solar variability for comparisons with stellar measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia GRP filters. Whereas the correction factors can be used to convert the variability amplitude from solar measurements into the values expected for stellar missions, the inclination affects the shapes of the light curves so that a much more sophisticated correction than simple scaling is needed to obtain light curves out of the ecliptic for the Sun. Title: Towards a more reliable reconstruction of the historical solar variability: a more realistic description of solar ephemeral magnetic regions Authors: Hofer, Bernhard; Krivova, Natalie A.; Wu, Chi-Ju; Usoskin, Ilya A.; Cameron, Robert Bibcode: 2020EGUGA..2217086H Altcode: Solar irradiance is a crucial input to climate models, but its measurements are only available since 1978. The variability of solar irradiance on climate-relevant time-scales is caused by the competition between bright and dark features formed by the magnetic fields emerging on the solar surface. Thus, models have been developed that reconstruct past irradiance variability from proxies of the solar magnetic activity. The longest direct proxy is the sunspot number. The common problem of such reconstructions is, however, that while sunspots adequately describe the evolution of the active regions (ARs) (large bipolar regions hosting sunspots), the evolution of their smaller counterparts, the ephemeral regions (ERs), is not directly featured by sunspots. At the same time, these small regions are much more numerous and are believed to be the main source of the long-term irradiance changes, which are of special interest to climate models. We develop an improved description of the ephemeral region emergence taking different solar observational constraints into account. The model builds on the SATIRE-T model, in which the emergence of ARs is described by the sunspot number and the emergence of the ERs is linearly linked to that of ARs. The latter, however, implies that whenever the sunspot number drops to zero, no magnetic field emerges in the model. In the new model, the emergence of the ERs is no longer linked to sunspots linearly. Instead, ARs and ERs are considered to be parts of a single power-law size distribution of the emerging magnetic regions. This ensures that even in the absence of ARs (e.g., during the grand minima of solar activity), the emergence rate of ERs remains non-zero. In particular, the solar open magnetic flux reconstructed using this approach does not drop to zero during the Maunder minimum, in agreement with independent reconstructions from the cosmogenic isotope data. Such an improved description of the ERs will allow a better constraint on the maximum solar irradiance drop during grand minima events. This, in turn, will allow a better constraint on the potential solar forcing in the future. Title: Historical solar Ca II K observations at the Kyoto and Sacramento Peak observatories Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020JPhCS1548a2007C Altcode: Archives of full-disc solar Ca II K observations covering even short periods of time can offer important data to fill observation gaps and to allow a better transition from historical to modern data. Two examples of such archives are those from the Kyoto and Sacramento Peak observatories. As most other historical Ca II K data, they suffer from artefacts that need to be accounted for to derive accurate plage areas. Here we present the results of our analysis of these archives of solar Ca II K images. Title: VizieR Online Data Catalog: Rotation periods of 97 solar-like stars (Witzke+, 2020) Authors: Witzke, V.; Reinhold, T.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020yCat..36349009W Altcode: The Kepler field of view was selected in order to contain a large fraction of solar-like stars. Focusing on stars in the effective temperature range of 5600K-5900K, it is challenging to determine their rotational periods.

Stellar fundamental parameters and rotation periods of the 97 periodic stars are presented.

(1 data file). Title: The Sun is less active than other solar-like stars Authors: Reinhold, Timo; Shapiro, Alexander I.; Solanki, Sami K.; Montet, Benjamin T.; Krivova, Natalie A.; Cameron, Robert H.; Amazo-Gómez, Eliana M. Bibcode: 2020Sci...368..518R Altcode: 2020arXiv200501401R The magnetic activity of the Sun and other stars causes their brightness to vary. We investigated how typical the Sun’s variability is compared with other solar-like stars, i.e., those with near-solar effective temperatures and rotation periods. By combining 4 years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we were able to measure photometric variabilities of 369 solar-like stars. Most of those with well-determined rotation periods showed higher variability than the Sun and are therefore considerably more active. These stars appear nearly identical to the Sun except for their higher variability. Therefore, we speculate that the Sun could potentially also go through epochs of such high variability. Title: VizieR Online Data Catalog: Plage area composite series (Chatzistergos+, 2020) Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K.; Banerjee, D.; Barata, T.; Belik, M.; Gafeira, R.; Garcia, A.; Hanaoka, Y.; Hegde, M.; Klimes, J.; Korokhin, V. V.; Lourenco, A.; Malherbe, J. -M.; Marchenko, G. P.; Peixinho, N.; Sakurai, T.; Tlatov, A. G. Bibcode: 2020yCat..36390088C Altcode: Annual, monthly, and daily values of the plage area composite series.

The archives included are those from the Arcetri (Ar), Baikal (Ba), Brussels (Br), Calern (CL), Catania (CT), Coimbra (Co), Kanzelhoehe (Ka), Kenwood (Ke), Kharkiv (Kh), Kislovodsk (Ki), Kodaikanal taken with the spectroheliograph (Ko), Kodaikanal taken with the Twin telescope (KT), Kodaikanal taken with the White-Light Active Region Monitor (WARM) telescope (KW), Kyoto (Ky), Manila (Ma), Mauna Loa (ML) taken with the Precision Solar Photometric Telescope (PSPT), McMath-Hulbert (MM), Mees (MS), Meudon taken with the spectroheliograph (MD1), Meudon taken with an interference filter (MD2), Mitaka taken with the spectroheliograph (Mi1), Mitaka taken with the Solar Flare Telescope with an interference filter (Mi2), Mt Wilson (MW), Pic du Midi (PM), SOlar Diameter Imager and Surface Mapper (SODISM) telescope on board the PICARD spacecraft (PS), Rome taken with the equatorial bar at Monte Mario (Ro), Rome taken with the PSPT (RP1), Rome taken with the PSPT with narrow bandwidth (RP2), Sacramento Peak (SP), San Fernando taken with the Cartesian Full-Disk Telescope (CFDT) 1 (SF1), San Fernando taken with the CFDT2 (SF2), Schauinsland (Sc), Teide (Te) taken with the Chromospheric Telescope (ChroTel), Upice (UP), Valasske Mezirici (VM), Wendelstein (WS), and Yerkes (YR) sites

(3 data files). Title: Power spectra of solar brightness variations at various inclinations Authors: Nèmec, N. -E.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K.; Tagirov, R. V.; Cameron, R. H.; Dreizler, S. Bibcode: 2020A&A...636A..43N Altcode: 2020arXiv200210895N Context. Magnetic features on the surfaces of cool stars lead to variations in their brightness. Such variations on the surface of the Sun have been studied extensively. Recent planet-hunting space telescopes have made it possible to measure brightness variations in hundred thousands of other stars. The new data may undermine the validity of setting the sun as a typical example of a variable star. Putting solar variability into the stellar context suffers, however, from a bias resulting from solar observations being carried out from its near-equatorial plane, whereas stars are generally observed at all possible inclinations.
Aims: We model solar brightness variations at timescales from days to years as they would be observed at different inclinations. In particular, we consider the effect of the inclination on the power spectrum of solar brightness variations. The variations are calculated in several passbands that are routinely used for stellar measurements.
Methods: We employ the surface flux transport model to simulate the time-dependent spatial distribution of magnetic features on both the near and far sides of the Sun. This distribution is then used to calculate solar brightness variations following the Spectral And Total Irradiance REconstruction approach.
Results: We have quantified the effect of the inclination on solar brightness variability at timescales down to a single day. Thus, our results allow for solar brightness records to be made directly comparable to those obtained by planet-hunting space telescopes. Furthermore, we decompose solar brightness variations into components originating from the solar rotation and from the evolution of magnetic features. Title: Inflection point in the power spectrum of stellar brightness variations. II. The Sun Authors: Amazo-Gómez, E. M.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Kopp, G.; Reinhold, T.; Oshagh, M.; Reiners, A. Bibcode: 2020A&A...636A..69A Altcode: 2020arXiv200203455A Context. Young and active stars generally have regular, almost sinusoidal, patterns of variability attributed to their rotation, while the majority of older and less active stars, including the Sun, have more complex and non-regular light curves, which do not have clear rotational-modulation signals. Consequently, the rotation periods have been successfully determined only for a small fraction of the Sun-like stars (mainly the active ones) observed by transit-based planet-hunting missions, such as CoRoT, Kepler, and TESS. This suggests that only a small fraction of such systems have been properly identified as solar-like analogues.
Aims: We aim to apply a new method of determining rotation periods of low-activity stars, such as the Sun. The method is based on calculating the gradient of the power spectrum (GPS) of stellar brightness variations and identifying a tell-tale inflection point in the spectrum. The rotation frequency is then proportional to the frequency of that inflection point. In this paper, we compare this GPS method to already-available photometric records of the Sun.
Methods: We applied GPS, auto-correlation functions, Lomb-Scargle periodograms, and wavelet analyses to the total solar irradiance (TSI) time series obtained from the Total Irradiance Monitor on the Solar Radiation and Climate Experiment and the Variability of solar IRradiance and Gravity Oscillations experiment on the SOlar and Heliospheric Observatory missions. We analysed the performance of all methods at various levels of solar activity.
Results: We show that the GPS method returns accurate values of solar rotation independently of the level of solar activity. In particular, it performs well during periods of high solar activity, when TSI variability displays an irregular pattern, and other methods fail. Furthermore, we show that the GPS and light curve skewness can give constraints on facular and spot contributions to brightness variability.
Conclusions: Our results suggest that the GPS method can successfully determine the rotational periods of stars with both regular and non-regular light curves.

The two movies are available at https://www.aanda.org Title: Solar-cycle irradiance variations over the last four billion years Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2020A&A...636A..83S Altcode: 2020arXiv200208806S Context. The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is strong evidence that the SSI variability has an effect on the Earth's atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could lead to a stronger response of the Earth's atmosphere as well as other solar system planets' atmospheres to the solar activity cycle.
Aims: We calculate the amplitude of the SSI and total solar irradiance (TSI) variability over the course of the solar activity cycle as a function of solar age.
Methods: We employed the relationship between the stellar magnetic activity and the age based on observations of solar twins. Using this relation, we reconstructed solar magnetic activity and the corresponding solar disk area coverages by magnetic features (i.e., spots and faculae) over the last four billion years. These disk coverages were then used to calculate the amplitude of the solar-cycle SSI variability as a function of wavelength and solar age.
Results: Our calculations show that the young Sun was significantly more variable than the present Sun. The amplitude of the solar-cycle TSI variability of the 600 Myr old Sun was about ten times larger than that of the present Sun. Furthermore, the variability of the young Sun was spot-dominated (the Sun being brighter at the activity minimum than in the maximum), that is, the Sun was overall brighter at activity minima than at maxima. The amplitude of the TSI variability decreased with solar age until it reached a minimum value at 2.8 Gyr. After this point, the TSI variability is faculae-dominated (the Sun is brighter at the activity maximum) and its amplitude increases with age. Title: VizieR Online Data Catalog: Sunspot area catalogue revisited (1874-2019) (Mandal+, 2020) Authors: Mandal, S.; Krivova, N.; Solanki, S. K.; Sinha, N.; Banerjee, D. Bibcode: 2020yCat..36400078M Altcode: A number of observatories around the globe have carried out measurements of sunspot areas and positions over the last century. RGO, the longest sunspot area database to date, started its campaign in 1874 and after continuing for a century, stopped it in 1976. Several other observatories from different parts of the world (e.g., Kodaikanal, Kislovodsk, Debrecen, Rome etc.) also carried out such observing programs throughout the 20th century. Sunspot area datasets are invaluable historical records of solar magnetic fields and are key to understanding the solar variability and its historical reconstructions.

In this work, we analyze and compare sunspot group areas from a total of nine observatories (RGO, Kislovodsk, Pulkovo, Debrecen, Kodaikanal, SOON, Rome, Catania, Yunnan). It turns out that data from only four observatories (RGO, Kislovodsk, Pulkovo, Debrecen) are sufficient to produce cross-calibrated, up-to-date (1874-2019) catalogs of daily total and individual group areas. The remaining gaps (776 days in total) could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the other archives cover these periods.

(2 data files). Title: Effect of metallicity on the detectability of rotational periods in solar-like stars Authors: Witzke, V.; Reinhold, T.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020A&A...634L...9W Altcode: 2020arXiv200101934W The accurate determination of stellar rotation periods is important for estimating stellar ages and for understanding stellar activity and evolution. While rotation periods can be determined for about thirty thousand stars in the Kepler field, there are over one hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars that are identical to our Sun with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature and photometric variability, and with metallicity between M/H = -0.35 and M/H = 0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we find a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87 solar-like Kepler stars for the first time. Title: Inflection point in the power spectrum of stellar brightness variations. I. The model Authors: Shapiro, A. I.; Amazo-Gómez, E. M.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020A&A...633A..32S Altcode: 2019arXiv191008351S Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods.
Aims: We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves.
Methods: We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features.
Results: The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log-log scale changes its sign, that is, by identifying the position of the inflection point.
Conclusions: The inflection point of the (log-log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate. Title: Readdressing the UV solar variability with SATIRE-S: non-LTE effects Authors: Tagirov, R. V.; Shapiro, A. I.; Krivova, N. A.; Unruh, Y. C.; Yeo, K. L.; Solanki, S. K. Bibcode: 2019A&A...631A.178T Altcode: 2019arXiv190911736T Context. Solar spectral irradiance (SSI) variability is one of the key inputs to models of the Earth's climate. Understanding solar irradiance fluctuations also helps to place the Sun among other stars in terms of their brightness variability patterns and to set detectability limits for terrestrial exoplanets.
Aims: One of the most successful and widely used models of solar irradiance variability is Spectral And Total Irradiance REconstruction model (SATIRE-S). It uses spectra of the magnetic features and surrounding quiet Sun that are computed with the ATLAS9 spectral synthesis code under the assumption of local thermodynamic equilibrium (LTE). SATIRE-S has been at the forefront of solar variability modelling, but due to the limitations of the LTE approximation its output SSI has to be empirically corrected below 300 nm, which reduces the physical consistency of its results. This shortcoming is addressed in the present paper.
Methods: We replaced the ATLAS9 spectra of all atmospheric components in SATIRE-S with spectra that were calculated using the Non-LTE Spectral SYnthesis (NESSY) code. To compute the spectrum of the quiet Sun and faculae, we used the temperature and density stratification models of the FAL set.
Results: We computed non-LTE contrasts of spots and faculae and combined them with the corresponding fractional disc coverages, or filling factors, to calculate the total and spectral irradiance variability during solar cycle 24. The filling factors have been derived from solar full-disc magnetograms and continuum images recorded by the Helioseismic and Magnetic Imager on Solar Dynamics Observatory (SDO/HMI).
Conclusions: The non-LTE contrasts yield total and spectral solar irradiance variations that are in good agreement with empirically corrected LTE irradiance calculations. This shows that the empirical correction applied to the SATIRE-S total and spectral solar irradiance is consistent with results from non-LTE computations. Title: Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A.; Banerjee, Dipankar; Jha, Bibhuti K.; Chatterjee, Subhamoy Bibcode: 2019SoPh..294..145C Altcode: 2019arXiv190805493C Full-disc Ca II K photographic observations of the Sun carry direct information as regards the evolution of solar-plage regions for more than a century and are therefore a unique dataset for solar-activity studies. For a long time Ca II K observations were barely explored, but recent digitizations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitized data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitization. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitized. The results obtained by different authors who analyzed the data from the new digitization of the Kodaikanal archive differ from each other and from those derived from the old digitization. Since the data were processed and analyzed using different techniques, it is not clear, however, whether the differences are due to the digitization or the processing of the data. To understand the reasons for such discrepancies, we analyze here the data from the two most recent digitizations of this archive. We use the same techniques to consistently process the images from both archives and to derive the plage areas from them. Some issues have been identified in both digitizations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitizations. Overall, the most recent 16-bit digitization offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred. Title: Solar irradiance variability over last four billion years Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2019EPSC...13.2071S Altcode: The action of dynamo generates magnetic field in the solar interior. This field then travels through the convective zone and emerges on the solar surface, leading to a various manifestations of solar magnetic activity. One of the most appealing among them is the variations of Spectral Solar Irradiance (SSI). There is an evidence that these variations have substantial effect on the Earth's climate system. The faster rotation of the Sun in the past led to a more vigorous dynamo and consequently larger amplitude of solar spectral irradiance variability. This could led to a stronger effect of the SSI variability on the Earth. The main goal of our study is to calculate the amplitude of the SSI variability over the course of the solar activity cycle (which presently lasts 11 years but could have different duration in the past) as a function of solar age. We utilise recently published relation between the stellar chromospheric activity and stellar age to reconstruct solar chromospheric activity back in time. It is used to calculate solar disk coverages by magnetic features, i.e. solar spots and faculae. Corresponding brightness variations are then computed using the SATIRE (which stands for Spectral and Total Irradiance Reconstruction) approach. Our study shows that the facular component of the irradiance variability over the solar activity cycle decreases slower with the solar age than the spot component. This makes the dependence of the amplitude of the solar variability on the age non-monotonic. The am- plitude decreases for the young Sun till it reaches minimum value and then gradually increases again. The variability of the Total Solar Irradiance (TSI, i.e. irradiance integrated over the entire spectral domain) changes from being spot- to facular-dominated at the solar age of about 2.8 Gyr. Our calculations show that the amplitude of the TSI variability of 600-Myr Sun was one order of magnitude larger than the present-day value. We have found that the age of the transition between spot- and facular-dominated regimes of the variability depends on the wavelength. For example, it is about 1.3 Gyr for the 210-400 nm spectral domain and becomes approximately 3.7 Gyr for the 400-700 nm spectral range. Our calculations of the past solar irradiance variability on the activity cycle timescale might be of interest for paleoclimate researchers as well as for modelling of atmospheres of exoplanets. Title: Cosmic-ray variability on the multi-millennial time scale: A new multi-proxy reconstruction Authors: Usoskin, I.; Wu, C. J.; Krivova, N.; Solanki, S. K.; Kovaltsov, G.; Baroni, M.; Bard, E. Bibcode: 2019ICRC...36.1164U Altcode: 2019PoS...358.1164U No abstract at ADS Title: Opacity distribution functions for stellar spectra synthesis Authors: Cernetic, M.; Shapiro, A. I.; Witzke, V.; Krivova, N. A.; Solanki, S. K.; Tagirov, R. V. Bibcode: 2019A&A...627A.157C Altcode: 2019arXiv190603112C Context. Stellar spectra synthesis is essential for the characterization of potential planetary hosts. In addition, comprehensive stellar variability calculations with fast radiative transfer are needed to disentangle planetary transits from stellar magnetically driven variability. The planet-hunting space telescopes, such as CoRoT, Kepler, and TESS, bring vast quantities of data, rekindling the interest in fast calculations of the radiative transfer.
Aims: We revisit the opacity distribution functions (ODF) approach routinely applied to speed up stellar spectral synthesis. To achieve a considerable speedup relative to the state of the art, we further optimize the approach and search for the best ODF configuration. Furthermore, we generalize the ODF approach for fast calculations of flux in various filters often used in stellar observations.
Methods: In a parameter-sweep fashion, we generated ODF in the spectral range from UV to IR with different setups. The most accurate ODF configuration for each spectral interval was determined. We adapted the wavelength grid based on the transmission curve for calculations of the radiative fluxes through filters before performing the normal ODF procedure.
Results: Our optimum ODF configuration allows for a three-fold speedup, compared to the previously used ODF configurations. The ODF generalization to calculate fluxes through filters results in a speedup of more than two orders of magnitude. Title: Recovering the unsigned photospheric magnetic field from Ca II K observations Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A.; Giorgi, Fabrizio; Yeo, Kok Leng Bibcode: 2019A&A...626A.114C Altcode: 2019arXiv190503453C Context. A number of studies have aimed at defining the exact form of the relation between magnetic field strength and Ca II H and K core brightness. All previous studies have however been restricted to isolated regions on the solar disc or to a limited set of observations.
Aims: We reassess the relationship between the photospheric magnetic field strength and the Ca II K intensity for a variety of surface features as a function of the position on the disc and the solar activity level. This relationship can be used to recover the unsigned photospheric magnetic field from images recorded in the core of Ca II K line.
Methods: We have analysed 131 pairs of high-quality, full-disc, near-co-temporal observations from the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To analytically describe the observationally determined relation, we considered three different functions: a power law with an offset, a logarithmic function, and a power-law function of the logarithm of the magnetic flux density. We used the obtained relations to reconstruct maps of the line-of-sight component of the unsigned magnetic field (unsigned magnetograms) from Ca II K observations, which were then compared to the original magnetograms.
Results: We find that both power-law functions represent the data well, while the logarithmic function is good only for quiet periods. We see no significant variation over the solar cycle or over the disc in the derived fit parameters, independently of the function used. We find that errors in the independent variable, which are usually not accounted for, introduce attenuation bias. To address this, we binned the data with respect to the magnetic field strength and Ca II K contrast separately and derived the relation for the bisector of the two binned curves. The reconstructed unsigned magnetograms show good agreement with the original ones. Root mean square differences are less than 90 G. The results were unaffected by the stray-light correction of the SDO/HMI and Rome/PSPT data.
Conclusions: Our results imply that accurately processed and calibrated Ca II K observations can be used to reconstruct unsigned magnetograms by using the relations derived in our study. Title: GPS, decrypting brightness variations of the Sun and Sun-like Authors: Amazo-Gómez, Eliana Maritza; Shapiro, Alexander I.; Solanki, Sami K.; Kopp, Greg; Oshagh, Mahmoudreza; Reinhold, Timo; Krivova, Natalie A.; Reiners, Ansgar Bibcode: 2019shin.confE.109A Altcode: The rotation period is in general detectable in the light curves of young and active stars. Even after successful stellar surveys stemmed from Kepler mission, there is still a lack of information in photometric records of rotation periods in Sun-like stars. Non-periodic light-curve profiles, low variability contrast -therefore low modulation amplitude- short lifetime evolution and random emergence of magnetic features (in comparison to the rotation time-scale) are the main reasons of unreliable determination of rotation periodicity in the Sun and its closer analogs. This indicates that only a small fraction of solar-like systems have been properly analyzed. We show that the rotation periods of those stars can be reliably determined from the profile of the gradient of the power spectra, GPS. By analysing periodic patterns in high-accuracy measurements of the total solar irradiance, TSI, by SORCE/TIM and SoHO/VIRGO missions, here we test and validate GPS, linking the variability by transits of magnetic features over the stellar surface with a clear and enhanced signal of the solar rotation. GPS method retrieves accurate and stable values of rotation period during different regimes of solar activity cycle and could be applied to stars of comparable and higher activity - where other methods underperform. Furthermore, GPS gives us constraints on the faculae to spot driver ratio and consequently help us to interpret the stellar surface. Title: Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2019A&A...625A..69C Altcode: 2019arXiv190207122C Context. Reconstructions of past irradiance variations require suitable data on solar activity. The longest direct proxy is the sunspot number, and it has been most widely employed for this purpose. These data, however, only provide information on the surface magnetic field emerging in sunspots, while a suitable proxy of the evolution of the bright magnetic features, specifically faculae/plage and network, is missing. This information can potentially be extracted from the historical full-disc observations in the Ca II K line.
Aims: We use several historical archives of full-disc Ca II K observations to derive plage areas over more than a century. Employment of different datasets allows the identification of systematic effects in the images, such as changes in instruments and procedures, as well as an assessment of the uncertainties in the results.
Methods: We have analysed over 100 000 historical images from eight digitised photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, and one archive of modern observations from the Rome/PSPT. The analysed data cover the period 1893-2018. We first performed careful photometric calibration and compensation for the centre-to-limb variation, and then segmented the images to identify plage regions. This has been consistently applied to both historical and modern observations.
Results: The plage series derived from different archives are generally in good agreement with each other. However, there are also clear deviations that most likely hint at intrinsic differences in the data and their digitisation. We showed that accurate image processing significantly reduces errors in the plage area estimates. Accurate photometric calibration also allows precise plage identification on images from different archives without the need to arbitrarily adjust the segmentation parameters. Finally, by comparing the plage area series from the various records, we found the conversion laws between them. This allowed us to produce a preliminary composite of the plage areas obtained from all the datasets studied here. This is a first step towards an accurate assessment of the long-term variation of plage regions.

The data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A69 Title: Evolution of Large-Scale Magnetic Fields From Near-Earth Space During the Last 11 Solar Cycles Authors: Pick, Leonie; Korte, Monika; Thomas, Yannik; Krivova, Natalie; Wu, Chi-Ju Bibcode: 2019JGRA..124.2527P Altcode: We use hourly mean magnetic field measurements from 34 midlatitude geomagnetic observatories between 1900 and 2015 to investigate the long-term evolution and driving mechanism of the large-scale external magnetic field at ground. The Hourly Magnetospheric Currents index (HMC) is derived as a refinement of the Annual Magnetospheric Currents index (HMC, Pick & Korte, 2017, https://doi.org/10.1093/gji/ggx367). HMC requires an extensive revision of the observatory hourly means. It depends on three third party geomagnetic field models used to eliminate the core, the crustal, and the ionospheric solar-quiet field contributions. We mitigate the dependency of HMC on the core field model by subtracting only nondipolar components of the model from the data. The separation of the residual (dipolar) signal into internal and external (HMC) parts is the main methodological challenge. Observatory crustal biases are updated with respect to AMC, and the solar-quiet field estimation is extended to the past based on a reconstruction of solar radio flux (F10.7). We find that HMC has more power at low frequencies (periods ≥ 1 year) than the Dcx index, especially at periods relevant to the solar cycle. Most of the slow variations in HMC can be explained by the open solar magnetic flux. There is a weakly decreasing linear trend in absolute HMC from 1900 to present, which depends sensitively on the data rejection criteria at early years. HMC is well suited for studying long-term variations of the geomagnetic field. Title: Intensity contrast of solar network and faculae. II. Implications for solar irradiance modelling Authors: Yeo, K. L.; Krivova, N. A. Bibcode: 2019A&A...624A.135Y Altcode: 2021arXiv210209530Y
Aims: We aim to gain insight into the effect of network and faculae on solar irradiance from their apparent intensity.
Methods: Taking full-disc observations from the Solar Dynamics Observatory, we examined the intensity contrast of network and faculae in the continuum and core of the Fe I 6173 Å line and 1700 Å, including the variation with magnetic flux density, distance from disc centre, nearby magnetic fields, and time.
Results: The brightness of network and faculae is believed to be suppressed by nearby magnetic fields from its effect on convection. We note that the degree of magnetically crowding of an area also affects the magnetic flux tube sizes and the depth at which magnetic concentrations are embedded in intergranular lanes, such that intensity contrast can be enhanced in magnetically crowded areas at certain flux densities and distances from disc centre. The difference in intensity contrast between the quiet-Sun network and active region faculae, noted by various studies, arises because active regions are more magnetically crowded and is not due to any fundamental physical differences between network and faculae. These results highlight that solar irradiance models need to include the effect of nearby magnetic fields on network and faculae brightness. We found evidence that suggests that departures from local thermal equilibrium (LTE) might have limited effect on intensity contrast. This could explain why solar irradiance models that are based on the intensity contrast of solar surface magnetic features calculated assuming LTE reproduce the observed spectral variability even where the LTE assumption breaks down. Certain models of solar irradiance employ chromospheric indices as direct indications of the effect of network and faculae on solar irradiance. Based on past studies of the Ca II K line and on the intensity contrast measurements derived here, we show that the fluctuations in chromospheric emission from network and faculae are a reasonable estimate of the emission fluctuations in the middle photosphere, but not of those in the lower photosphere. This is due to the different physical mechanisms that underlie the magnetic intensity enhancement in the various atmospheric regimes, and represents a fundamental limitation of these solar irradiance models. Any time variation in the radiant properties of network and faculae is, of course, relevant to their effect on solar irradiance. The data set, which extends from 2010 to 2018, indicates that their intensity contrast was stable to about 3% in this period.
Conclusions: This study offers new insights into the radiant behaviour of network and faculae, with practical implications for solar irradiance modelling. Title: VizieR Online Data Catalog: Plage area composite series (Chatzistergos+, 2019) Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2019yCat..36250069C Altcode: Annual, monthly, and daily values of the plage area composite series produced in the paper.

(3 data files). Title: Historical solar Ca II K observations at the Rome and Catania observatories Authors: Chatzistergos, T.; Ermolli, I.; Falco, M.; Giorgi, F.; Guglielmino, S. L.; Krivova, N. A.; Romano, P.; Solanki, S. K. Bibcode: 2019NCimC..42....5C Altcode: 2019arXiv190207483C Here we present the little explored Ca II K archives from the Rome and the Catania observatories and analyse the digitised images from these archives to derive plage areas. Title: Solar total and spectral irradiance reconstruction over the last 9000 years Authors: Wu, C. -J.; Krivova, N. A.; Solanki, S. K.; Usoskin, I. G. Bibcode: 2018A&A...620A.120W Altcode: 2018arXiv181103464W Context. Changes in solar irradiance and in its spectral distribution are among the main natural drivers of the climate on Earth. However, irradiance measurements are only available for less than four decades, while assessment of solar influence on Earth requires much longer records.
Aims: The aim of this work is to provide the most up-to-date physics-based reconstruction of the solar total and spectral irradiance (TSI/SSI) over the last nine millennia.
Methods: The concentrations of the cosmogenic isotopes 14C and 10Be in natural archives have been converted to decadally averaged sunspot numbers through a chain of physics-based models. TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are carried out for each isotope record separately, as well as for their composite.
Results: We present the first ever SSI reconstruction over the last 9000 years from the individual 14C and 10Be records as well as from their newest composite. The reconstruction employs physics-based models to describe the involved processes at each step of the procedure.
Conclusions: Irradiance reconstructions based on two different cosmogenic isotope records, those of 14C and 10Be, agree well with each other in their long-term trends despite their different geochemical paths in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular variability in TSI is of the order of 0.11%, or 1.5 W m-2. After the Maunder minimum, the reconstruction from the cosmogenic isotopes is consistent with that from the direct sunspot number observation. Furthermore, over the nineteenth century, the agreement of irradiance reconstructions using isotope records with the reconstruction from the sunspot number by Chatzistergos et al. (2017, A&A, 602, A69) is better than that with the reconstruction from the WDC-SILSO series (Clette et al. 2014, Space Sci. Rev., 186, 35), with a lower χ2-value. Title: Activity variation driven by flux emergence and transport on Sun-like stars Authors: Işık, Emre; Solanki, Sami K.; Krivova, Natalie A.; Shapiro, Alexander I. Bibcode: 2018arXiv181208976I Altcode: In G dwarfs, the surface distribution, coverage and lifetimes of starspots deviate from solar-like patterns as the rotation rate increases. We set up a numerical platform which includes the large-scale rotational and surface flow effects, aiming to simulate evolving surface patterns over an activity cycle for up to 8 times the solar rotation and flux emergence rates. At the base of the convection zone, we assume a solar projected butterfly diagram. We then follow the rotationally distorted trajectories of rising thin flux tubes to obtain latitudes and tilt angles. Using them as source distributions, we run a surface flux transport model with solar parameters. Our model predicts surface distributions of the signed radial fields and the starspots that qualitatively agree with observations. Title: Forward modelling of brightness variations in Sun-like stars. I. Emergence and surface transport of magnetic flux Authors: Işık, E.; Solanki, S. K.; Krivova, N. A.; Shapiro, A. I. Bibcode: 2018A&A...620A.177I Altcode: 2018arXiv181006728I Context. The latitudinal distribution of starspots deviates from the solar pattern with increasing rotation rate. Numerical simulations of magnetic flux emergence and transport can help model the observed stellar activity patterns and the associated brightness variations.
Aims: We set up a composite model for the processes of flux emergence and transport on Sun-like stars to simulate stellar brightness variations for various levels of magnetic activity and rotation rates.
Methods: Assuming that the distribution of magnetic flux at the base of the convection zone follows solar scaling relations, we calculate the emergence latitudes and tilt angles of bipolar regions at the surface for various rotation rates, using thin-flux-tube simulations. Taking these two quantities as input to a surface flux transport (SFT) model, we simulate the diffusive-advective evolution of the radial field at the stellar surface, including effects of active region nesting.
Results: As the rotation rate increases, (1) magnetic flux emerges at higher latitudes and an inactive gap opens around the equator, reaching a half-width of 20° for 8 Ω⊙; and (2) the tilt angles of freshly emerged bipolar regions show stronger variations with latitude. Polar spots can form at 8 Ω⊙ by accumulation of follower-polarity flux from decaying bipolar regions. From 4 Ω⊙ to 8 Ω⊙, the maximum spot coverage changes from 3 to 20%, respectively, compared to 0.4% in the solar model. Nesting of activity can lead to strongly non-axisymmetric spot distributions.
Conclusions: On Sun-like stars rotating at 8 Ω⊙ (Prot ≃ 3 days), polar spots can form, owing to higher levels of flux emergence rate and tilt angles. Defining spots by a threshold field strength yields global spot coverages that are roughly consistent with stellar observations. Title: From solar to stellar brightness variations. The effect of metallicity Authors: Witzke, V.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Schmutz, W. Bibcode: 2018A&A...619A.146W Altcode: 2018arXiv180904360W Context. Comparison studies of Sun-like stars with the Sun suggest an anomalously low photometric variability of the Sun compared to Sun-like stars with similar magnetic activity. Comprehensive understanding of stellar variability is needed to find a physical reason for this observation.
Aims: We investigate the effect of metallicity and effective temperature on the photometric brightness change of Sun-like stars seen at different inclinations. The considered range of fundamental stellar parameters is sufficiently small so the stars investigated here still count as Sun-like or even as solar twins.
Methods: To model the brightness change of stars with solar magnetic activity, we extended a well-established model of solar brightness variations based on solar spectra, Spectral And Total Irradiance REconstruction (SATIRE), to stars with different fundamental parameters. For this we calculated stellar spectra for different metallicities and effective temperature using the radiative transfer code ATLAS9.
Results: We show that even a small change (e.g. within the observational error range) of metallicity or effective temperature significantly affects the photometric brightness change compared to the Sun. We find that for Sun-like stars, the amplitude of the brightness variations obtained for Strömgren (b + y)/2 reaches a local minimum for fundamental stellar parameters close to the solar metallicity and effective temperature. Moreover, our results show that the effect of inclination decreases for metallicity values greater than the solar metallicity. Overall, we find that an exact determination of fundamental stellar parameters is crucially important for understanding stellar brightness changes. Title: ACTReSS: ACTive Rotating Star Simulator Authors: Johnson, Luke Jonathan; Norris, Charlotte; Unruh, Yvonne; Solanki, Sami; Krivova, Natalie Bibcode: 2018csss.confE..58J Altcode: Variability of late-type stars on timescales of days or longer is largely due to magnetic surface features (starspots and faculae). We present ACTReSS, a software tool to calculate rotational lightcurves of stars. The model uses limb-dependent intensities derived from MURaM 3-D magneto-convection simulations (Beeck et al. 2014) for quiet stellar photospheres and bright active regions. This allows us to investigate the dependence of the flux variation on spectral type, inclination and wavelength. Title: Solar irradiance variability on time scales of decades to millennia Authors: Krivova, Natalie; Solanki, Sami K.; Leng Yeo, Kok; Wu, Chi-Ju Bibcode: 2018cosp...42E1847K Altcode: Reconstructions of past solar irradiance changes over as long periodsof time as possible are crucial to our understanding of solarinfluence on climate. They are only possible with the help of suitablemodels, based on detailed understanding of the mechanisms of thevariability. With the advance of such models, also the pastreconstructions are becoming more reliable. Nevertheless, theremaining uncertainties spread out when extrapolating back over longperiods of time. They are further amplified by the increasingly pooreramount and quality of the available data that bear information on pastsolar activity. We will discuss the progress and the uncertainties ofirradiance reconstructions on time scales of decades to millennia. Title: Solar Activity and Irradiance Reconstruction over the Holocene Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami K.; Kovaltsov, Gennady A.; Baroni, Melanie; Bard, Edouard Bibcode: 2018cosp...42E3695W Altcode: Direct measurements of the solar irradiance are only available since 1978. To understand the solar influence on Earth's climate, longer records and thus reconstructions of the solar variability in the past are needed. The directly observed sunspot number allows going back to 1610 A.D, although with progressively increasing uncertainty. To assess solar variability at earlier times, one has to rely on indirect proxies of solar activity, such as concentrations of cosmogenic isotopes 10Be and 14C in terrestrial archives. They are produced mostly in the upper atmosphere by impinging galactic cosmic rays (GCRs). The flux of GCRs is modulated by both the heliospheric magnetic field and the geomagnetic field. Therefore, the isotope signals retrieved from various sites around the globe show a very high degree of similarity, reflecting changes in the solar activity. Still, short-and mid-term deviations can be observed due to various systematic effects, such as different geochemical production, atmospheric distribution processes and local climatic conditions. To account for these differences, we have constructed a state-of-the-art consistent multi-isotope composite from one global 14C and six regional 10Be data sets. This composite is then used to reconstruct decadal values of the total and spectral solar irradiance over the Holocene with the semi-empirical SATIRE-M model, while the quasi-11 year solar cycle has been simulated statistically. Title: Solar activity over nine millennia: A consistent multi-proxy reconstruction Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.; Baroni, M.; Bard, E.; Solanki, S. K. Bibcode: 2018A&A...615A..93W Altcode: 2018arXiv180401302W
Aims: The solar activity in the past millennia can only be reconstructed from cosmogenic radionuclide proxy records in terrestrial archives. However, because of the diversity of the proxy archives, it is difficult to build a homogeneous reconstruction. All previous studies were based on individual, sometimes statistically averaged, proxy datasets. Here we aim to provide a new consistent multi-proxy reconstruction of the solar activity over the last 9000 yr, using all available long-span datasets of 10Be and 14C in terrestrial archives.
Methods: A new method, based on a Bayesian approach, was applied for the first time to solar activity reconstruction. A Monte Carlo search (using the χ2 statistic) for the most probable value of the modulation potential was performed to match data from different datasets for a given time. This provides a straightforward estimate of the related uncertainties. We used six 10Be series of different lengths (from 500-10 000 yr) from Greenland and Antarctica, and the global 14C production series. The 10Be series were resampled to match wiggles related to the grand minima in the 14C reference dataset. The stability of the long data series was tested.
Results: The Greenland Ice-core Project (GRIP) and the Antarctic EDML (EPICA Dronning Maud Land) 10Be series diverge from each other during the second half of the Holocene, while the 14C series lies in between them. A likely reason for the discrepancy is the insufficiently precise beryllium transport and deposition model for Greenland, which leads to an undercorrection of the GRIP series for the geomagnetic shielding effect. A slow 6-7 millennia variability with lows at ca. 5500 BC and 1500 AD in the long-term evolution of solar activity is found. Two components of solar activity can be statistically distinguished: the main component, corresponding to the "normal" moderate level, and a component corresponding to grand minima. A possible existence of a component representing grand maxima is indicated, but it cannot be separated from the main component in a statistically significant manner.
Conclusions: A new consistent reconstruction of solar activity over the last nine millennia is presented with the most probable values of decadal sunspot numbers and their realistic uncertainties. Independent components of solar activity corresponding to the main moderate activity and the grand-minimum state are identified; they may be related to different operation modes of the dynamo.

A table with the reconstructed SN series is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/A93 Title: Reconstructing Total Solar Irradiance with the help of MHD simulations Authors: Leng Yeo, Kok; Solanki, Sami; Noris, Charlotte; Unruh, Yvonne; Krivova, Natalie; Beeck, Benjamin Bibcode: 2018EGUGA..20.8166L Altcode: The variation in the solar irradiance is an important input into the climate system. Whereas it is commonly thought that solar irradiance variability is driven by the evolving surface magnetism, verifying this assumption has been more difficult. Making use of 3D MHD simulations of the solar atmosphere layers and state-of-the-art solar magnetograms from the Solar Dynamics Observatory, we present a model of total solar irradiance (TSI) that does not require any calibration of the modeled TSI by the measured TSI. I.e., the modeled irradiance variability is entirely independent of the observational record, except for the absolute level. The model replicates 95% of the observed variability between April 2010 and July 2016, leaving little scope for alternative drivers of solar irradiance variability, at least over the time scales examined (days to years). Title: VizieR Online Data Catalog: Reconstructed decadal sunspot numbers (Wu+, 2018) Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.; Baroni, M.; Bard, E.; Solanki, S. K. Bibcode: 2018yCat..36150093W Altcode: The file contains the decadal numbers of sunspot groups including the decade central year [Y], the low [Sl], the mean [SM] and the upper [S_u] 1-sigma (68% bounds).

(1 data file). Title: Ca II K spectroheliograms for studies of long-term changes in solar irradiance Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2018IAUS..340..125C Altcode: 2018arXiv180503928C We address the importance of historical full-disc Ca II K spectroheliograms for solar activity and irradiance reconstruction studies. We review our work on processing such data to enable them to be used in irradiance reconstructions. We also present our preliminary estimates of the plage areas from five of the longest available historical Ca II K archives. Title: The potential of Ca II K observations for solar activity and variability studies Authors: Ermolli, Ilaria; Chatzistergos, Theodosios; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2018IAUS..340..115E Altcode: 2018arXiv180504483E Several observatories around the globe started regular full-disc imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. These observations are continued today at a few sites with either old spectroheliographs or modern telescopes equipped with narrow-band filters. The Ca II K time series are unique in representing long-term variations of the Sun's chromospheric magnetic field. However, meaningful results from their analysis require accurate processing of the available data and robust merging of the information stored in different archives. This paper provides an overview of the historical and modern full-disc Ca II K observations, with focus on their quality and the main results obtained from their analysis over the last decade. Title: The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity Authors: Karoff, Christoffer; Metcalfe, Travis S.; Santos, Ângela R. G.; Montet, Benjamin T.; Isaacson, Howard; Witzke, Veronika; Shapiro, Alexander I.; Mathur, Savita; Davies, Guy R.; Lund, Mikkel N.; Garcia, Rafael A.; Brun, Allan S.; Salabert, David; Avelino, Pedro P.; van Saders, Jennifer; Egeland, Ricky; Cunha, Margarida S.; Campante, Tiago L.; Chaplin, William J.; Krivova, Natalie; Solanki, Sami K.; Stritzinger, Maximilian; Knudsen, Mads F. Bibcode: 2018ApJ...852...46K Altcode: 2017arXiv171107716K Observations of Sun-like stars over the past half-century have improved our understanding of how magnetic dynamos, like that responsible for the 11 yr solar cycle, change with rotation, mass, and age. Here we show for the first time how metallicity can affect a stellar dynamo. Using the most complete set of observations of a stellar cycle ever obtained for a Sun-like star, we show how the solar analog HD 173701 exhibits solar-like differential rotation and a 7.4 yr activity cycle. While the duration of the cycle is comparable to that generated by the solar dynamo, the amplitude of the brightness variability is substantially stronger. The only significant difference between HD 173701 and the Sun is its metallicity, which is twice the solar value. Therefore, this provides a unique opportunity to study the effect of the higher metallicity on the dynamo acting in this star and to obtain a comprehensive understanding of the physical mechanisms responsible for the observed photometric variability. The observations can be explained by the higher metallicity of the star, which is predicted to foster a deeper outer convection zone and a higher facular contrast, resulting in stronger variability. Title: Analysis of full disc Ca II K spectroheliograms. I. Photometric calibration and centre-to-limb variation compensation Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2018A&A...609A..92C Altcode: 2017arXiv171004949C Context. Historical Ca II K spectroheliograms (SHG) are unique in representing long-term variations of the solar chromospheric magnetic field. They usually suffer from numerous problems and lack photometric calibration. Thus accurate processing of these data is required to get meaningful results from their analysis.
Aims: In this paper we aim at developing an automatic processing and photometric calibration method that provides precise and consistent results when applied to historical SHG.
Methods: The proposed method is based on the assumption that the centre-to-limb variation of the intensity in quiet Sun regions does not vary with time. We tested the accuracy of the proposed method on various sets of synthetic images that mimic problems encountered in historical observations. We also tested our approach on a large sample of images randomly extracted from seven different SHG archives.
Results: The tests carried out on the synthetic data show that the maximum relative errors of the method are generally <6.5%, while the average error is <1%, even if rather poor quality observations are considered. In the absence of strong artefacts the method returns images that differ from the ideal ones by <2% in any pixel. The method gives consistent values for both plage and network areas. We also show that our method returns consistent results for images from different SHG archives.
Conclusions: Our tests show that the proposed method is more accurate than other methods presented in the literature. Our method can also be applied to process images from photographic archives of solar observations at other wavelengths than Ca II K. Title: Solar Spectral Irradiance Reconstruction over 9 Millennia from a Composite 14C and 10Be Series Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G.; Solanki, S. K. Bibcode: 2017AGUFMSH42A..03W Altcode: The Sun is the main external energy source to the Earth and thus the knowledge of solar variability on different time scales is important for understanding the solar influence on the terrestrial atmosphere and climate. The overall energy input and its spectral distribution are described by the total (TSI) and spectral (SSI) solar irradiance, respectively. Direct measurements of the solar irradiance provide information on solar variability on the decadal and shorter time scales, while the sunspot number record covers four centuries. On yet longer time scales only indirect proxies can be used, such as the concentrations of the cosmogenic isotopes 10Be and 14C in terrestrial archives. These isotopes are produced in the terrestrial atmosphere by impinging cosmic rays, whose flux is modulated by solar activity. Therefore the isotope data retrieved from various natural archives around the globe show a very high degree of similarity reflecting changes in the solar activity. Nevertheless, significant short-term deviations can be observed due to the different geochemical production processes and local climatic conditions. We will present the newest TSI/SSI reconstruction over the last 9000 years based on a new consistent composite multi-isotope proxy series. The solar irradiance reconstruction reveals the global and robust pattern of solar variability in the past. Title: Spectral variability of photospheric radiation due to faculae. I. The Sun and Sun-like stars Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne C.; Solanki, Sami K.; Krivova, Natalie A.; Yeo, Kok Leng Bibcode: 2017A&A...605A..45N Altcode: 2017arXiv170504455N Context. Stellar spectral variability on timescales of a day and longer, arising from magnetic surface features such as dark spots and bright faculae, is an important noise source when characterising extra-solar planets. Current 1D models of faculae do not capture the geometric properties and fail to reproduce observed solar facular contrasts. Magnetoconvection simulations provide facular contrasts accounting for geometry.
Aims: We calculate facular contrast spectra from magnetoconvection models of the solar photosphere with a view to improve (a) future parameter determinations for planets with early G type host stars and (b) reconstructions of solar spectral variability.
Methods: Regions of a solar twin (G2, log g = 4.44) atmosphere with a range of initial average vertical magnetic fields (100 to 500 G) were simulated using a 3D radiation-magnetohydrodynamics code, MURaM, and synthetic intensity spectra were calculated from the ultraviolet (149.5 nm) to the far infrared (160 000 nm) with the ATLAS9 radiative transfer code. Nine viewing angles were investigated to account for facular positions across most of the stellar disc.
Results: Contrasts of the radiation from simulation boxes with different levels of magnetic flux relative to an atmosphere with no magnetic field are a complicated function of position, wavelength and magnetic field strength that is not reproduced by 1D facular models. Generally, contrasts increase towards the limb, but at UV wavelengths a saturation and decrease are observed close to the limb. Contrasts also increase strongly from the visible to the UV; there is a rich spectral dependence, with marked peaks in molecular bands and strong spectral lines. At disc centre, a complex relationship with magnetic field was found and areas of strong magnetic field can appear either dark or bright, depending on wavelength. Spectra calculated for a wide variety of magnetic fluxes will also serve to improve total and spectral solar irradiance reconstructions. Title: Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface Authors: Yeo, K. L.; Solanki, S. K.; Norris, C. M.; Beeck, B.; Unruh, Y. C.; Krivova, N. A. Bibcode: 2017PhRvL.119i1102Y Altcode: 2017arXiv170900920Y The variation in the radiative output of the Sun, described in terms of solar irradiance, is important to climatology. A common assumption is that solar irradiance variability is driven by its surface magnetism. Verifying this assumption has, however, been hampered by the fact that models of solar irradiance variability based on solar surface magnetism have to be calibrated to observed variability. Making use of realistic three-dimensional magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art solar magnetograms from the Solar Dynamics Observatory, we present a model of total solar irradiance (TSI) that does not require any such calibration. In doing so, the modeled irradiance variability is entirely independent of the observational record. (The absolute level is calibrated to the TSI record from the Total Irradiance Monitor.) The model replicates 95% of the observed variability between April 2010 and July 2016, leaving little scope for alternative drivers of solar irradiance variability at least over the time scales examined (days to years). Title: The Solar Ultraviolet Imaging Telescope on-board Aditya-L1 Authors: Tripathi, Durgesh; Ramaprakash, A. N.; Khan, Aafaque; Ghosh, Avyarthana; Chatterjee, Subhamoy; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K. Bibcode: 2017CSci..113..616T Altcode: 2022arXiv220407732T The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 mission of ISRO that will measure and monitor the solar radiation emitted in the near-ultraviolet wavelength range (200-400 nm). SUIT will simultaneously map the photosphere and the chromosphere of the Sun using 11 filters sensitive to different wavelengths and covering different heights in the solar atmosphere and help us understand the processes involved in the transfer of mass and energy from one layer to the other. SUIT will also allow us to measure and monitor spatially resolved solar spectral irradiance that governs the chemistry of oxygen and ozone in the stratosphere of Earth's atmosphere. This is central to our understanding of the Sun climate relationship. Title: The nature of solar brightness variations Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Yeo, K. L.; Schmutz, W. K. Bibcode: 2017NatAs...1..612S Altcode: 2017arXiv171104156S Determining the sources of solar brightness variations1,2, often referred to as solar noise3, is important because solar noise limits the detection of solar oscillations3, is one of the drivers of the Earth's climate system4,5 and is a prototype of stellar variability6,7—an important limiting factor for the detection of extrasolar planets. Here, we model the magnetic contribution to solar brightness variability using high-cadence8,9 observations from the Solar Dynamics Observatory (SDO) and the Spectral And Total Irradiance REconstruction (SATIRE)10,11 model. The brightness variations caused by the constantly evolving cellular granulation pattern on the solar surface were computed with the Max Planck Institute for Solar System Research (MPS)/University of Chicago Radiative Magnetohydrodynamics (MURaM)12 code. We found that the surface magnetic field and granulation can together precisely explain solar noise (that is, solar variability excluding oscillations) on timescales from minutes to decades, accounting for all timescales that have so far been resolved or covered by irradiance measurements. We demonstrate that no other sources of variability are required to explain the data. Recent measurements of Sun-like stars by the COnvection ROtation and planetary Transits (CoRoT)13 and Kepler14 missions uncovered brightness variations similar to that of the Sun, but with a much wider variety of patterns15. Our finding that solar brightness variations can be replicated in detail with just two well-known sources will greatly simplify future modelling of existing CoRoT and Kepler as well as anticipated Transiting Exoplanet Survey Satellite16 and PLAnetary Transits and Oscillations of stars (PLATO)17 data. Title: The variability of magnetic activity in solar-type stars Authors: Fabbian, D.; Simoniello, R.; Collet, R.; Criscuoli, S.; Korhonen, H.; Krivova, N. A.; Oláh, K.; Jouve, L.; Solanki, S. K.; Alvarado-Gómez, J. D.; Booth, R.; García, R. A.; Lehtinen, J.; See, V. Bibcode: 2017AN....338..753F Altcode: No abstract at ADS Title: New reconstruction of the sunspot group numbers since 1739 using direct calibration and "backbone" methods Authors: Chatzistergos, Theodosios; Usoskin, Ilya G.; Kovaltsov, Gennady A.; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2017A&A...602A..69C Altcode: 2017arXiv170206183C Context. The group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. This database is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity.
Aims: We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period.
Methods: We used a daisy chain process with backbone (BB) observers and calibrated all overlapping observers to them. We performed the calibration of each individual observer with a probability distribution function (PDF) matrix constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs was carried out in a similar manner. The final series was constructed by merging different BB series. We modelled the propagation of errors straightforwardly with Monte Carlo simulations. A potential bias due to the selection of BBs was investigated and the effect was shown to lie within the 1σ interval of the produced series. The exact selection of the reference period was shown to have a rather small effect on our calibration as well.
Results: The final series extends back to 1739 and includes data from 314 observers. This series suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions.
Conclusions: The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the modern grand maximum of solar activity in the second half of the 20th century.

Values of the group sunspot number series are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A69 Title: Assessment of different sunspot number series using the cosmogenic isotope 44Ti in meteorites Authors: Asvestari, Eleanna; Usoskin, Ilya G.; Kovaltsov, Gennady A.; Owens, Mathew J.; Krivova, Natalie A.; Rubinetti, Sara; Taricco, Carla Bibcode: 2017MNRAS.467.1608A Altcode: 2017MNRAS.tmp..193A Many sunspot number series exist suggesting different levels of solar activity during the past centuries. Their reliability can be assessed only by comparing them with alternative indirect proxies. We test different sunspot number series against the updated record of cosmogenic radionuclide 44Ti measured in meteorites. Two bounding scenarios of solar activity changes have been considered: the HH-scenario (based on the series by Svalgaard and Schatten), in particular, predicting moderate activity during the Maunder minimum, and the LL-scenario (based on the RG series by Lockwood et al.) predicting moderate activity for the 18th-19th centuries and the very low activity level for the Maunder minimum. For each scenario, the magnetic open solar flux, the heliospheric modulation potential and the expected production of 44Ti were computed. The calculated production rates were compared with the corresponding measurements of 44Ti activity in stony meteorites fallen since 1766. The analysis reveals that the LL-scenario is fully consistent with the measured 44Ti data, in particular, recovering the observed secular trend between the 17th century and the Modern grand maximum. On the contrary, the HH-scenario appears significantly inconsistent with the data, mostly due to the moderate level of activity during the Maunder minimum. It is concluded that the HH-scenario sunspot number reconstruction significantly overestimates solar activity prior to the mid-18th century, especially during the Maunder minimum. The exact level of solar activity after 1750 cannot be distinguished with this method, since both H- and L- scenarios appear statistically consistent with the data. Title: EMPIRE: A robust empirical reconstruction of solar irradiance variability Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2017JGRA..122.3888Y Altcode: 2017arXiv170407652Y We present a new empirical model of total and spectral solar irradiance (TSI and SSI) variability entitled EMPirical Irradiance REconstruction (EMPIRE). As with existing empirical models, TSI and SSI variability is given by the linear combination of solar activity indices. In empirical models, UV SSI variability is usually determined by fitting the rotational variability in activity indices to that in measurements. Such models have to date relied on ordinary least squares regression, which ignores the uncertainty in the activity indices. In an advance from earlier efforts, the uncertainty in the activity indices is accounted for in EMPIRE by the application of an error-in-variables regression scheme, making the resultant UV SSI variability more robust. The result is consistent with observations and unprecedentedly, with that from other modeling approaches, resolving the long-standing controversy between existing empirical models and other types of models. We demonstrate that earlier empirical models, by neglecting the uncertainty in activity indices, underestimate UV SSI variability. The reconstruction of TSI and visible and IR SSI from EMPIRE is also shown to be consistent with observations. The EMPIRE reconstruction is of utility to climate studies as a more robust alternative to earlier empirical reconstructions. Title: Climate responses to SATIRE and SIM-based spectral solar forcing in a 3D atmosphere-ocean coupled GCM Authors: Wen, Guoyong; Cahalan, Robert F.; Rind, David; Jonas, Jeffrey; Pilewskie, Peter; Wu, Dong L.; Krivova, Natalie A. Bibcode: 2017JSWSC...7A..11W Altcode: We apply two reconstructed spectral solar forcing scenarios, one SIM (Spectral Irradiance Monitor) based, the other the SATIRE (Spectral And Total Irradiance REconstruction) modeled, as inputs to the GISS (Goddard Institute for Space Studies) GCMAM (Global Climate Middle Atmosphere Model) to examine climate responses on decadal to centennial time scales, focusing on quantifying the difference of climate response between the two solar forcing scenarios. We run the GCMAM for about 400 years with present day trace gas and aerosol for the two solar forcing inputs. We find that the SIM-based solar forcing induces much larger long-term response and 11-year variation in global averaged stratospheric temperature and column ozone. We find significant decreasing trends of planetary albedo for both forcing scenarios in the 400-year model runs. However the mechanisms for the decrease are very different. For SATIRE solar forcing, the decreasing trend of planetary albedo is associated with changes in cloud cover. For SIM-based solar forcing, without significant change in cloud cover on centennial and longer time scales, the apparent decreasing trend of planetary albedo is mainly due to out-of-phase variation in shortwave radiative forcing proxy (downwelling flux for wavelength >330 nm) and total solar irradiance (TSI). From the Maunder Minimum to present, global averaged annual mean surface air temperature has a response of 0.1 °C to SATIRE solar forcing compared to 0.04 °C to SIM-based solar forcing. For 11-year solar cycle, the global surface air temperature response has 3-year lagged response to either forcing scenario. The global surface air 11-year temperature response to SATIRE forcing is about 0.12 °C, similar to recent multi-model estimates, and comparable to the observational-based evidence. However, the global surface air temperature response to 11-year SIM-based solar forcing is insignificant and inconsistent with observation-based evidence. Title: VizieR Online Data Catalog: Group sunspot number series since 1739 (Chatzistergos+, 2017) Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G. A.; Krivova, N. A.; Solanki, S. K. Bibcode: 2017yCat..36020069C Altcode: Annual, monthly, and daily values of the Group sunspot number series produced in the paper.

(3 data files). Title: Handling Historical Ca II K Spectroheliogram Observation Authors: Solanki, S. K.; Chatzistergos, T.; Ermolli, I.; Krivova, N. Bibcode: 2016AGUFMSA51B2429S Altcode: The total solar irradiance has been continuously monitored since 1978, but climate studies require time-series extending further back in time. Models assuming that the irradiance variations are due to the evolution of the solar surface magnetic field manifesting itself as bright faculae and dark sunspots have been quite successful in reproducing the measured changes. Unfortunately full-disk observations of the magnetic field exist only for the last four decades. Sunspot observations (areas or numbers) have been employed to reconstruct past irradiance changes, however they provide information about the plage regions only indirectly. Information about the evolution of the plage regions can potentially be obtained from Ca II K spectroheliograms that are available for the last century. Such observational programs have been carried out at several observatories and some of these photographic archives have been digitised. However, analysis of such data is plagued by numerous problems affecting the images and the lack of photometric calibration. Here we present a new method to process historical Ca II K spectroheliograms in order to perform the photometric calibration and compensate for the centre-to-limb variations and artefacts affecting the data. The method is tested on synthetic images with known artefacts and a sample of images from various historical archives. Title: Solar Total and Spectral Irradiance Reconstruction over Last 9000 Years Authors: Wu, C. J.; Krivova, N.; Solanki, S. K.; Usoskin, I. G. Bibcode: 2016AGUFMSA51B2430W Altcode: Although the mechanisms of solar influence on Earth climate system are not yet fully understood, solar total and spectral irradiance are considered to be among the main determinants. Solar total irradiance is the total flux of solar radiative energy entering Earth's climate system, whereas the spectral irradiance describes this energy is distributed over the spectrum. Solar irradiance in the UV band is of special importance since it governs chemical processes in the middle and upper atmosphere. On timescales of the 11-year solar cycle and shorter, solar irradiance is measured by space-based instruments while models are needed to reconstruct solar irradiance on longer timescale. The SATIRE-M model (Spectral And Total Irradiance Reconstruction over millennia) is employed in this study to reconstruct solar irradiance from decadal radionuclide isotope data such as 14C and 10Be stored in tree rings and ice cores, respectively. A reconstruction over the last 9000 years will be presented. Title: Non-linear re-calibration of group sunspot number series back to 1819 Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G.; Solanki, S. K.; Krivova, N. Bibcode: 2016AGUFMSH31B2547C Altcode: Sunspot numbers form the longest series of direct observations that give information about the activity of the Sun. It consists in numerous records from different observers with different instruments and techniques, and unavoidably requires a calibration of individual observers to a standard reference condition. Such calibrations pose a challenge and are still a subject of debate. Most of the calibration methods are based on a simple linear scaling of data from one observer to the other, proposed in the mid-19th century by Rudolf Wolf. This method is still used in many recent reconstructions. However, this linear method is shown, on both real and synthetic data, to be very rough and incorrect because of the essentially non-linearity of the relation.Here we present a new reconstruction of the group sunspot number series back to 1819 attempting to take into account the non-linearity of the relation between data of different observers. We use an approach incorporating several overlapping backbone observers (similar to Svalgaard and Schatten, 2016) and perform the straightforward non-linear calibration of each observer to the backbone (and the backbones with each other) based on the actual statistics of the common daily values. Title: The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions Authors: Kopp, G.; Krivova, N.; Wu, C. J.; Lean, J. Bibcode: 2016SoPh..291.2951K Altcode: 2016SoPh..tmp...42K; 2016arXiv160105397K Reliable historical records of the total solar irradiance (TSI) are needed to assess the extent to which long-term variations in the Sun's radiant energy that is incident upon Earth may exacerbate (or mitigate) the more dominant warming in recent centuries that is due to increasing concentrations of greenhouse gases. We investigate the effects that the new Sunspot Index and Long-term Solar Observations (SILSO) sunspot-number time series may have on model reconstructions of the TSI. In contemporary TSI records, variations on timescales longer than about a day are dominated by the opposing effects of sunspot darkening and facular brightening. These two surface magnetic features, retrieved either from direct observations or from solar-activity proxies, are combined in TSI models to reproduce the current TSI observational record. Indices that manifest solar-surface magnetic activity, in particular the sunspot-number record, then enable reconstructing historical TSI. Revisions of the sunspot-number record therefore affect the magnitude and temporal structure of TSI variability on centennial timescales according to the model reconstruction methods that are employed. We estimate the effects of the new SILSO record on two widely used TSI reconstructions, namely the NRLTSI2 and the SATIRE models. We find that the SILSO record has little effect on either model after 1885, but leads to solar-cycle fluctuations with greater amplitude in the TSI reconstructions prior. This suggests that many eighteenth- and nineteenth-century cycles could be similar in amplitude to those of the current Modern Maximum. TSI records based on the revised sunspot data do not suggest a significant change in Maunder Minimum TSI values, and from comparing this era to the present, we find only very small potential differences in the estimated solar contributions to the climate with this new sunspot record. Title: The statistical study of global properties of sunspots observed by SoHO/MDI continuum images over solar cycle 23 Authors: Goel, Suruchi; Krivova, Natalie; Solanki, Sami K.; Mathew, Shibu K. Bibcode: 2016cosp...41E.687G Altcode: A better understanding of inter-dependency of various sunspot parameters such as magnetic field, intensity, temperature, size etc., and also their variation with strength of solar activity cycle is important to understand the magneto-convection process involved in sunspot formation and evolution and hence to develop a consistent sunspot model. We have investigated global sunspot properties using parameters of sunspots identified from stray-light-corrected continuum images from SoHO/MDI spanning from years 1996 to 2011. We find that the non-linear relation between umbral core (minimum) intensity and sunspot area is best represented by an exponential function, which reaches an asymptotic value at 600 MSH. For the first time we have also observed that the core intensity depends on shape of umbrae, i.e., circular umbrae are statistically darker compared to the elongated ones. The core intensity increases slightly towards the limb (by value of ~0.1 from disk center to the limb). From sunspots sampled over the complete solar cycle 23 and during the rising phase of cycle 24, we did not find any solar-cycle variation in umbral core intensity. The penumbra to umbra area ratio is found to be not a constant parameter, instead it shows a quadratic decrease with sunspot area. Leading and following sunspots usually have different morphological features, however in this study we did not observe significant differences in their core intensity and penumbra-umbra area ratio relation with the sunspot area. Title: Solar total and spectral irradiance reconstruction over last 9000 years Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami K. Bibcode: 2016cosp...41E2072W Altcode: Although the mechanisms of solar influence on Earth climate system are not yet fully understood, solar total and spectral irradiance are considered to be among the main determinants. Solar total irradiance is the total flux of solar radiative energy entering Earth's climate system, whereas the spectral irradiance describes this energy is distributed over the spectrum. Solar irradiance in the UV band is of special importance since it governs chemical processes in the middle and upper atmosphere. On timescales of the 11-year solar cycle and shorter, solar irradiance is measured by space-based instruments while models are needed to reconstruct solar irradiance on longer timescale. The SATIRE-M model (Spectral And Total Irradiance Reconstruction over millennia) is employed in this study to reconstruct solar irradiance from decadal radionuclide isotope data such as 14C and 10Be stored in tree rings and ice cores, respectively. A reconstruction over the last 9000 years will be presented. Title: The Solar Ultraviolet Imaging Telescope onboard Aditya-L1 Authors: Ghosh, Avyarthana; Chatterjee, Subhamoy; Khan, Aafaque R.; Tripathi, Durgesh; Ramaprakash, A. N.; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim M.; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K.; Sriram, S. Bibcode: 2016SPIE.9905E..03G Altcode: The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 spacecraft, the first dedicated solar mission of the Indian Space Research Organization (ISRO), which will be put in a halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis Ritchey-Chrétien configuration with a combination of 11 narrow and broad bandpass filters which will be used for full-disk solar imaging in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun's influence on climate. Title: Long-term solar irradiance variability: knowns and unknowns Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria; Leng Yeo, Kok; Wu, Chi-Ju Bibcode: 2016cosp...41E1066K Altcode: Long-term reconstructions of solar irradiance variability are crucial to our understanding of solar influence on climate. They are only possible with the help of suitable models, which in turn require a thorough understanding of the mechanisms of this variability. With the advance of such models, also the past reconstructions are becoming more reliable. Nevertheless, the remaining uncertainties spread out when extrapolating back over long periods of time, amplified by the increasingly poorer quality and reliability of the available data that bear information on past solar activity. We will discuss the progress and the reliability of irradiance reconstructions on time scales of decades to millennia. Title: EMPIRE reconstruction of TSI and SSI since 1947 Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K. Bibcode: 2016cosp...41E2107L Altcode: TSI and SSI, at least in the UV, have been monitored from space since 1978. This is accompanied by the development of models aimed at reproducing TSI and SSI by relating the variability to solar magnetism. We present a reconstruction of TSI and SSI, spanning 1947 to 2016, based on the EMPirical Irradiance REconstruction (EMPIRE) model. EMPIRE is what is termed a proxy model, based on the regression of solar activity indices to measured solar irradiance. One of the main open questions in our understanding of solar irradiance variability, and consequently its influence on the Earth's climate, is the variation in UV SSI over the solar cycle. Solar cycle variability in UV SSI in preceding proxy models, such as NRLSSI, NRLSSI2 and MGNM, is significantly weaker than that indicated by other types of models longwards of around 240 nm. We demonstrate here that this discrepancy arises from previous proxy models neglecting the influence of measurement uncertainty on regression analyses. EMPIRE is based on the same activity indices as NRLSSI(2) but by taking measurement uncertainty into account, reproduces variability consistent with other modelling approaches and measurements. As such, EMPIRE supersedes earlier proxy models, including NRLSSI(2) and MGNM. Title: The origin of Total Solar Irradiance variability on timescales less than a day Authors: Shapiro, Alexander; Krivova, Natalie; Schmutz, Werner; Solanki, Sami K.; Leng Yeo, Kok; Cameron, Robert; Beeck, Benjamin Bibcode: 2016cosp...41E1774S Altcode: Total Solar Irradiance (TSI) varies on timescales from minutes to decades. It is generally accepted that variability on timescales of a day and longer is dominated by solar surface magnetic fields. For shorter time scales, several additional sources of variability have been proposed, including convection and oscillation. However, available simplified and highly parameterised models could not accurately explain the observed variability in high-cadence TSI records. We employed the high-cadence solar imagery from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and the SATIRE (Spectral And Total Irradiance Reconstruction) model of solar irradiance variability to recreate the magnetic component of TSI variability. The recent 3D simulations of solar near-surface convection with MURAM code have been used to calculate the TSI variability caused by convection. This allowed us to determine the threshold timescale between TSI variability caused by the magnetic field and convection. Our model successfully replicates the TSI measurements by the PICARD/PREMOS radiometer which span the period of July 2010 to February 2014 at 2-minute cadence. Hence, we demonstrate that solar magnetism and convection can account for TSI variability at all timescale it has ever been measured (sans the 5-minute component from p-modes). Title: Modelling the Spectral Contrasts of Stellar Faculae. Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne; Solanki, Sami; Yeo, Kok Leng; Krivova, Natalie Bibcode: 2016csss.confE..63N Altcode: Facular contrasts are an important parameter in modelling stellar variability and exoplanet transits. The ultimate goal of this work will be to model the contrasts of faculae for different spectral types and thus improve the modelling of solar and stellar photospheric variability. This is done by using a radiative transfer algorithm (ATLAS9) on magneto-convection simulations. Starting with solar twins, we derive facular contrasts as a function of limb angle and discuss their wavelength dependence. Title: Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K.; Unruh, Y. C.; Yeo, K. L. Bibcode: 2016A&A...590A..63D Altcode: 2016arXiv160502039D
Aims: We present a reconstruction of the spectral solar irradiance since 1700 using the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope era version 2) model. This model uses as input magnetograms simulated with a surface flux transport model fed with semi-synthetic records of emerging sunspot groups.
Methods: The record of sunspot group areas and positions from the Royal Greenwich Observatory (RGO) is only available since 1874. We used statistical relationships between the properties of sunspot group emergence, such as the latitude, area, and tilt angle, and the sunspot cycle strength and phase to produce semi-synthetic sunspot group records starting in the year 1700. The semi-synthetic records are fed into a surface flux transport model to obtain daily simulated magnetograms that map the distribution of the magnetic flux in active regions (sunspots and faculae) and their decay products on the solar surface. The magnetic flux emerging in ephemeral regions is accounted for separately based on the concept of extended cycles whose length and amplitude are linked to those of the sunspot cycles through the sunspot number. The magnetic flux in each surface component (sunspots, faculae and network, and ephemeral regions) was used to compute the spectral and total solar irradiance (TSI) between the years 1700 and 2009. This reconstruction is aimed at timescales of months or longer although the model returns daily values.
Results: We found that SATIRE-T2, besides reproducing other relevant observations such as the total magnetic flux, reconstructs the TSI on timescales of months or longer in good agreement with the PMOD composite of observations, as well as with the reconstruction starting in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI of 1.2+0.2-0.3 Wm-2 between 1700 and the present. The spectral irradiance reconstruction is in good agreement with the UARS/SUSIM measurements as well as the Lyman-α composite.

The complete total and spectral (115 nm-160 μm) irradiance reconstructions since 1700 will be available from http://www2.mps.mpg.de/projects/sun-climate/data.html Title: Are solar brightness variations faculae- or spot-dominated? Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Yeo, K. L.; Schmutz, W. K. Bibcode: 2016A&A...589A..46S Altcode: 2016arXiv160204447S Context. Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to a surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability.
Aims: To understand whether the broader pattern of stellar variations is consistent with the solar paradigm, we assess relative contributions of faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness variability and its facular and spot contributions depend on the wavelength, timescale of variability, and position of the observer relative to the ecliptic plane.
Methods: We performed calculations with the SATIRE model, which returns solar brightness with daily cadence from solar disc area coverages of various magnetic features. We took coverages as seen by an Earth-based observer from full-disc SoHO/MDI and SDO/HMI data and projected them to mimic out-of-ecliptic viewing by an appropriate transformation.
Results: Moving the observer away from the ecliptic plane increases the amplitude of 11-year variability as it would be seen in Strömgren (b + y)/2 photometry, but decreases the amplitude of the rotational brightness variations as it would appear in Kepler and CoRoT passbands. The spot and facular contributions to the 11-year solar variability in the Strömgren (b + y)/2 photometry almost fully compensate each other so that the Sun appears anomalously quiet with respect to its stellar cohort. Such a compensation does not occur on the rotational timescale.
Conclusions: The rotational solar brightness variability as it would appear in the Kepler and CoRoT passbands from the ecliptic plane is spot-dominated, but the relative contribution of faculae increases for out-of-ecliptic viewing so that the apparent brightness variations are faculae-dominated for inclinations less than about I = 45°. Over the course of the 11-year activity cycle, the solar brightness variability is faculae-dominated shortwards of 1.2 μm independently of the inclination. Title: Semi-empirical Long-term Reconstruction of the Heliospheric Parameters: Validation by Cosmogenic Radionuclide Records Authors: Asvestari, E.; Usoskin, I. G.; Cameron, R. H.; Krivova, N. A. Bibcode: 2016ASPC..504..269A Altcode: We have developed a semi-empirical model that describes the heliospheric modulation of galactic cosmic rays considering different heliospheric parameters. This model is an improvement of a previous model. The parameters of the model are fitted using the observations and reconstructions of the heliospheric parameters for the period 1976 - 2013, which includes the latest very weak solar minimum. The modulation potential is computed since 1610 using different reconstructions of the open solar magnetic flux and it is then used to compute the production and distribution of cosmogenic isotope 14C, which was subsequently compared with terrestrial archives in tree rings. It is shown that the group sunspot number series by Svalgaard & Schatten (2015) is inconsistent with the data, while other series agree well. Title: Modelling Solar and Stellar Brightness Variabilities Authors: Yeo, K. L.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2016ASPC..504..273Y Altcode: Total and spectral solar irradiance, TSI and SSI, have been measured from space since 1978. This is accompanied by the development of models aimed at replicating the observed variability by relating it to solar surface magnetism. Despite significant progress, there remains persisting controversy over the secular change and the wavelength-dependence of the variation with impact on our understanding of the Sun's influence on the Earth's climate. We highlight the recent progress in TSI and SSI modelling with SATIRE. Brightness variations have also been observed for Sun-like stars. Their analysis can profit from knowledge of the solar case and provide additional constraints for solar modelling. We discuss the recent effort to extend SATIRE to Sun-like stars. Title: Exploiting Four Historical Ca II K Spectroheliogram Archives Authors: Chatzistergos, T.; Ermolli, I.; Solanki, S. K.; Krivova, N. A. Bibcode: 2016ASPC..504..227C Altcode: Here we briefly review the status of the project aimed to analyse the potential of historical Ca II K archives for studies of long-term (decades to a century) solar variability. Title: Commission 12: Solar Radiation and Structure Authors: Cauzzi, Gianna; Shchukina, Nataliya; Kosovichev, Alexander; Bianda, Michele; Brandenburg, Axel; Chou, Dean-Yi; Dasso, Sergio; Ding, Ming-De; Jefferies, Stuart; Krivova, Natalie; Kuznetsov, Vladimir D.; Moreno-Insertis, Fernando Bibcode: 2016IAUTA..29..278C Altcode: Commission 12 of the International Astronomical Union encompasses investigations of the internal structure and dynamics of the Sun, the quiet solar atmosphere, solar radiation and its variability, and the nature of relatively stable magnetic structures like sunspots, faculae and the magnetic network. The Commission sees participation of over 300 scientists worldwide. Title: Variation of the Mn I 539.4 nm line with the solar cycle Authors: Danilovic, S.; Solanki, S. K.; Livingston, W.; Krivova, N.; Vince, I. Bibcode: 2016A&A...587A..33D Altcode: 2015arXiv151101286D Context. As a part of the long-term program at Kitt Peak National Observatory (KPNO), the Mn I 539.4 nm line has been observed for nearly three solar cycles using the McMath telescope and the 13.5 m spectrograph in double-pass mode. These full-disk spectrophotometric observations revealed an unusually strong change of this line's parameters over the solar cycle.
Aims: Optical pumping by the Mg II k line was originally proposed to explain these variations. More recent studies have proposed that this is not required and that the magnetic variability (I.e., the changes in solar atmospheric structure due to faculae) might explain these changes. Magnetic variability is also the mechanism that drives the changes in total solar irradiance variations (TSI). With this work we investigate this proposition quantitatively by using the same model that was earlier successfully employed to reconstruct the irradiance.
Methods: We reconstructed the changes in the line parameters using the model SATIRE-S, which takes only variations of the daily surface distribution of the magnetic field into account. We applied exactly the same model atmospheres and value of the free parameter as were used in previous solar irradiance reconstructions to now model the variation in the Mn I 539.4 nm line profile and in neighboring Fe I lines. We compared the results of the theoretical model with KPNO observations.
Results: The changes in the Mn I 539.4 nm line and a neighbouring Fe I 539.52 nm line over approximately three solar cycles are reproduced well by the model without additionally tweaking the model parameters, if changes made to the instrument setup are taken into account. The model slightly overestimates the change for the strong Fe I 539.32 nm line.
Conclusions: Our result confirms that optical pumping of the Mn II 539.4 nm line by Mg II k is not the main cause of its solar cycle change. It also provides independent confirmation of solar irradiance models which are based on the assumption that irradiance variations are caused by the evolution of the solar surface magnetic flux. The result obtained here also supports the spectral irradiance variations computed by these models. Title: Sunspot areas and tilt angles for solar cycles 7-10 Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova, N. A.; Solanki, S. K. Bibcode: 2015A&A...584A..73S Altcode: 2015arXiv150807849S
Aims: Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims to estimate areas of sunspots observed by Schwabe in 1825-1867 and to calculate the tilt angles of sunspot groups.
Methods: The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century.
Results: Umbral areas for about 130 000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, average to 5̊.85 ± 0, with the leading polarity located closer to the equator, in good agreement with tilt angles obtained from 20th century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided at the CDS.

The sunspot area and tilt angle data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/584/A73 Title: The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions Authors: Kopp, G.; Krivova, N.; Lean, J.; Wu, C. J. Bibcode: 2015AGUFMSH23C2451K Altcode: We describe the expected effects of the new sunspot number time series on proxy model based reconstructions of the total solar irradiance (TSI), which is largely explained by the opposing effects of dark sunspots and bright faculae. Regressions of indices for facular brightening and sunspot darkening with time series of direct TSI observations during the recent 37-year spacecraft TSI measurement era determine the relative contributions from each. Historical TSI reconstructions are enabled by extending these proxy models back in time prior to the start of the measurement record using a variety of solar activity indices including the sunspot number time series alone prior to 1882. Such reconstructions are critical for Earth climate research, which requires knowledge of the incident energy from the Sun to assess climate sensitivity to the natural influence of solar variability. Two prominent TSI reconstructions that utilize the sunspot record starting in 1610 are the NRLTSI and the SATIRE models. We review the indices that each currently uses and estimate the effects the revised sunspot record has on these reconstructions. Title: VizieR Online Data Catalog: Sunspot areas and tilt angles (Senthamizh Pavai+, 2015) Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova, N.; Solanki, S. Bibcode: 2015yCat..35840073S Altcode: We present sunspot positions and areas from historical observations of sunspots by Samuel Heinrich Schwabe from Dessau, Germany. He has recorded his observations of sunspots from 1825-1867 as drawings in small circles of about 5cm diameter (representing the solar disk). Even though he has used quite a number of telescopes for his observations, the majority of the full-disk drawings were made with a 3-1/2-foot telescope from Fraunhofer. His observing log books are stored in the library of the Royal Astronomical Society in London. Those drawings were digitized photographically with a resolution of 2912x4378 pixels per page. The sizes and positions of the sunspots were measured using a dozen of circular mouse cursor shapes with different diameters. The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century. Umbral areas for about 130,000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar (two or more spots). There is, of course, no polarity information in the observations.

Both an updated sunspot database and a tilt angle database are available at http://www.aip.de/Members/rarlt/ sunspots for further study.

(2 data files). Title: The Maunder minimum (1645-1715) was indeed a grand minimum: A reassessment of multiple datasets Authors: Usoskin, Ilya G.; Arlt, Rainer; Asvestari, Eleanna; Hawkins, Ed; Käpylä, Maarit; Kovaltsov, Gennady A.; Krivova, Natalie; Lockwood, Michael; Mursula, Kalevi; O'Reilly, Jezebel; Owens, Matthew; Scott, Chris J.; Sokoloff, Dmitry D.; Solanki, Sami K.; Soon, Willie; Vaquero, José M. Bibcode: 2015A&A...581A..95U Altcode: 2015arXiv150705191U
Aims: Although the time of the Maunder minimum (1645-1715) is widely known as a period of extremely low solar activity, it is still being debated whether solar activity during that period might have been moderate or even higher than the current solar cycle #24. We have revisited all existing evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum.
Methods: We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of the sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum.
Results: The level of solar activity during the Maunder minimum is reassessed on the basis of all available datasets.
Conclusions: We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely lower than during the Dalton minimum of around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected with a high confidence level. Title: The role of the Fraunhofer lines in solar brightness variability Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Tagirov, R. V.; Schmutz, W. K. Bibcode: 2015A&A...581A.116S Altcode: 2015arXiv150705437S Context. The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections.
Aims: We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales.
Methods: Our approach is based on the assumption that variability of the solar brightness on timescales greater than a day is driven by the evolution of the solar surface magnetic field. We calculated the solar brightness variations employing the solar disc area coverage of magnetic features deduced from the MDI/SOHO observations. The brightness contrasts of magnetic features relative to the quiet Sun were calculated with a non-LTE radiative transfer code as functions of disc position and wavelength. By consecutive elimination of molecular and atomic lines from the radiative transfer calculations, we assessed the role of these lines in producing solar brightness variability.
Results: We show that the variations in Fraunhofer lines define the amplitude of the solar brightness variability on timescales greater than a day and even the phase of the total solar irradiance variability over the 11-year cycle. We also demonstrate that molecular lines make substantial contribution to solar brightness variability on the 11-year activity cycle and centennial timescales. In particular, our model indicates that roughly a quarter of the total solar irradiance variability over the 11-year cycle originates in molecular lines. The maximum of the absolute spectral brightness variability on timescales greater than a day is associated with the CN violet system between 380 and 390 nm. Title: Modelling stellar brightness variations Authors: Shapiro, Alexander; Solanki, Sami K.; Krivova, Natalie Bibcode: 2015IAUGA..2256741S Altcode: We develop a model that attributes the variability of the stellar brightness to the imbalance between starspot darkening and facular brightening. Our approach is based on the assumption that the photometric variability of the Sun and Sun-like stars have the same fundamental causes so that we can describe stellar variability by extrapolating the solar model. Our results suggest that the solar paradigm is remarkably successful in explaining the stellar variability on the activity cycle time-scale. In particular, the model reproduces the observed reversal of the in-phase activity-brightness relationship for low-activity stars to an anti-phase one for more active stars.We simulate the solar variability as it would be measured out-of-ecliptic by Kepler and CoRoT and discuss the relative contributions of spots and faculae to the photometric stellar variability. Title: UV solar irradiance in observations and the NRLSSI and SATIRE-S models Authors: Yeo, K. L.; Ball, W. T.; Krivova, N. A.; Solanki, S. K.; Unruh, Y. C.; Morrill, J. Bibcode: 2015JGRA..120.6055Y Altcode: 2015arXiv150701224Y Total solar irradiance and UV spectral solar irradiance has been monitored since 1978 through a succession of space missions. This is accompanied by the development of models aimed at replicating solar irradiance by relating the variability to solar magnetic activity. The Naval Research Laboratory Solar Spectral Irradiance (NRLSSI) and Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE-S) models provide the most comprehensive reconstructions of total and spectral solar irradiance over the period of satellite observation currently available. There is persistent controversy between the various measurements and models in terms of the wavelength dependence of the variation over the solar cycle, with repercussions on our understanding of the influence of UV solar irradiance variability on the stratosphere. We review the measurement and modeling of UV solar irradiance variability over the period of satellite observation. The SATIRE-S reconstruction is consistent with spectral solar irradiance observations where they are reliable. It is also supported by an independent, empirical reconstruction of UV spectral solar irradiance based on Upper Atmosphere Research Satellite/Solar Ultraviolet Spectral Irradiance Monitor measurements from an earlier study. The weaker solar cycle variability produced by NRLSSI between 300 and 400 nm is not evident in any available record. We show that although the method employed to construct NRLSSI is principally sound, reconstructed solar cycle variability is detrimentally affected by the uncertainty in the SSI observations it draws upon in the derivation. Based on our findings, we recommend, when choosing between the two models, the use of SATIRE-S for climate studies. Title: The Maunder minimum: A reassessment from multiple dataset Authors: Usoskin, Ilya; Arlt, Rainer; Asvestari, Eleanna; Kovaltsov, Gennady; Krivova, Natalie; Lockwood, Michael; Käpylä, Maarit; Owens, Matthew; Sokoloff, Dmitry D.; Solanki, Sami; Soon, Willie; Vaquero, Jose; Scott, Chris Bibcode: 2015IAUGA..2253036U Altcode: The Maunder minimum (MM) in 1645-1715 was a period of the lowest ever known solar activity recorded via sunspot numbers since 1610. Since it is the only Grand minimum of solar activity directly observed, it forms a benchmark for the solar variability studies. Therefore, it is crucially important to assess the level and other features of temporal and spatial solar magnetic variability during that time. However, because of uncertainties related mostly to ambiguity of some historical sunspot observation records, the exact level of solar activity during the MM is somewhat unclear, leaving room for continuous discussions and speculations. Many of these issues have been addressed by Jack Eddy in his cornerstone papers of 1976 and 1983, but since then numerous new pieces of evidence and datasets have appeared, making it possible to verify the paradigm of the Maunder minimum with far greater certainty than before.Here we provide a full reassessment of the Maunder minimum using all the available datasets: augmented sunspot counts and drawings; revisited historical archives; both well-known and newly revealed records of auroral observations; cosmic ray variability via cosmogenic isotope records of 14C in tree trunks, 10Be in ice cores and 44Ti in fallen meteorites. We show that, while the exact level of the activity is not easy to determine, the Sun indeed exhibited exceptionally low magnetic activity during the MM, in comparison to other periods of moderate or decreased activity, such as the Dalton minimum (ca. 1800), the Gleissberg minimum (ca. 1900) and the present weak solar cycle # 24. We show that a scenario of moderate or strong activity during the MM contradicts all the available datasets.Thus, we confirm, using all the presently available datasets of different nature, that the period of the Maunder minimum in 1645-1715 was indeed a Grand minimum, with very low solar surface magnetic activity, low intensity of the interplanetary magnetic field, as well as lower frequency and higher geographical latitude of auroral occurrence. Meanwhile some indications of the continuation, but at a very low level, of the 11-year solar cycle can be found in the data. Title: Solar cycle variation in UV solar spectral irradiance Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K. Bibcode: 2015IAUGA..2253798L Altcode: Solar spectral irradiance, SSI, in the UV has been measured from space, almost without interruption, since 1978. This is accompanied by the development of models aimed at reconstructing SSI by relating its variability to solar magnetic activity. The various satellite records and model reconstructions differ significantly in terms of the variation over the solar cycle, with the consequence that their application to climate models yield qualitatively different results. Here, we highlight the key discrepancies between available records and reconstructions, and discuss the possible underlying causes. Title: Solar Cycle Variation in Solar Irradiance Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2015sac..book..137Y Altcode: No abstract at ADS Title: GISS GCMAM Modeled Climate Responses to Total and Spectral Solar Forcing on Decadal and Centennial Time Scales Authors: Wen, G.; Cahalan, R. F.; Rind, D. H.; Jonas, J.; Pilewskie, P.; Harder, J. W.; Krivova, N. Bibcode: 2014AGUFMSA53A4108W Altcode: We examine the influence of the SORCE (Solar Radiation and Climate Experiment) SIM (Spectral Irradiance Monitor) observed spectral solar irradiance (SSI) variations on Earth's climate. We apply two reconstructed spectral solar forcing scenarios, one SIM based, the other based on the SATIRE (Spectral And Total Irradiance REconstruction) model, as inputs to the GISS (Goddard Institute for Space Studies) GCMAM (Global Climate Middle Atmosphere Model) to examine the climate responses on decadal and centennial time scales. We show that the atmosphere has different temperature, ozone, and dynamic responses to the two solar spectral forcing scenarios, even when the variations in TSI (Total Solar Irradiance) are the same. We find that solar variations under either scenario contribute a small fraction of the observed temperature increase since the industrial revolution. The trend of global averaged surface air temperature response to the SIM-based solar forcing is 0.02 °C/century, about half of the temperature trend to the SATIRE-based SSI. However the temporal variation of the surface air temperature for the SIM-based solar forcing scenario is much larger compared to its SATIRE counterpart. Further research is required to examine TSI and SSI variations in the ascending phase of solar cycle 24, to assess their implications for the solar influence on climate. Title: Solar Spectral Irradiance Variability in November/December 2012: Comparison of Observations by Instruments on the International Space Station and Models Authors: Thuillier, G.; Schmidtke, G.; Erhardt, C.; Nikutowski, B.; Shapiro, A. I.; Bolduc, C.; Lean, J.; Krivova, N.; Charbonneau, P.; Cessateur, G.; Haberreiter, M.; Melo, S.; Delouille, V.; Mampaey, B.; Yeo, K. L.; Schmutz, W. Bibcode: 2014SoPh..289.4433T Altcode: 2014SoPh..tmp..120T Onboard the International Space Station (ISS), two instruments are observing the solar spectral irradiance (SSI) at wavelengths from 16 to 2900 nm. Although the ISS platform orientation generally precludes pointing at the Sun more than 10 - 14 days per month, in November/December 2012 a continuous period of measurements was obtained by implementing an ISS `bridging' maneuver. This enabled observations to be made of the solar spectral irradiance (SSI) during a complete solar rotation. We present these measurements, which quantify the impact of active regions on SSI, and compare them with data simultaneously gathered from other platforms, and with models of spectral irradiance variability. Our analysis demonstrates that the instruments onboard the ISS have the capability to measure SSI variations consistent with other instruments in space. A comparison among all available SSI measurements during November-December 2012 in absolute units with reconstructions using solar proxies and observed solar activity features is presented and discussed in terms of accuracy. Title: Solar Cycle Variation in Solar Irradiance Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2014SSRv..186..137Y Altcode: 2014arXiv1407.4249Y; 2014SSRv..tmp...25Y The correlation between solar irradiance and the 11-year solar activity cycle is evident in the body of measurements made from space, which extend over the past four decades. Models relating variation in solar irradiance to photospheric magnetism have made significant progress in explaining most of the apparent trends in these observations. There are, however, persistent discrepancies between different measurements and models in terms of the absolute radiometry, secular variation and the spectral dependence of the solar cycle variability. We present an overview of solar irradiance measurements and models, and discuss the key challenges in reconciling the divergence between the two. Title: The PLATO 2.0 mission Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz, W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon, L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse, M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.; Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.; Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.; Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.; Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.; Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.; Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.; Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.; Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.; Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer, R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille, T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt, M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels, A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker, S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko, M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.; Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal, P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur, S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro, M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.; Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack, L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics, P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.; Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.; Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne, A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio, A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.; Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó, Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair, S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.; Werner, S. C.; Wheatley, P. J.; Zwintz, K. Bibcode: 2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence) providing a wide field-of-view (2232 deg 2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmosphere. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science. Title: A New SATIRE-S Spectral Solar Irradiance Reconstruction for Solar Cycles 21-23 and Its Implications for Stratospheric Ozone* Authors: Ball, William T.; Krivova, Natalie A.; Unruh, Yvonne C.; Haigh, Joanna D.; Solanki, Sami K. Bibcode: 2014JAtS...71.4086B Altcode: 2014arXiv1408.0365B We present a revised and extended total and spectral solar irradiance (SSI) reconstruction, which includes a wavelength-dependent uncertainty estimate, spanning the last three solar cycles using the SATIRE-S model. The SSI reconstruction covers wavelengths between 115 and 160,000 nm and all dates between August 1974 and October 2009. This represents the first full-wavelength SATIRE-S reconstruction to cover the last three solar cycles without data gaps and with an uncertainty estimate. SATIRE-S is compared with the NRLSSI model and SORCE/SOLSTICE ultraviolet (UV) observations. SATIRE-S displays similar cycle behaviour to NRLSSI for wavelengths below 242 nm and almost twice the variability between 242 and 310 nm. During the decline of last solar cycle, between 2003 and 2008, SSI from SORCE/SOLSTICE version 12 and 10 typically displays more than three times the variability of SATIRE-S between 200 and 300 nm. All three datasets are used to model changes in stratospheric ozone within a 2D atmospheric model for a decline from high solar activity to solar minimum. The different flux changes result in different modelled ozone trends. Using NRLSSI leads to a decline in mesospheric ozone, while SATIRE-S and SORCE/SOLSTICE result in an increase. Recent publications have highlighted increases in mesospheric ozone when considering version 10 SORCE/SOLSTICE irradiances. The recalibrated SORCE/SOLSTICE version 12 irradiances result in a much smaller mesospheric ozone response than when using version 10 and now similar in magnitude to SATIRE-S. This shows that current knowledge of variations in spectral irradiance is not sufficient to warrant robust conclusions concerning the impact of solar variability on the atmosphere and climate. Title: Modelling total solar irradiance since 1878 from simulated magnetograms Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K. Bibcode: 2014A&A...570A..23D Altcode: 2014arXiv1409.1941D
Aims: We present a new model of total solar irradiance (TSI) based on magnetograms simulated with a surface flux transport model (SFTM) and the Spectral And Total Irradiance REconstructions (SATIRE) model. Our model provides daily maps of the distribution of the photospheric field and the TSI starting from 1878.
Methods: The modelling is done in two main steps. We first calculate the magnetic flux on the solar surface emerging in active and ephemeral regions. The evolution of the magnetic flux in active regions (sunspots and faculae) is computed using a surface flux transport model fed with the observed record of sunspot group areas and positions. The magnetic flux in ephemeral regions is treated separately using the concept of overlapping cycles. We then use a version of the SATIRE model to compute the TSI. The area coverage and the distribution of different magnetic features as a function of time, which are required by SATIRE, are extracted from the simulated magnetograms and the modelled ephemeral region magnetic flux. Previously computed intensity spectra of the various types of magnetic features are employed.
Results: Our model reproduces the PMOD composite of TSI measurements starting from 1978 at daily and rotational timescales more accurately than the previous version of the SATIRE model computing TSI over this period of time. The simulated magnetograms provide a more realistic representation of the evolution of the magnetic field on the photosphere and also allow us to make use of information on the spatial distribution of the magnetic fields before the times when observed magnetograms were available. We find that the secular increase in TSI since 1878 is fairly stable to modifications of the treatment of the ephemeral region magnetic flux. Title: Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.; Glassmeier, K. H. Bibcode: 2014A&A...570A..85Y Altcode: 2014arXiv1408.1229Y Context. Total and spectral solar irradiance are key parameters in the assessment of solar influence on changes in the Earth's climate.
Aims: We present a reconstruction of daily solar irradiance obtained using the SATIRE-S model spanning 1974 to 2013 based on full-disc observations from the KPVT, SoHO/MDI, and SDO/HMI.
Methods: SATIRE-S ascribes variation in solar irradiance on timescales greater than a day to photospheric magnetism. The solar spectrum is reconstructed from the apparent surface coverage of bright magnetic features and sunspots in the daily data using the modelled intensity spectra of these magnetic structures. We cross-calibrated the various data sets, harmonizing the model input so as to yield a single consistent time series as the output.
Results: The model replicates 92% (R2 = 0.916) of the variability in the PMOD TSI composite including the secular decline between the 1996 and 2008 solar cycle minima. The model also reproduces most of the variability in observed Lyman-α irradiance and the Mg II index. The ultraviolet solar irradiance measurements from the UARS and SORCE missions are mutually consistent up to about 180 nm before they start to exhibit discrepant rotational and cyclical variability, indicative of unresolved instrumental effects. As a result, the agreement between model and measurement, while relatively good below 180 nm, starts to deteriorate above this wavelength. As with earlier similar investigations, the reconstruction cannot reproduce the overall trends in SORCE/SIM SSI. We argue, from the lack of clear solar cycle modulation in the SIM record and the inconsistency between the total flux recorded by the instrument and TSI, that unaccounted instrumental trends are present.
Conclusions: The daily solar irradiance time series is consistent with observations from multiple sources, demonstrating its validity and utility for climate models. It also provides further evidence that photospheric magnetism is the prime driver of variation in solar irradiance on timescales greater than a day. Title: Variability of Sun-like stars: reproducing observed photometric trends Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Schmutz, W. K.; Ball, W. T.; Knaack, R.; Rozanov, E. V.; Unruh, Y. C. Bibcode: 2014A&A...569A..38S Altcode: 2014arXiv1406.2383S Context. The Sun and stars with low magnetic activity levels become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behavior and become fainter when their activity increases.
Aims: We reproduce the observed photometric trends in stellar variations with a model thattreats stars as hypothetical suns with coverage by magnetic features different from that of the Sun.
Methods: The model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and bright faculae. A stellar spectrum is calculated from spectra of the individual components by weighting them with corresponding disk-area coverages. The latter are obtained by extrapolating the solar dependences of spot and facular disk-area coverages on chromospheric activity to stars with different levels of mean chromospheric activity.
Results: We find that the contribution by starspots to the variability increases faster with chromospheric activity than the facular contribution. This causes the transition from faculae-dominated variability and direct activity-brightness correlation to spot-dominated variability and inverse activity-brightness correlation with increasing chromospheric activity level. We show that the regime of the variability also depends on the angle between the stellar rotation axis and the line-of-sight and on the latitudinal distribution of active regions on the stellar surface. Our model can be used as a tool for extrapolating the observed photometric variability of the Sun to Sun-like stars at different activity levels, which makes a direct comparison between solar and stellar irradiance data possible.

Appendices are available in electronic form at http://www.aanda.org Title: Analysis of Different Solar Spectral Irradiance Reconstructions and Their Impact on Solar Heating Rates Authors: Thuillier, G.; Melo, S. M. L.; Lean, J.; Krivova, N. A.; Bolduc, C.; Fomichev, V. I.; Charbonneau, P.; Shapiro, A. I.; Schmutz, W.; Bolsée, D. Bibcode: 2014SoPh..289.1115T Altcode: Proper numerical simulation of the Earth's climate change requires reliable knowledge of solar irradiance and its variability on different time scales, as well as the wavelength dependence of this variability. As new measurements of the solar spectral irradiance have become available, so too have new reconstructions of historical solar irradiance variations, based on different approaches. However, these various solar spectral irradiance reconstructions have not yet been compared in detail to quantify differences in their absolute values, variability, and implications for climate and atmospheric studies. In this paper we quantitatively compare five different reconstructions of solar spectral irradiance changes during the past four centuries, in order to document and analyze their differences. The impact on atmosphere and climate studies is discussed in terms of the calculation of short wave solar heating rates. Title: Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena Authors: Yeo, K. L.; Feller, A.; Solanki, S. K.; Couvidat, S.; Danilovic, S.; Krivova, N. A. Bibcode: 2014A&A...561A..22Y Altcode: 2013arXiv1310.4972Y
Aims: We present a point spread function (PSF) for the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data.
Methods: The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the Venusian disc and solar background with a guess PSF. We described the PSF as the sum of five Gaussian functions, the amplitudes of which vary sinusoidally with azimuth. This relatively complex functional form was required by the data. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. We also examined the variation in the shape of the solar aureole in daily data, as an indication of PSF changes over time.
Results: Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 6173 Å line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. The shape of the solar aureole varied, with time and between the two CCDs. There are also indications that the PSF varies across the FOV. However, all these variations were found to be relatively small, such that a single PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error.
Conclusions: Restoring HMI observations with the PSF presented here returns a reasonable estimate of the stray light-free intensity contrast. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation. Title: Reconstructions of solar irradiance on centennial time scales Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria; Kok Leng, Yeo Bibcode: 2014cosp...40E1664K Altcode: Solar irradiance is the main external source of energy to Earth's climate system. The record of direct measurements covering less than 40 years is too short to study solar influence on Earth's climate, which calls for reconstructions of solar irradiance into the past with the help of appropriate models. An obvious requirement to a competitive model is its ability to reproduce observed irradiance changes, and a successful example of such a model is presented by the SATIRE family of models. As most state-of-the-art models, SATIRE assumes that irradiance changes on time scales longer than approximately a day are caused by the evolving distribution of dark and bright magnetic features on the solar surface. The surface coverage by such features as a function of time is derived from solar observations. The choice of these depends on the time scale in question. Most accurate is the version of the model that employs full-disc spatially-resolved solar magnetograms and reproduces over 90% of the measured irradiance variation, including the overall decreasing trend in the total solar irradiance over the last four cycles. Since such magnetograms are only available for about four decades, reconstructions on time scales of centuries have to rely on disc-integrated proxies of solar magnetic activity, such as sunspot areas and numbers. Employing a surface flux transport model and sunspot observations as input, we have being able to produce synthetic magnetograms since 1700. This improves the temporal resolution of the irradiance reconstructions on centennial time scales. The most critical aspect of such reconstructions remains the uncertainty in the magnitude of the secular change. Title: Total solar irradiance reconstruction since 1700 using a flux transport model Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.; Jiang, Jie Bibcode: 2014cosp...40E.634D Altcode: Reconstructions of solar irradiance into the past are crucial for studies of solar influence on climate. Models based on the assumption that irradiance changes are caused by the evolution of the photospheric magnetic fields have been most successful in reproducing the measured irradiance variations. Daily magnetograms, such as those from MDI and HMI, provide the most detailed information on the changing distribution of the photospheric magnetic fields. Since such magnetograms are only available from 1974, we used a surface flux transport model to describe the evolution of the magnetic fields on the solar surface due to the effects of differential rotation, meridional circulation, and turbulent diffusivity, before 1974. In this model, the sources of magnetic flux are the active regions, which are introduced based on sunspot group areas, positions, and tilt angles. The RGO record is, however, only available since 1874. Here we present a model of solar irradiance since 1700, which is based on a semi-synthetic sunspot record. The semi-synthetic record was obtained using statistical relationships between sunspot group properties (areas, positions, tilt angles) derived from the RGO record on one hand, and the cycle strength and phase derived from the sunspot group number (Rg) on the other. These relationships were employed to produce daily records of sunspot group positions, areas, and tilt angles before 1874. The semi-synthetic records were fed into the surface flux transport model to simulate daily magnetograms since 1700. By combining the simulated magnetograms with a SATIRE-type model, we then reconstructed total solar irradiance since 1700. Title: Solar Irradiance Variability and Climate Authors: Solanki, Sami K.; Krivova, Natalie A.; Haigh, Joanna D. Bibcode: 2013ARA&A..51..311S Altcode: 2013arXiv1306.2770S The brightness of the Sun varies on all timescales on which it has been observed, and there is increasing evidence that this has an influence on climate. The amplitudes of such variations depend on the wavelength and possibly the timescale. Although many aspects of this variability are well established, the exact magnitude of secular variations (going beyond a solar cycle) and the spectral dependence of variations are under discussion. The main drivers of solar variability are thought to be magnetic features at the solar surface. The climate response can be, on a global scale, largely accounted for by simple energetic considerations, but understanding the regional climate effects is more difficult. Promising mechanisms for such a driving have been identified, including through the influence of UV irradiance on the stratosphere and dynamical coupling to the surface. Here, we provide an overview of the current state of our knowledge, as well as of the main open questions. Title: Sunspot group tilt angles and the strength of the solar cycle (Corrigendum) Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron, R.; Peñuela, T. Bibcode: 2013A&A...556C...3D Altcode: No abstract at ADS Title: The latitudinal dependence of the solar radiance Authors: Finsterle, Wolfgang; Shapiro, Alexander; Schmutz, Werner; Krivova, Natalie Bibcode: 2013EGUGA..1511672F Altcode: Active regions and sunspots occur predominantly at low to mid heliographic latitudes. Hence, it seems reasonable to assume that the radiant output of the sun is not spherically symmetrical. Due to the relatively small inclination (~7.25°) of the solar rotation axis this asphericity is difficult to detect in integrated disk data taken from an ecliptic-bound vantage point. A histogram analysis of 13 years of VIRGO TSI data revealed a slight north-south asymmetry with maximal deviations of ±4 parts in 10^5. Interestingly, the north-south asymmetry persists even after subtracting the simulated TSI data by Krivova et al. (2003) from the VIRGO TSI measurements. The Krivova time series attributes the TSI to magnetic activity patterns as observed by MDI (sunspots, faculae, and plage). The asymmetry thus seems to be of a different origin, i.e. unrelated to sunspots, faculae, or plage, although smaller magnetic structures might contribute to the asymmetry. We will also investigate a potential asymmetry in the equator-to-pole temperature gradient. At this point we can only speculate if the observed asymmetry is characteristic of solar cycle 23, which is covered by the VIRGO time series, or more fundamental. In any case it would be very interesting to extend the TSI vs. latitude curve towards higher heliographic latitudes. Title: A Collaborative FP7 Effort towards the First European Comprehensive SOLar Irradiance Data Exploitation (SOLID) Authors: Haberreiter, Margit; Dasi, Maria; Delouille, Veronique; Del Zanna, Giulio; Dudok de Wit, Thierry; Ermolli, Ilaria; Kretzschmar, Matthieu; Krivova, Natalie; Mason, Helen; Qahwaji, Rami; Schmutz, Werner; Solanki, Sami; Thuillier, Gerard; Tourpali, Kleareti; Unruh, Yvonne; Verbeeck, Cis; Weber, Mark; Woods, Tom Bibcode: 2013EGUGA..1513079H Altcode: Variations of solar irradiance are the most important natural factor in the terrestrial climate and as such, the time dependent spectral solar irradiance is a crucial input to any climate modelling. There have been previous efforts to compile solar irradiance but it is still uncertain by how much the spectral and total solar irradiance changed on yearly, decadal and longer time scales. Observations of irradiance data exist in numerous disperse data sets. Therefore, it is important to bring together the European expertise in the field to analyse and merge the complete set of European irradiance data, complemented by archive data that include data from non-European missions. We report on the initiation of a collaborative effort to unify representatives from all European solar space experiments and European teams specialized in multi-wavelength solar image processing. It is intended to include the European groups involved in irradiance modelling and reconstruction. They will work with two different state of the art approaches to produce reconstructed spectral and total solar irradiance data as a function of time. These results will be used to bridge gaps in time and wavelength coverage of the observational data. This will allow the proposing SOLID team to reduce the uncertainties in the irradiance time series - an important requirement by the climate community - and to provide uniform data sets of modelled and observed solar irradiance data from the beginning of the space era to the present including proper error and uncertainty estimates. Climate research needs these data sets and therefore, the primary benefit is for the climate community, but the stellar community, planetary, lunar, and ionospheric researchers are also interested in having at their disposition incident radiation of the Sun. The proposing team plans to realize a wide international synergy in solar physics from 7 European countries, and collaborators from the US, complemented by representatives from the climate community, who will accompany their research work with wide dissemination activities. Title: Modeling the variability of Sun-like stars Authors: Shapiro, Alexander; Knaack, Reto; Krivova, Natalie; Schmutz, Werner; Solanki, Sami; Unruh, Yvonne Bibcode: 2013EGUGA..15.9981S Altcode: We present a model which attributes the variability of the stellar radiative energy flux to the imbalance between the contributions from dark starspots and bright faculae. The stellar radiative energy flux variations are modeled from the individual component's spectra, by weighting them with corresponding filling factors. The filling factors are deduced by extrapolating the sunspot and facular filling factors dependencies on solar CaII S-index to stars with different levels of the chromospheric activity. Our approach allows us to model the stellar photometric variability vs. activity dependency and reproduce the transition from spot-dominated to facula-dominated regimes of variability. We show how the effect of inclination (arising due to the random position of the Earth-bound observer relative to the directions of stellar rotational axis) can affect these dependencies and present the modeling of the individual stellar photometric light curves. Title: What can we learn about the Sun with PREMOS/PICARD? Authors: Cessateur, Gaël; Shapiro, Alexander; Schmutz, Werner; Krivova, Natalie; Solanki, Sami K.; Yeo, Kok Leng; Thuillier, Gérard Bibcode: 2013EGUGA..1511720C Altcode: Total and Spectral Solar Irradiance are key input parameters to atmospheric/oceanic and space weather models. We present here spectral solar irradiance data from the radiometer PREMOS onboard the PICARD satellite. This instrument convers the solar spectrum from the Ultraviolet to near-infrared, and provides valuable information and nourishes theoretical models. Based on redundancy strategies, instrumental degradation has been mostly corrected, revealing surprising behavior from the visible and near-infrared filters. We compare these data with those from the VIRGO/SOHO and SOLSTIC/SORCE experiments. Finally we use COSI to model the variability of the irradiance, assuming that the latter is determined by the evolution of the solar surface magnetic field as seen with SDO/HMI data. A direct comparison shows a very good correlation for most of channels from PREMOS. Title: Analysis of Ca II K images aiming to determine long-term trends in solar irradiance variability Authors: Kar, Anuradha; Ermolli, Ilaria; Krivova, Natalie; Solanki, Sami Bibcode: 2013EGUGA..15..330K Altcode: The change in radiative output of the Sun on time scales longer than a day is attributed to the variability in solar surface magnetic fields. Direct irradiance measurements are only available for less than four decades. To reconstruct long term trends in solar total and spectral irradiance, proxies of solar surface magnetism like sunspot, facular and network areas are needed. Currently, sunspot records alone are used for this purpose, from which the deduction of facular and network areas is rather indirect. Historical records of full disk images of the Sun taken in the Ca II K spectral line (393.3 nm) have the potential to provide far more direct information about the distribution and evolution of faculae and network elements. The latter appear as bright regions in the Ca II K spectroheliograms and their intensity is correlated with the magnetic field strength of the features on the solar surface. Solar full disk images in the Ca II K line have been recorded since the beginning of the 20th century at a number of solar observatories such as at Arcetri (Italy), Mount Wilson(California, US) and Kodaikanal (India). The images are available in digitized archives that contain the data processed for standard instrumental calibrations. To utilize these records for irradiance studies, the next step is to identify the bright magnetic features from the images using feature recognition techniques. We test different feature identification methods which are first applied to a set of recent images from the PSPT instrument at the Osservatorio Astronomico di Roma, taken during three periods characterized by high, medium and low levels of activity. Then the performance of these methods to historical images from Arcetri, Mt. Wilson and Kodaikanal archives is tested. The results will be presented and discussed here. Title: Recent variability of the solar spectral irradiance and its impact on climate modelling Authors: Ermolli, I.; Matthes, K.; Dudok de Wit, T.; Krivova, N. A.; Tourpali, K.; Weber, M.; Unruh, Y. C.; Gray, L.; Langematz, U.; Pilewskie, P.; Rozanov, E.; Schmutz, W.; Shapiro, A.; Solanki, S. K.; Woods, T. N. Bibcode: 2013ACP....13.3945E Altcode: 2012ACPD...1224557E; 2013arXiv1303.5577E The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult. Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earth's atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earth's climate.

We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earth's atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks. We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities.

We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earth's climate in CCMs.

We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation.

The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response.

Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments. Title: Intensity contrast of solar network and faculae Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2013A&A...550A..95Y Altcode: 2013arXiv1302.1442Y
Aims: This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position.
Methods: Full-disc magnetograms and intensity images by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae.
Results: In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores.
Conclusions: HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account. Title: Climate and Weather of the Sun - Earth System Authors: Usoskin, Ilya; Krivova, Natalie Bibcode: 2012aaa2.book..397U Altcode: The Sun is a variable star whose output, including electromagnetic radiation, magnetic fields and energetic particles varies at different time scales, from seconds to millennia. Solar variability affects the interplanetary medium but also planetary environments, including that of Earth. The state of the near-Earth environment is collectively called the Space weather, while its long-term changes make the concept of Space Climate. This forms the field of an interdisciplinary research focused on a wide range of topics: from solar physics, solar wind, cosmic rays, to planetary atmospheres and climate. Special emphasis is paid upon the processes that inter-relate solar variability and terrestrial environment - the Sun-Earth system. In this work, we present an overview on the state of the art in the field of the weather and climate of the Sun-Earth system. Title: Solar Irradiance Models and Measurements: A Comparison in the 220-240 nm wavelength band Authors: Unruh, Yvonne C.; Ball, Will T.; Krivova, Natalie A. Bibcode: 2012SGeo...33..475U Altcode: 2011arXiv1111.2068U; 2011SGeo..tmp..121U Solar irradiance models that assume solar irradiance variations to be due to changes in the solar surface magnetic flux have been successfully used to reconstruct total solar irradiance on rotational as well as cyclical and secular time scales. Modelling spectral solar irradiance is not yet as advanced, and also suffers from a lack of comparison data, in particular on solar cycle time scales. Here, we compare solar irradiance in the 220-240 nm band as modelled with SATIRE-S and measured by different instruments on the UARS and SORCE satellites. We find good agreement between the model and measurements on rotational time scales. The long-term trends, however, show significant differences. Both SORCE instruments, in particular, show a much steeper gradient over the decaying part of cycle 23 than the modelled irradiance or that measured by UARS/SUSIM. Title: A reconstruction of solar irradiance using a flux transport model Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.; Jiang, Jie Bibcode: 2012cosp...39..418D Altcode: 2012cosp.meet..418D Solar irradiance is one of the important drivers of the Earth's global climate, but it has only been measured for the past 33 years. Its reconstructions are therefore crucial to study longer term variations relevant to climate timescales. Most successful in reproducing the measured irradiance variations have being the models that are based on the assumption that irradiance changes are caused by the evolution of the photospheric magnetic field. Our SATIRE-S model is one of these, which uses solar full-disc magnetograms as an input, and these are available for less than four decades. To reconstruct the irradiance back to times when no observed magnetograms are available, we combine the SATIRE-S model with synthetic magnetograms, produced using a surface flux transport model. The model is fed with daily, observed or modelled statistically, records of sunspot positions, areas, and tilt angles. The concept of overlapping ephemeral region cycles is used to describe the secular change in the irradiance. Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and PREMOS/PICARD Authors: Cessateur, Gaël.; Kretzschmar, Matthieu; Krivova, Natalie; Schmutz, Werner; Solanki, Sami K.; Thuillier, Gerard; Shapiro, Alexander; Schoell, Micha; Shapiro, Anna; Dominique, Marie; Tagirov, Rinat; Wehrli, Christoph; Yeo, Kok Leng Bibcode: 2012cosp...39..287C Altcode: 2012cosp.meet..287C No abstract at ADS Title: Long-term reconstructions of total solar irradiance Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria Bibcode: 2012cosp...39..985K Altcode: 2012cosp.meet..985K Solar irradiance is the main external driver of the Earth's climate, although its relative contribution compared to other internal and anthropogenic factors is not yet well determined. Variations of total solar irradiance have being measured for over three decades and are relatively well understood. Reconstructions of the irradiance into the past remain, however, rather uncertain. In particular, the magnitude of the secular change is highly debated. The reason is the lack of direct and well-sampled proxies of solar magnetic activity on time scales longer than a few decades. Reconstructions on time scales of centuries rely on sunspot observations available since 1610. Reconstructions on millennial time scales use concentrations of the cosmogenic isotopes in terrestrial archives. We will review long-term reconstructions of the solar irradiance using the SATIRE set of models, compare them with other recent models and discuss the remaining uncertainties. Title: Reconstruction of total solar irradiance 1974-2009 Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.; Wenzler, T.; Mortlock, D. J.; Jaffe, A. H. Bibcode: 2012A&A...541A..27B Altcode: 2012arXiv1202.3554B Context. The study of variations in total solar irradiance (TSI) is important for understanding how the Sun affects the Earth's climate.
Aims: Full-disk continuum images and magnetograms are now available for three full solar cycles. We investigate how modelled TSI compares with direct observations by building a consistent modelled TSI dataset. The model, based only on changes in the photospheric magnetic flux can then be tested on rotational, cyclical and secular timescales.
Methods: We use Kitt Peak and SoHO/MDI continuum images and magnetograms in the SATIRE-S model to reconstruct TSI over cycles 21-23. To maximise independence from TSI composites, SORCE/TIM TSI data are used to fix the one free parameter of the model. We compare and combine the separate data sources for the model to estimate an uncertainty on the reconstruction and prevent any additional free parameters entering the model.
Results: The reconstruction supports the PMOD composite as being the best historical record of TSI observations, although on timescales of the solar rotation the IRMB composite provides somewhat better agreement. Further to this, the model is able to account for 92% of TSI variations from 1978 to 2009 in the PMOD composite and over 96% during cycle 23. The reconstruction also displays an inter-cycle, secular decline of 0.20+0.12-0.09 W m-2 between cycle 23 minima, in agreement with the PMOD composite.
Conclusions: SATIRE-S is able to recreate TSI observations on all timescales of a day and longer over 31 years from 1978. This is strong evidence that changes in photospheric magnetic flux alone are responsible for almost all solar irradiance variations over the last three solar cycles. Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and PREMOS/PICARD Authors: Cessateur, G.; Shapiro, A. I.; Dominique, M.; Kretzschmar, M.; Krivova, N.; Shapiro, A. V.; Schmutz, W.; Schoell, M.; Solanki, S.; Tagirov, R.; Thuillier, G.; Wehrli, C.; Yeo, K. L. Bibcode: 2012EGUGA..14.8254C Altcode: Total and Spectral Solar Irradiance are key input parameters to atmospheric/oceanic and space weather models. Both vary on time-scales ranging from days to millennia, although a complete picture of the solar irradiance variability is still missing. The recent launch of two European missions PROBA-2 with LYRA radiometer onboard and PICARD with PREMOS package onboard provides therefore valuable information and nourishes theoretical models. Both instruments covers the solar spectrum from the EUV to near-infrared. Here we present spectral solar irradiance data from these two recent missions. After a proper correction of the degradation and non-solar signatures, we compare these data with those from the VIRGO/SOHO and SOLSTICE+SIM/SORCE experiments. Both LYRA and PREMOS have also observed several solar eclipses, which allows us to accurately retrieve the center-to-limb variations (CLVs) of the quiet Sun's brightness at the wavelengths of the corresponding channels. CLVs play indeed an important role in modelling of the solar irradiance variability. We show that calculations with the published COde for Solar Irradiance (COSI) yield CLVs that are in good agreement with measurements. Finally we use COSI to model the variability of the irradiance, assuming that the latter is determined by the evolution of the solar surface magnetic field as seen with SDO/HMI data. These theoretical results are compared to PREMOS and LYRA measurements. Title: The influence of spectral solar irradiance data on stratospheric heating rates during the 11 year solar cycle Authors: Oberländer, S.; Langematz, U.; Matthes, K.; Kunze, M.; Kubin, A.; Harder, J.; Krivova, N. A.; Solanki, S. K.; Pagaran, J.; Weber, M. Bibcode: 2012GeoRL..39.1801O Altcode: 2012GeoRL..3901801O Heating rate calculations with the FUBRad shortwave (SW) radiation parameterization have been performed to examine the effect of prescribed spectral solar fluxes from the NRLSSI, MPS and IUP data sets on SW heating rates over the 11 year solar cycle 22. The corresponding temperature response is derived from perpetual January General Circulation Model (GCM) simulations with prescribed ozone concentrations. The different solar flux input data sets induce clear differences in SW heating rates at solar minimum, with the established NRLSSI data set showing the smallest solar heating rates. The stronger SW heating in the middle and upper stratosphere in the MPS data warms the summer upper stratosphere by 2 K. Over the solar cycle, SW heating rate differences vary up to 40% between the irradiance data sets, but do not result in a significant change of the solar temperature signal. Lower solar fluxes in the newer SIM data lead to a significantly cooler stratosphere and mesosphere when compared to NRLSSI data for 2007. Changes in SW heating from 2004 to 2007 are however up to six times stronger than for the NRLSSI data. Title: Solar Irradiance Models and Measurements: A Comparison in the 220–240 nm wavelength band Authors: Unruh, Yvonne C.; Ball, Will T.; Krivova, Natalie A. Bibcode: 2012omee.book..143U Altcode: No abstract at ADS Title: Modeling of the Solar Spectral Irradiance as observed by LYRA/PROBA2 and PREMOS/PICARD Authors: Shapiro, A.; Cessateur, G.; Dominique, M.; Krivova, N. A.; Lachat, D.; Rozanov, E.; Schmutz, W. K.; Shapiro, A. V.; Tagirov, R. V.; Thuillier, G. O.; Wehrli, C. Bibcode: 2011AGUFMGC22A..07S Altcode: Measurements and modeling of the solar irradiance have gained an increased attention during the last few decades. Nevertheless a complete picture of the solar variability is still missing. Therefore a launch of every new space mission devoted to the measurements of the spectral solar irradiance provides a crucial piece of complementary information and nourishes the theoretical models. We present here spectral solar irradiance data from the recent European missions PROBA-2 (launched on November 2, 2009) and PICARD (launched on June 15, 2010) and their theoretical interpretation. The PREMOS package onboard PICARD comprises two experiments, one observing solar irradiance in five (two UV, one visible and two near infrared) spectral channels with filter radiometers the other measuring TSI with absolute radiometers. LYRA is a solar VUV radiometer onboard PROBA-2, which is a technologically oriented ESA micro-mission, and is observing the solar irradiance in two UV and two EUV spectral channels. The passbands of the UV channels in the both experiments were selected on the ground of relevance for the terrestrial ozone concentration. The PREMOS and LYRA measurements were carefully corrected for the degradation and cleaned for non-solar signatures. We provide a comparison with the VIRGO/SOHO and SOLSTICE+SIM/SORCE data. Both LYRA and PREMOS have observed several solar eclipses. The analysis of these observations allows us to accurately retrieve the center-to-limb variations (CLV) of the solar brightness, which play an important role in the modeling of the solar irradiance variability on the time scale of the solar rotation. We show that the calculations with the recently developed and published COde for Solar Irradiance (COSI) yield the CLV which are in a good agreement with the measurements. The irradiance in all channels shows a clear variability on time-scale of the solar rotation. The amplitude and the profile of the variability strongly depend on the wavelength. We use COSI as a tool for modeling the variability of the irradiance, assuming that the latter is determined by the evolution of the solar surface magnetic field. The theoretical results are compared with the PREMOS and LYRA measurements. Finally we show that COSI and other semi-empirical and empirical models allow to reconstruct the entire solar spectrum from the observations in a few passbands. Hence the PREMOS and LYRA measurements can be used as a proxies for the nowcasting of the solar irradiance (UV-visible-IR) with the high cadence rate and spectral resolution. Title: Analyzing Solar Cycles Authors: Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2011Sci...334..916S Altcode: No abstract at ADS Title: Modelling solar irradiance variability on time scales from minutes to months Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A. Bibcode: 2011A&A...532A.108S Altcode: 2013arXiv1303.1961S We analyze and model total solar irradiance variability on time scales from minutes to months, excluding variations due to p-mode oscillations, using a combination of convective and magnetic components. These include granulation, the magnetic network, faculae and sunspots. Analysis of VIRGO data shows that on periods of a day or longer solar variability depends on magnetic activity, but is nearly independent at shorter periods. We assume that only granulation affects the solar irradiance variability on time scales from minutes to hours. Granulation is described as a large sample of bright cells and dark lanes that evolve according to rules deduced from observations and radiation hydrodynamic simulations. Comparison of this model combined with a high time resolution magnetic-field based irradiance reconstruction, with solar data reveals a good correspondence except at periods of 10 to 30 h. This suggests that the model is missing some power at these periods, which may be due to the absence of supergranulation or insufficient sensitivity of MDI magnetograms used for the reconstruction of the magnetic field-based irradiance reconstructions. Our model also shows that even for spatially unresolved data (such as those available for stars) the Fourier or wavelet transform of time series sampled at high cadence may allow properties of stellar granulation, in particular granule lifetimes to be determined. Title: Evolution of the solar irradiance during the Holocene Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I. Bibcode: 2011A&A...531A...6V Altcode: 2011arXiv1103.4958V Context. Long-term records of solar radiative output are vital for understanding solar variability and past climate change. Measurements of solar irradiance are available for only the last three decades, which calls for reconstructions of this quantity over longer time scales using suitable models.
Aims: We present a physically consistent reconstruction of the total solar irradiance for the Holocene.
Methods: We extend the SATIRE (Spectral And Total Irradiance REconstruction) models to estimate the evolution of the total (and partly spectral) solar irradiance over the Holocene. The basic assumption is that the variations of the solar irradiance are due to the evolution of the dark and bright magnetic features on the solar surface. The evolution of the decadally averaged magnetic flux is computed from decadal values of cosmogenic isotope concentrations recorded in natural archives employing a series of physics-based models connecting the processes from the modulation of the cosmic ray flux in the heliosphere to their record in natural archives. We then compute the total solar irradiance (TSI) as a linear combination of the jth and jth + 1 decadal values of the open magnetic flux. In order to evaluate the uncertainties due to the evolution of the Earth's magnetic dipole moment, we employ four reconstructions of the open flux which are based on conceptually different paleomagnetic models.
Results: Reconstructions of the TSI over the Holocene, each valid for a different paleomagnetic time series, are presented. Our analysis suggests that major sources of uncertainty in the TSI in this model are the heritage of the uncertainty of the TSI since 1610 reconstructed from sunspot data and the uncertainty of the evolution of the Earth's magnetic dipole moment. The analysis of the distribution functions of the reconstructed irradiance for the last 3000 years, which is the period that the reconstructions overlap, indicates that the estimates based on the virtual axial dipole moment are significantly lower at earlier times than the reconstructions based on the virtual dipole moment. We also present a combined reconstruction, which represents our best estimate of total solar irradiance for any given time during the Holocene.
Conclusions: We present the first physics-based reconstruction of the total solar irradiance over the Holocene, which will be of interest for studies of climate change over the last 11 500 years. The reconstruction indicates that the decadally averaged total solar irradiance ranges over approximately 1.5 W/m2 from grand maxima to grand minima.

Appendix A is available in electronic form at http://www.aanda.orgThe TSI data is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A6 Title: Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.; Harder, J. W. Bibcode: 2011A&A...530A..71B Altcode: 2011arXiv1104.0885B
Aims: We investigate how well modeled solar irradiances agree with measurements from the SORCE satellite, both for total solar irradiance and broken down into spectral regions on timescales of several years.
Methods: We use the SATIRE model and compare modeled total solar irradiance (TSI) with TSI measurements over the period 25 February 2003 to 1 November 2009. Spectral solar irradiance over 200-1630 nm is compared with the SIM instrument on SORCE over the period 21 April 2004 to 1 November 2009. We discuss the overall change in flux and the rotational and long-term trends during this period of decline from moderate activity to the recent solar minimum in ~10 nm bands and for three spectral regions of significant interest: the UV integrated over 200-300 nm, the visible over 400-691 nm and the IR between 972-1630 nm.
Results: The model captures 97% of the observed TSI variation. This is on the order at which TSI detectors agree with each other during the period considered. In the spectral comparison, rotational variability is well reproduced, especially between 400 and 1200 nm. The magnitude of change in the long-term trends is many times larger in SIM at almost all wavelengths while trends in SIM oppose SATIRE in the visible between 500 and 700 nm and again between 1000 and 1200 nm. We discuss the remaining issues with both SIM data and the identified limits of the model, particularly with the way facular contributions are dealt with, the limit of flux identification in MDI magnetograms during solar minimum and the model atmospheres in the IR employed by SATIRE. However, it is unlikely that improvements in these areas will significantly enhance the agreement in the long-term trends. This disagreement implies that some mechanism other than surface magnetism is causing SSI variations, in particular between 2004 and 2006, if the SIM data are correct. Since SATIRE was able to reproduce UV irradiance between 1991 and 2002 from UARS, either the solar mechanism for SSI variation fundamentally changed around the peak of cycle 23, or there is an inconsistency between UARS and SORCE UV measurements. We favour the second explanation. Title: Solar total irradiance in cycle 23 Authors: Krivova, N. A.; Solanki, S. K.; Schmutz, W. Bibcode: 2011A&A...529A..81K Altcode: 2011arXiv1102.3077K Context. The most recent minimum of solar activity was deeper and longer than the previous two minima as indicated by different proxies of solar activity. This is also true for the total solar irradiance (TSI) according to the PMOD composite.
Aims: The apparently unusual behaviour of the TSI has been interpreted as evidence against solar surface magnetism as the main driver of the secular change in the TSI. We test claims that the evolution of the solar surface magnetic field does not reproduce the observed TSI in cycle 23.
Methods: We use sensitive, 60-min averaged MDI magnetograms and quasi-simultaneous continuum images as an input to our SATIRE-S model and calculate the TSI variation over cycle 23, sampled roughly every two weeks. The computed TSI is then compared with the PMOD composite of TSI measurements and with the data from two individual instruments, SORCE/TIM and UARS/ACRIM II, that monitored the TSI during the declining phase of cycle 23 and over the previous minimum in 1996, respectively.
Results: Excellent agreement is found between the trends shown by the model and almost all sets of measurements. The only exception is the early, i.e. 1996 to 1998, PMOD data. Whereas the agreement between the model and the PMOD composite over the period 1999-2009 is almost perfect, the modelled TSI shows a steeper increase between 1996 and 1999 than implied by the PMOD composite. On the other hand, the steeper trend in the model agrees remarkably well with the ACRIM II data. A closer look at the VIRGO data, which are the basis of the PMOD composite after 1996, reveals that only one of the two VIRGO instruments, the PMO6V, shows the shallower trend present in the composite, whereas the DIARAD measurements indicate a steeper trend.
Conclusions: Based on these results, we conclude that (1) the sensitivity changes of the PMO6V radiometers within VIRGO during the first two years have very likely not been correctly evaluated; and that (2) the TSI variations over cycle 23 and the change in the TSI levels between the minima in 1996 and 2008 are consistent with the solar surface magnetism mechanism. Title: VizieR Online Data Catalog: Evolution of solar irradiance during Holocene (Vieira+, 2011) Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I. Bibcode: 2011yCat..35310006V Altcode: 2011yCat..35319006V This is a composite total solar irradiance (TSI) time series for 9495BC to 2007AD constructed as described in Sect. 3.3 of the paper. Since the TSI is the main external heat input into the Earth's climate system, a consistent record covering as long period as possible is needed for climate models. This was our main motivation for constructing this composite TSI time series.

In order to produce a representative time series, we divided the Holocene into four periods according to the available data for each period. Table 4 (see below) summarizes the periods considered and the models available for each period. After the end of the Maunder Minimum we compute daily values, while prior to the end of the Maunder Minimum we compute 10-year averages. For the period for which both solar disk magnetograms and continuum images are available (period 1) we employ the SATIRE-S reconstruction (Krivova et al. 2003A&A...399L...1K; Wenzler et al. 2006A&A...460..583W). SATIRE-T (Krivova et al. 2010JGRA..11512112K) reconstruction is used from the beginning of the Maunder Minimum (approximately 1640AD) to 1977AD. Prior to 1640AD reconstructions are based on cosmogenic isotopes (this paper). Different models of the Earth's geomagnetic field are available before and after approximately 5000BC. Therefore we treat periods 3 and 4 (before and after 5000BC) separately. Further details can be found in the paper. We emphasize that the reconstructions based on different proxies have different time resolutions.

(1 data file). Title: Towards a long-term record of solar total and spectral irradiance Authors: Krivova, N. A.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2011JASTP..73..223K Altcode: 2009arXiv0911.4002K The variation of total solar irradiance (TSI) has been measured since 1978 and that of the spectral irradiance for an even shorter amount of time. Semi-empirical models are now available that reproduce over 80% of the measured irradiance variations. An extension of these models into the more distant past is needed in order to serve as input to climate simulations. Here we review our most recent efforts to model solar total and spectral irradiance on time scales from days to centuries and even longer. Solar spectral irradiance has been reconstructed since 1947. Reconstruction of solar total irradiance goes back to 1610 and suggests a value of about 1-1.5W/m2 for the increase in the cycle-averaged TSI since the end of the Maunder minimum, which is significantly lower than previously assumed but agrees with other modern models. First steps have also been made towards reconstructions of solar total and spectral irradiance on time scales of millennia. Title: Reconstruction of solar spectral irradiance since the Maunder minimum Authors: Krivova, N. A.; Vieira, L. E. A.; Solanki, S. K. Bibcode: 2010JGRA..11512112K Altcode: Solar irradiance is the main external driver of the Earth's climate. Whereas the total solar irradiance is the main source of energy input into the climate system, solar UV irradiance exerts control over chemical and physical processes in the Earth's upper atmosphere. The time series of accurate irradiance measurements are, however, relatively short and limit the assessment of the solar contribution to the climate change. Here we reconstruct solar total and spectral irradiance in the range 115-160,000 nm since 1610. The evolution of the solar photospheric magnetic flux, which is a central input to the model, is appraised from the historical record of the sunspot number using a simple but consistent physical model. The model predicts an increase of 1.25 W/m2, or about 0.09%, in the 11-year averaged solar total irradiance since the Maunder minimum. Also, irradiance in individual spectral intervals has generally increased during the past four centuries, the magnitude of the trend being higher toward shorter wavelengths. In particular, the 11-year averaged Ly-α irradiance has increased by almost 50%. An exception is the spectral interval between about 1500 and 2500 nm, where irradiance has slightly decreased (by about 0.02%). Title: Reply to comment by P. Foukal on “A homogeneous database of sunspot areas covering more than 130 years” Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S. Bibcode: 2010JGRA..115.9103B Altcode: 2010JGRA..11509103B Abstract Available from http://www.agu.org Title: Sunspot group tilt angles and the strength of the solar cycle Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron, R.; Peñuela, T. Bibcode: 2010A&A...518A...7D Altcode: 2010arXiv1005.1774D Context. It is well known that the tilt angles of active regions increase with their latitude (Joy's law). It has never been checked before, however, whether the average tilt angles change from one cycle to the next. Flux transport models show the importance of tilt angles for the reversal and build up of magnetic flux at the poles, which is in turn correlated to the strength of the next cycle.
Aims: Here we analyse time series of tilt angle measurements and look for a possible relationship of the tilt angles with other solar cycle parameters, in order to glean information on the solar dynamo and to estimate their potential for predicting solar activity.
Methods: We employed tilt angle data from Mount Wilson and Kodaikanal observatories covering solar cycles 15 to 21. We analyse the latitudinal distribution of the tilt angles (Joy's law), their variation from cycle to cycle, and their relationship to other solar cycle parameters, such as the strength (or total area covered by sunspots in a cycle), amplitude, and length.
Results: The two main results of our analysis follow. 1. We find an anti-correlation between the mean normalised tilt angle of a given cycle and the strength (or amplitude) of that cycle, with a correlation coefficient of rc = -0.95 (99.9% confidence level) and rc = -0.93 (99.76% confidence level) for Mount Wilson and Kodaikanal data, respectively. 2. The product of the cycle's averaged tilt angle and the strength of the same cycle displays a significant correlation with the strength of the next cycle (rc = 0.65 at 89% confidence level and rc = 0.70 at 92% confidence level for Mount Wilson and Kodaikanal data, respectively). An even better correlation is obtained between the source term of the poloidal flux in Babcock-Leighton-type dynamos (which contains the tilt angle) and the amplitude of the next cycle. Further we confirm the linear relationship (Joy's law) between the tilt angle and latitude with slopes of 0.26 and 0.28 for Mount Wilson and Kodaikanal data, respectively. In addition, we obtain good positive correlations between the normalised-area-weighted tilt angle and the length of the following cycle, whereas the strength or the amplitude of the next cycle does not appear to be correlated to the tilt angles of the current cycle alone.
Conclusions: The results of this study indicate that, in combination with the cycle strength, the active region tilt angles play an important role in building up the polar fields at cycle minimum. Title: Evolution of the solar radiative forcing on climate during the Holocene Authors: Vieira, Luis Eduardo; Solanki, Sami K.; Krivova, Natalie Bibcode: 2010cosp...38.1785V Altcode: 2010cosp.meet.1785V The main external heating source of the Earth's coupled atmosphere-ocean system is the solar radiative energy input. The variability of this energy source produces corresponding changes on the coupled system. However, there is still significant uncertainty on the level of changes. One way to distinguish the influence of the Sun on the climate from other sources is to search for its influence in the pre-industrial period, when the influence of human activities on the atmosphere composition and Earth's surface properties can be neglected. Such studies require long time series of solar and geophysical parameters, ideally covering the whole Holocene. Here, we compute the total and spectral irradiance for the Holocene employing the reconstructions of the open flux and sunspot number obtained from the cosmogenic isotope 14C. The model employed in this study is identical to the spectral and total irradiance reconstruction (SATIRE) models employed to study these parameters on time scales from days to centuries, but adapted to work with decadal averaged data. The model is tested by comparing to the total and spectral solar irradiance reconstructions from the sunspot number for the last 4 centuries. We also discuss limits and uncertainties of the model. Title: Is the current solar activity minimum special? Authors: Solanki, Sami K.; Krivova, Natalie; Antunes Vieira, Luis Eduardo Bibcode: 2010cosp...38.1693S Altcode: 2010cosp.meet.1693S Is the current solar activity minimum special? Sami K. Solanki(1,2), Natalie Krivova(1), Luis E. Vieira(1,3) (1) Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany (2) Kyung-Hee University, Yongin, Gyeonggi, Korea (3) Laboratory for Physics and Chemistry of the Terrestrial Environment/CNRS, Or-leans, France In many respects the current minimum of solar activity is rather different from other minima during the last half century, which has seen the Sun in a state of unusually high activity. This uncommon minimum has fascinated solar physicists. Of particular importance for our understanding is the behaviour of the magnetic field, which is the source of all activity observed in the solar atmosphere, including the heliosphere. Furthermore, the evolution of dark and bright magnetic features on the solar surface modulates the solar irradiance, which could directly influence the Earth's climate. In this presentation observations and models of the evolution of the Sun's magnetic field, sunspots and irradiance are reviewed. After discussing the variation of these quantities over the solar cycle and from cycle to cycle, the current minimum is considered in greater detail. It is compared to earlier minima going back to the Maunder minimum and, in a more averaged sense, to solar activity and irradiance during the whole Holocene. Questions that will be onsidered are: Are we leaving the period of high solar activity (a grand maximum) to enter into a phase of more moderate activity, or even into a grand minimum? How could such a change in the level of the magnetic field and irradiance aid us in gaining further insight into the nature of the Sun's influence on the Earth's climate? Title: Analysis of Ca II K spectroheliogram time-series for solar irradiance studies Authors: Ermolli, Ilaria; Krivova, Natalie; Solanki, Sami K. Bibcode: 2010cosp...38..133E Altcode: 2010cosp.meet..133E Historic Ca II K observations constitute an extremely valuable resource for many research topics, including investigations of solar activity and irradiance variations on century timescales. In fact, the intensities measured on these observations can serve as proxies for magnetic activity, due to the strong correlation found between the Ca II K emission and the line-of-sight magnetic flux density. We analyzed three time-series of historic Ca II K observations, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson spectroheliohram archives. We examined the quality of these data, in order to estimate their value for irradiance studies focusing on time-scales longer than the solar cycle. We employ different processing techniques in order to investigate whether uniform results on the evolution of the solar magnetic field, both in quiet and active regions, can be obtained for the three analyzed series. ——-Session A1.1 Atmosphere Chemistry and Physics (i) Solar Spectral Irradiance Variations and Their Influence on the Earth's Atmosphere Title: Total and spectral irradiance comparisons between SIM and the SATIRE model in the declining phase of cycle 23 Authors: Ball, Will; Unruh, Yvonne; Krivova, Natalie; Solanki, Sami K.; Harder, Jerald Bibcode: 2010cosp...38..136B Altcode: 2010cosp.meet..136B Climate models rely on accurate total and spectral solar irradiance inputs, but until 2003 con-tinuous spectral irradiance information across a large portion of the solar spectrum was lacking. Since the launch of the Solar Radiation and Climate Experiment (SORCE), with the Spectral Irradiance Monitor (SIM) observing the UV, visible and IR, data have been accumulating and now cover a significant portion of a cycle. For the first time this allows spectral models to be tested over periods greater than a solar rotation. We present six years of total and spectral irradiance reconstructions using the SATIRE model that incorporates SOHO/MDI continuum and magnetogram images for the period April 2004 to November 2009 in the declining phase of cycle 23 and through the recent unusual minimum. We compare these results with the SIM instrument and so cover the spectral region 200 -1600 nm. While detrended, short-term, variation is recreated well by the model, there are discrepancies in longer-term trends between observations and the model. This may become important when considering the radiative forcing from the Sun used in climate research and so understanding why there is such a significant disagreement is an important area of investigation. Title: Solar magnetic field and irradiance: how unusual is the current minimum? (Invited) Authors: Solanki, S. K.; Krivova, N.; Vieira, L. A. Bibcode: 2009AGUFM.U34A..01S Altcode: In many respects the current minimum of solar activity is rather different from other minima during the last half century, which has seen the Sun in a state of unusually high activity. This uncommon minimum has fascinated solar physicists. Of particular importance for our understanding is the behaviour of the magnetic field, which is the source of all activity observed in the solar atmosphere, including the heliosphere. Furthermore, the evolution of dark and bright magnetic features on the solar surface modulates the solar irradiance, which could directly influence the Earth's climate. In this presentation observations and models of the evolution of the Sun's magnetic field and irradiance are reviewed. After discussing the variation of these quantities over the solar cycle and from cycle to cycle, the current minimum is considered in greater detail. It is compared to earlier minima going back to the Maunder minimum and, in a more averaged sense, to solar activity and irradiance during the whole Holocene. Questions that will be considered are: Are we leaving the period of high solar activity (a grand maximum) to enter into a phase of more moderate activity, or even into a grand minimum? How could such a change in the level of the magnetic field and irradiance aid us in gaining further insight into the nature of the Sun's influence on the Earth's climate? Title: ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996? Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T. Bibcode: 2009GeoRL..3620101K Altcode: 2009arXiv0911.3817K A gap in the total solar irradiance (TSI) measurements between ACRIM-1 and ACRIM-2 led to the ongoing debate on the presence or not of a secular trend between the minima preceding cycles 22 (in 1986) and 23 (1996). It was recently proposed to use the SATIRE model of solar irradiance variations to bridge this gap. When doing this, it is important to use the appropriate SATIRE-based reconstruction, which we do here, employing a reconstruction based on magnetograms. The accuracy of this model on months to years timescales is significantly higher than that of a model developed for long-term reconstructions used by the ACRIM team for such an analysis. The constructed ‘mixed’ ACRIM — SATIRE composite shows no increase in the TSI from 1986 to 1996, in contrast to the ACRIM TSI composite. Title: Reconstruction of solar UV irradiance since 1974 Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T.; Podlipnik, B. Bibcode: 2009JGRD..114.0I04K Altcode: 2009arXiv0907.1500K; 2009JGRD..11400I04K Variations of the solar UV irradiance are an important driver of chemical and physical processes in the Earth's upper atmosphere and may also influence global climate. Here we reconstruct solar UV irradiance in the range 115-400 nm over the period 1974-2007 by making use of the recently developed empirical extension of the Spectral And Total Irradiance Reconstruction (SATIRE) models employing Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) data. The evolution of the solar photospheric magnetic flux, which is a central input to the model, is described by the magnetograms and continuum images recorded at the Kitt Peak National Solar Observatory between 1974 and 2003 and by the Michelson Doppler Imager instrument on SOHO since 1996. The reconstruction extends the available observational record by 1.5 solar cycles. The reconstructed Ly-α irradiance agrees well with the composite time series by Woods et al. (2000). The amplitude of the irradiance variations grows with decreasing wavelength and in the wavelength regions of special interest for studies of the Earth's climate (Ly-α and oxygen absorption continuum and bands between 130 and 350 nm) is 1-2 orders of magnitude stronger than in the visible or if integrated over all wavelengths (total solar irradiance). Title: Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter, M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.; Unruh, Y.; Vögler, A. Bibcode: 2009SSRv..145..337D Altcode: The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we review the current status of solar irradiance measurements and modelling efforts based on solar photospheric magnetic fields. Thereby we restrict ourselves to the study of solar variations from days to the solar cycle. Phenomenological models of the solar atmosphere in combination with imaging observations of solar electromagnetic radiation and measurements of the photospheric magnetic field have reached high enough quality to show that a large fraction (at least, about 80%) of the solar irradiance variability can be explained by the radiative effects of the magnetic activity present in the photosphere. Also, significant progress has been made with magnetohydrodynamic simulations of convection that allow us to relate the radiance of the photospheric magnetic structures to the observations. Title: A homogeneous database of sunspot areas covering more than 130 years Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S. Bibcode: 2009JGRA..114.7104B Altcode: 2009JGRA..11407104B; 2009arXiv0906.0942B The historical record of sunspot areas is a valuable and widely used proxy of solar activity and variability. The Royal Greenwich Observatory regularly measured this and other parameters between 1874 and 1976. After that time records from a number of different observatories are available. These, however, show systematic differences and often have significant gaps. Our goal is to obtain a uniform and complete sunspot area time series by combining different data sets. A homogeneous composite of sunspot areas is essential for different applications in solar physics, among others for irradiance reconstructions. Data recorded simultaneously at different observatories are statistically compared in order to determine the intercalibration factors. Using these data we compile a complete and cross-calibrated time series. The Greenwich data set is used as a basis until 1976, the Russian data (a compilation of observations made at stations in the former USSR) are used between 1977 and 1985, and data compiled by the USAF network are used since 1986. Other data sets (Rome, Yunnan, and Catania) are used to fill up the remaining gaps. Using the final sunspot areas record the Photometric Sunspot Index is calculated. We also show that the use of uncalibrated sunspot areas data sets can seriously affect the estimate of irradiance variations. Our analysis implies that there is no basis for the claim that UV irradiance variations have a much smaller influence on climate than total solar irradiance variations. Title: Reconstructed and measured total solar irradiance: Is there a secular trend between 1978 and 2003? Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A. Bibcode: 2009GeoRL..3611102W Altcode: 2009arXiv0909.2975W Total solar irradiance reconstructed between 1978 and 2003 using solar surface magnetic field distributions is compared with three composites of total solar irradiance measurements. A good correspondence is found with the total solar irradiance composite from PMOD/WRC, with no bias between the three cycles. The agreement with the other composites (the ACRIM composite, mainly based on the Active Cavity Radiometer Irradiance Monitors I, II, and III, and the IRMB composite from the Institut Royal Meteorologique Belgique) is significantly poorer. In particular, a secular increase in the irradiance exhibited by these composites is not present in the reconstructions. Hence any secular trend in total solar irradiance between 1978 and 2003 is not due to magnetic fields at the solar surface. Title: Comparison Among Ca II K Spectroheliogram Time Series with an Application to Solar Activity Studies Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.; Ulrich, R. K.; Singh, J. Bibcode: 2009ApJ...698.1000E Altcode: Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca II K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca II K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger. Title: Faculae and Plague Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..153S Altcode: 2009LanB...4B.4124S No abstract at ADS Title: Sunspots Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..141S Altcode: 2009LanB...4B.4123S No abstract at ADS Title: Solar Activity Cycle Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..124S Altcode: 2009LanB...4B.4122S No abstract at ADS Title: Spectral irradiance variations: comparison between observations and the SATIRE model on solar rotation time scales Authors: Unruh, Y. C.; Krivova, N. A.; Solanki, S. K.; Harder, J. W.; Kopp, G. Bibcode: 2008A&A...486..311U Altcode: 2008arXiv0802.4178U Aims: We test the reliability of the observed and calculated spectral irradiance variations between 200 and 1600 nm over a time span of three solar rotations in 2004.
Methods: We compare our model calculations to spectral irradiance observations taken with SORCE/SIM, SoHO/VIRGO, and UARS/SUSIM. The calculations assume LTE and are based on the SATIRE (Spectral And Total Irradiance REconstruction) model. We analyse the variability as a function of wavelength and present time series in a number of selected wavelength regions covering the UV to the NIR. We also show the facular and spot contributions to the total calculated variability.
Results: In most wavelength regions, the variability agrees well between all sets of observations and the model calculations. The model does particularly well between 400 and 1300 nm, but fails below 220 nm, as well as for some of the strong NUV lines. Our calculations clearly show the shift from faculae-dominated variability in the NUV to spot-dominated variability above approximately 400 nm. We also discuss some of the remaining problems, such as the low sensitivity of SUSIM and SORCE for wavelengths between approximately 310 and 350 nm, where currently the model calculations still provide the best estimates of solar variability. Title: Models of solar irradiance variations: Current status Authors: Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2008JApA...29..151K Altcode: Regular monitoring of solar irradiance has been carried out since 1978 to show that solar total and spectral irradiance varies at different time scales. Whereas variations on time scales of minutes to hours are due to solar oscillations and granulation, variations on longer time scales are driven by the evolution of the solar surface magnetic field. Here the most recent advances in modelling of solar irradiance variations on time scales longer than a day are briefly reviewed. Title: On the potential value of Ca II K spectroheliogram time-series for solar activity and irradiance studies Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.; Ulrich, R. K.; Singh, J. Bibcode: 2008arXiv0802.3806E Altcode: Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line since the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image contents of three Ca II K spectroheliogram time-series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives. We describe the main problems afflicting these data and analyze their quality by expressing the image contents through several quantities. We compare the results obtained with those for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. Our results imply that the main challenge for the analysis of historic data is their accurate photometric calibration. This problem must be solved before they can provide reliable information about solar magnetism and activity over the last century. Moreover, inter-calibration of results obtained from independent time-series is required to reliably trace changes of solar properties with time from the analysis of such data. Title: Variability of solar irradiance from the UV to the NIR from GOME and SCIAMACHY for use in atmospheric models Authors: Ambrose Pagaran, Joseph; Weber, Mark; Ambrose Pagaran, Joseph; Burrows, John P.; Krivova, Natalie; Solanki, Sami; Floyd, Linton Bibcode: 2008cosp...37.2329A Altcode: 2008cosp.meet.2329A The Sun is the primary energy source that drives the Earth's climate system. Its radiative output is known to vary in time, for instance, with the 11-year solar cycle and 27-day solar rotation period. Variations in the solar spectral irradiance (SSI) affect the thermal structure and chemical composition of the Earth's atmosphere. Although the largest solar variations are observed in the UV spectral region, a large fraction of the total solar irradiance (TSI, solar constant) variation over a solar cycle comes from the visible and near IR spectral range. In order to understand how SSI variations cause a detectable change in climate, we need to quantify UV, visible, and near IR variation in the solar spectral irradiance to a high level of certainty both over the short term solar rotation 27-day period and 11-year solar cycle. Using daily solar irradiance observations from SUSIM (1992-2005), GOME (1995-present), and SCIAMACHY (2002-present), we study solar variations over 27-day solar rotations from 120 nm to 1600 nm. The variability is modelled by parameterizing SSI in terms of faculae brightening (using the Mg II core-to-wing ratio proxy) and sunspot darkening (using the photospheric sunspot index). Since the variations in the visible and NIR are well below 1 Title: Investigation of influence of hypomagnetic conditions closely similar to interplanetary magnetic filed on behavioral and vegetative reactions of higher mammals Authors: Krivova, Natalie; Trukhanov, Kiril; Zamotshina, Tatyana; Zaeva, Olga; Khodanovich, Marina; Misina, Tatyana; Tukhvatulin, Ravil; Suhko, Valery Bibcode: 2008cosp...37.1624K Altcode: 2008cosp.meet.1624K To study the influence of long being under reduced magnetic field on behavioral and vegetative reactions of higher mammals the white rat males were put into the 700-1000 times reduced geomagnetic field (50-70 nT) for 25 days. Such field was obtained by using automatic compensation of the horizontal and vertical components of the GMF at a frequencies up to 10 Hz by means of solenoids of the experimental magnetic system. Control animals were located in the same room under usual laboratory GMF conditions (52 uT). Two days before the experiment the behavioral reactions were studied in the "open field" by means of a set of tests, characterizing the level of emotionality, moving and orientational-investigative activities of the animals under conditions of unimpeded behavior. 60 white underbred rat males with the initial body mass of 200 g were divided into three clusters. Animals with average indices were selected for the experiment. We have judged behavioral reaction disturbances of the rats under hypomagnetic conditions using videotape recordings carried out in the entire course of the chronic experiment. According to the obtained results during the period of maximum activity (from 230 to 330 a.m.) the number of interrelations between the individuals increased appreciably for experimental rats including interrelations with aggressive character. This was real during all 25 days of observation. We observed a certain dynamics of this index differed from that of the control group. We have also analyzed the final period of observation from the 21th to the 25th days. In this period we studied the 24 hours' dynamics of interrelations which were noted during 5 minutes in every hour around the clock. In the control group the number of interrelation was at a constantly low level. For experimental animals the number of interrelations was higher in the night hours than in the day ones. Moreover it exceeded the similar indexes observed from the 1st to the 20th day. For example from 300 to 305 a.m. on the 23th day we recorded 27 contacts of aggressive character between the individuals. So, in hypomagnetic field conditions the irritability of the animals' central nervous system grows, that expresses itself in the increase of contacts of aggressive and non-aggressive character between the individuals. Also we have carried out the Spirman correlation analysis between studied indices of moving activity and chemiluminescence of blood plasma and urine, electrolytic composition of urine and muscles. For control animals the quantity of correlation connections between electrolyte concentrations in studied substrata was higher than for experimental animals. The physiological sense of these correlation connections is discussed. Title: Solar Activity and Irradiance Studies with Ca II Spectroheliograms: Potential and Problems Authors: Ermolli, I.; Tlatov, A.; Solanki, S. K.; Krivova, N. A.; Singh, J. Bibcode: 2007ASPC..368..533E Altcode: Various observatories around the globe carried out synoptic full-disk observations of the Sun since the beginning of the 20th century. The archives created by these observations, especially those including Ca II spectroheliograms, have the potential of providing far more detailed information on solar activity than the indices usually used to study activity variations, solar cycle and irradiance changes. However, these data suffer significantly from various problems including numerous defects in the photographic plates, missing or inaccurate calibration of the blackening curve, changes in the positioning of the exit slit with respect to the spectral line and variable seeing. Here we discuss the quality of images obtained by the digitization of three historic Ca II K time series, specifically those stored by the Arcetri, Kodaikanal and Mt Wilson Observatories. The aim of this work is to evaluate the potential value of these data for studies of solar activity and variability. It also shows the importance of the detailed and accurate image processing technique, in order to obtain uniform and trustable results from images coming from different historic archives. Title: Reconstruction of the Long-Term Irradiance Variations Authors: Balmaceda, L.; Krivova, N.; Solanki, S. Bibcode: 2007AGUSMGP54A..05B Altcode: Solar irradiance variations have been recorded only since 1978. Clearly, there is a need to extend these records into the past in order to evaluate their possible influence on the Earth's climate. Here, a reconstruction of solar irradiance back to the Maunder minimum from the surface magnetic flux is presented. The reconstruction is based on a simple physical model that builds on the sunspot number records and sunspot areas where available. Since the sunspot area records generally consist of a compilation of data from multiple observatories, a proper cross-calibration is essential. The use of data of different sources directly combined can lead to errors in estimating the increase of solar irradiance during the past centuries. Thus, a brief description of the cross-calibration of sunspot areas is also presented. Title: Reconstruction of solar total irradiance since 1700 from the surface magnetic flux Authors: Krivova, N. A.; Balmaceda, L.; Solanki, S. K. Bibcode: 2007A&A...467..335K Altcode: Context: Total solar irradiance changes by about 0.1% between solar activity maximum and minimum. Accurate measurements of this quantity are only available since 1978 and do not provide information on longer-term secular trends.
Aims: In order to reliably evaluate the Sun's role in recent global climate change, longer time series are, however, needed. They can only be assessed with the help of suitable models.
Methods: The total solar irradiance is reconstructed from the end of the Maunder minimum to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the sunspot number using a simple but consistent physical model.
Results: Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux since 1974 and the open magnetic flux since 1868 empirically reconstructed using the geomagnetic aa-index. The model predicts an increase in the solar total irradiance since the Maunder minimum of 1.3^+0.2_-0.4 Wm-2. Title: Properties of sunspots in cycle 23. I. Dependence of brightness on sunspot size and cycle phase Authors: Mathew, S. K.; Martínez Pillet, V.; Solanki, S. K.; Krivova, N. A. Bibcode: 2007A&A...465..291M Altcode: 2007astro.ph..1401M Aims:In this paper we investigate the dependence of umbral core brightness, as well as the mean umbral and penumbral brightness on the phase of the solar cycle and on the size of the sunspot.
Methods: Albregtsen & Maltby (1978, Nature, 274, 41) reported an increase in umbral core brightness from the early to the late phase of solar cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here we revisit this topic by analysing continuum images of more than 160 sunspots observed by the MDI instrument on board the SOHO spacecraft for the period between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The advantage of this data set is its homogeneity, with no seeing fluctuations. A careful stray light correction, which is validated using the Mercury transit of 7th May, 2003, is carried out before the umbral and penumbral intensities are determined. The influence of the Zeeman splitting of the nearby Ni I spectral line on the measured "continuum" intensity is also taken into account.
Results: We did not observe any significant variation in umbral core, mean umbral and mean penumbral intensities with solar cycle, which is in contrast to earlier findings for the umbral core intensity. We do find a strong and clear dependence of the umbral brightness on sunspot size, however. The penumbral brightness also displays a weak dependence. The brightness-radius relationship has numerous implications, some of which, such as those for the energy transport in umbrae, are pointed out. Title: Magnetic source of the solar cycle variation of the Mn I 539.4 nm line Authors: Danilović, S.; Solanki, S. K.; Livingston, W.; Krivova, N.; Vince, I. Bibcode: 2007msfa.conf..189D Altcode: As a part of the long term program at KPNO, the Mn I 539.4 nm line has been observed for nearly three solar cycles using the McMath telescope and the 13.5 m spectrograph in double pass mode. These full-disk spectrophotometric observations revealed an unusually large amplitude change of its parameters over the solar cycle and its correlation with Ca II K intensity. One of the proposed explanations for this phenomenon is the optical pumping by the Mg II k line. With this work we would like to show that this may not be the main mechanism behind the change. We reconstructed the changes of the line parameters using a model that takes into account only changes of the daily surface distributions of magnetic field. This model has already been used to successfully model total solar irradiance. We now apply it for modelling the Mn I line, as well as its neighboring Fe I line using exactly the same value of the free parameter as used for the reconstruction of total solar irradiance. We reproduce well the Mn I and Fe I line changes over the cycle purely with LTE modelling. This indicates that optical pumping of the Mn I line by Mg II k is not the main cause of its solar cycle change and sets an independent constraint on solar irradiance models. Title: Reconstruction of solar irradiance using the Group sunspot number Authors: Balmaceda, L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2007AdSpR..40..986B Altcode: 2007astro.ph..3147B We present a reconstruction of total solar irradiance since 1610 to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the Group sunspot number using a simple but consistent physical model. Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux from 1974 and the open magnetic flux since 1868 (as empirically reconstructed from the geomagnetic aa-index). The model predicts an increase in the total solar irradiance since the Maunder Minimum of about 1.3 Wm -2. Title: Reconstruction of solar irradiance variations in cycles 21-23 based on surface magnetic fields Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fröhlich, C. Bibcode: 2006A&A...460..583W Altcode: Aims.We present a reconstruction of total solar irradiance (TSI) back to 1974, i.e. from the minimum of cycle 21 to the declining phase of cycle 23. We also present a cross-calibration between the magnetograms obtained by the 512 channel magnetograph and the spectromagnetograph at Kitt Peak.
Methods: .The TSI reconstruction is carried out using data from the 512-channel Diode Array Magnetograph and the newer spectromagnetograph on Kitt Peak. The model is based on the assumption that all irradiance changes on time-scales of a day and longer are entirely due to the variations of the surface distribution of the solar magnetic field. The reconstructed irradiance is compared with the composite of total solar irradiance measurements from PMOD/WRC (version 41).
Results: .A good correspondence is found with the PMOD TSI composite, with no bias between the three cycles on time-scales longer than the solar rotation period, although the accuracy of the TSI reconstruction is somewhat lower when 512 channel magnetograph data are used. This suggests that the same driver of the irradiance variations, namely the evolution of the magnetic flux at the solar surface, is acting in cycles 21-23. Different methods of comparing the magnetograms obtained by the two Kitt Peak magnetographs give somewhat different results, with factors by which 512 channel data must be divided in the range 1.38-1.63 being found. This is due to the non-linearity of the relationship between the magnetic field measured by the two instruments.
Title: Solar Variability of Possible Relevance for Planetary Climates Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2006SSRv..125...25S Altcode: 2006SSRv..tmp...97S The global variability of the Sun of relevance for planetary climates has been directly measured for the past few decades. For longer stretches of time models are required. Semi-empirical models can now accurately reproduce the measured records of solar total and spectral irradiance, as well as of the magnetic flux. They can also provide reconstructions of these quantities on longer time scales. Here a summary is given of some of the modelling efforts and of the results achieved so far. Title: Reconstruction of solar UV irradiance in cycle 23 Authors: Krivova, N. A.; Solanki, S. K.; Floyd, L. Bibcode: 2006A&A...452..631K Altcode: Solar irradiance variations show a strong wavelength dependence. Whereas the total solar irradiance varies by about 0.1% during the course of the solar cycle, variations at the wavelengths around the Ly-α emission line near 121.6 nm range up to 50-100%. These variations may have a significant impact on the Earth's climate system. Being almost completely absorbed in the upper atmosphere, solar UV radiation below 300 nm affects stratospheric chemistry and controls production and destruction of ozone. Models of the solar UV irradiance remain far from perfect, even though considerable progress has been made in modelling the irradiance variations longwards of about 200-300 nm. We show that after correcting for the exposure dependent degradation of the SUSIM channels sampling irradiance at λ >240 nm (making use of the Mg II core-to-wing ratio) the agreement between model and measurement is significantly improved. At shorter wavelengths the LTE approximation usually made in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. The model reproduces observed solar cycle variations of the irradiance at wavelengths down to 115 nm and indicates an important role of UV irradiance variability: up to 60% of the total irradiance variations over the solar cycle might be produced at wavelengths below 400 nm. Title: Can surface magnetic fields reproduce solar irradiance variations in cycles 22 and 23? Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A. Bibcode: 2005A&A...432.1057W Altcode: Total solar irradiance is reconstructed using data from the spectromagnetograph on Kitt Peak for 1992 2003 (covering parts of cycles 22 and 23) and compared with observational data. The model assumes that solar irradiance variations are caused by surface magnetism and its single free parameter is kept at the same value for both cycles. A good correspondence is found for the whole period of time, with no bias between the two cycles on time-scales longer than the solar rotation period. This is in agreement with the recent results of [CITE], but it is in contrast to the earlier analysis of [CITE], and suggests that the source of the irradiance variations is the same for cycles 22 and 23, namely the evolution of the magnetic flux at the solar surface. Title: Reconstruction of the solar UV irradiance back to 1974 Authors: Haberreiter, M.; Krivova, N. A.; Schmutz, W.; Wenzler, T. Bibcode: 2005AdSpR..35..365H Altcode: The variability of the solar UV irradiance has strong effects on the terrestrial atmosphere. In order to study the solar influence for times when no UV observations are available, it is necessary to reconstruct the variation of the UV irradiance with time on the basis of proxies. We present reconstructions of the solar UV irradiance based on the analysis of space-based and ground-based magnetograms of the solar disk going back to 1974. With COde for Solar Irradiance (COSI) we calculate solar intensity spectra for the quiet Sun and different active regions and combine them according to their fractional area on the solar disk, whereby their time-dependent contributions over the solar cycle lead to a variability in radiation. COSI calculates the continuum and line formation under conditions which are out of local thermodynamic equilibrium (non-LTE). The applied temperature and density structures include the chromosphere and transition region, which is particularly important for the UV. The reconstructions are compared with observations. Title: Modelling of irradiance variations through atmosphere models . Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2005MmSAI..76..834K Altcode: Regular space-based measurements of solar total and spectral irradiance reveal its variations on time scales from minutes to decades. About 90% of these variations are reproduced by recent models assuming that the evolution of the solar surface magnetic fields is their main cause. Circumstantial evidence suggests that variations on yet longer time scales, which are of special interest for climate studies, are also possible. Once good understanding of the directly observed variations has been gained, we can then attempt to extend the models back in time. Here a brief overview of our recent efforts to reconstruct solar total and spectral irradiance on time scales of days to centuries is given. Title: A cross-calibrated sunspot areas time series since 1874 Authors: Balmaceda, L.; Solanki, S. K.; Krivova, N. Bibcode: 2005MmSAI..76..929B Altcode: A complete and homogeneous historical record of sunspot areas is a valuable proxy of solar variability, and is widely used, e.g., to understand the behaviour of total and spectral solar irradiance at earlier times. Since 1874, the Royal Greenwich Observatory (RGO) regularly carried out these and other measurements until December 1976. After that time the records from a number of different observatories are available. These, however, show some systematic differences and often have a lot of gaps. In order to compile a complete and cross-calibrated time series we compare the data from different observatories when they overlap and find the corresponding correction factors. The Greenwich data set is used as a basis until 1976, the Russian data (stations from the former USSR) between 1977 and 1985 and the Mt. Wilson data since 1986. Other data sets (Rome, Yunnan, Catania) are used to fill in the remaining gaps. Title: Reconstruction of solar UV irradiance Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2005AdSpR..35..361K Altcode: Understanding solar influence on the Earth’s climate requires a reconstruction of solar irradiance for the pre-satellite period. Considerable advances have been made in modelling the irradiance variations at wavelengths longer than 200 nm. At shorter wavelengths, however, the LTE approximation usually taken in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. Title: Irradiance models Authors: Solanki, S. K.; Krivova, N. A.; Wenzler, T. Bibcode: 2005AdSpR..35..376S Altcode: Measurements of solar irradiance have revealed variations at all the sampled time scales (ranging from minutes to the length of the solar cycle). One important task of models is to identify the causes of the observed (total and spectral) irradiance variations. Another major aim is to reconstruct irradiance over time scales longer than sampled by direct measurements in order to consider if and to what extent solar irradiance variations may be responsible for global climate change. Here, we describe recent efforts to model solar irradiance over the current and the previous two solar cycles. These irradiance models are remarkably successful in reproducing the observed total and spectral irradiance, although further improvements are still possible. Title: Comparison between KPVT/SPM and SoHO/MDI magnetograms with an application to solar irradiance reconstructions Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fluri, D. M. Bibcode: 2004A&A...427.1031W Altcode: To be able to use both space- and ground-based solar magnetograms and construct long time series of derived parameters it is important to cross-calibrate them so that we can estimate their reliability and combine them. Using two different techniques, we compare magnetograms as well as continuum images recorded by the Spectropolarimeter (SPM) on Kitt Peak and the Michelson Doppler Interferometer (MDI) on board SoHO. We find that the result obtained depends on the method used. The method we favour gives almost identical umbral and penumbral areas and very similar total magnetic fluxes in faculae. The magnetic fluxes in umbrae and penumbrae returned by the two instruments, however, differ considerably. We also demonstrate that SPM data can be employed to reconstruct total solar irradiance variations with almost the same accuracy as recently shown for MDI data. Title: Solar Irradiance Variations: From Current Measurements to Long-Term Estimates Authors: Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2004SoPh..224..197S Altcode: 2005SoPh..224..197S Variations of solar total and spectral irradiance are prime solar quantities purported to have an influence on the Earth's climate. Quantitative estimates of irradiance over as long a time as possible are needed to judge their effectiveness in forcing the climate. In order to do this reliably, first the measured record must be reproduced and a feeling for the physics underlying the irradiance variations must be developed. With the help of this knowledge combined with the available proxy data, reconstructions of irradiance in the past, generally since the Maunder minimum, are attempted. Here a brief introduction to some of the irradiance reconstruction work aiming at irradiance on time scales of days to the solar cycle is given, followed by a brief and incomplete overview of the longer-term reconstructions. Title: Effect of spatial resolution on estimating the Sun's magnetic flux Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004A&A...417.1125K Altcode: A critical question related to a possible secular trend in the Sun's total magnetic flux and consequently in solar irradiance is the total amount of magnetic flux present on the Sun and how it is distributed between active regions and the quiet Sun. NSO/Kitt Peak synoptic charts have in the past been used to estimate the total flux and the fraction of the flux in active regions and in the quiet Sun. Since a single pixel of these synoptic charts is much bigger than individual small-scale magnetic elements and opposite polarities may be present within the same pixel, some magnetic flux escaped notice. Here we estimate the fraction of the magnetic flux escaping detection in Kitt Peak synoptic charts. By artificially reducing the spatial resolution of MDI full-disc and high-resolution magnetograms we study the influence of the resolution on the measured total magnetic flux. Noise in the data poses the main difficulty to this approach and is carefully studied. It is concluded that at least half of the magnetic flux in the quiet Sun remains undetected in Kitt Peak synoptic charts and that the total flux present on the solar surface at maxima of activity is around twice the flux present at activity minima. Title: Towards understanding the β Pictoris dust stream Authors: Krivov, A. V.; Krivova, N. A.; Solanki, S. K.; Titov, V. B. Bibcode: 2004A&A...417..341K Altcode: The recent radar detection by \citet{baggaley-2000} of a collimated stream of interstellar meteoroids postulated to be sourced at β Pictoris, a nearby star with a prominent dust disk, presents a challenge to theoreticians. Two mechanisms of possible dust ejection from β Pic have been proposed: ejection of dust by radiation pressure from comets in eccentric orbits and by gravity of a hypothetical planet in the disk. Here we re-examine observational data and reconsider theoretical scenarios, substantiating them with detailed modeling to test whether they can explain quantitatively and simultaneously the masses, speeds, and fluxes. Our analysis of the stream geometry and kinematics confirms that β Pic is the most likely source of the stream and suggests that an intensive dust ejection phase took place ∼0.7 Myr ago. Our dynamical simulations show that high ejection speeds retrieved from the observations can be explained by both planetary ejection and radiation pressure mechanisms, providing, however, several important constraints. In the planetary ejection scenario, only a ``hot Jupiter''-type planet with a semimajor axis of less than 1 AU can be responsible for the stream, and only if the disk was dynamically ``heated'' by a more distant massive planet. The radiation pressure scenario also requires the presence of a relatively massive planet at several AU or more, that had heated the cometesimal disk before the ejection occurred. Finally, the dust flux measured at Earth can be brought into reasonable agreement with both scenarios, provided that β Pic's protoplanetary disk recently passed through an intensive short-lasting (∼0.1 Myr) clearance stage by nascent giant planets, similar to what took place in the early solar system. Title: Reconstruction of solar UV irradiance Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004cosp...35..504K Altcode: 2004cosp.meet..504K Understanding solar influence on the Earth's climate requires a reconstruction of solar irradiance for the pre-satellite period. Considerable advances have been made in modelling the irradiance variations at wavelengths longer than 300 nm. At shorter wavelengths, however, the LTE approximation usually taken in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. Title: Understanding Solar Variability as a Groundwork for Planet Transit Detection Authors: Seleznyov, A. D.; Krivova, N. A.; Solanki, S. K. Bibcode: 2004IAUS..219..815S Altcode: 2003IAUS..219E...7K Detection of planetary transits holds the greatest promise for the search of terrestrial planets. However intrinsic stellar variability can mask real transits or lead to 'false' planet transit detections. Understanding the origin of stellar variability can help to estimate the minimum sizes of planets detectable with this technique around different types of stars and to identify the best wavelength range for such measurements. The only star for which data with sufficient photometric accuracy and temporal sampling exist is the Sun. We analyze and model solar variability on timescales relevant for planetary transits (hours to several days) using a variety of components such as granulation network (supergranulation) faculae and sunspots. This study extends our successful work to model solar irradiance variations on days to years timescales (Fligge et al. 2000; Krivova et al. 2003). Title: Reconstruction of solar UV irradiance back to 1974 Authors: Haberreiter, M.; Krivova, N. A.; Schmutz, W.; Wenzler, T. Bibcode: 2004cosp...35.2707H Altcode: 2004cosp.meet.2707H The variability of the solar UV irradiance has strong effects on the terrestrial atmosphere. In order to study the solar influence for times when no UV observations are available, it is necessary to reconstruct the variation of the UV irradiance with time on the basis of proxies. We present reconstructions of the solar UV irradiance based on the analysis of space-based and ground-based magnetograms of the solar disk going back to 1974. With COSI (COde for Solar Irradiance) we calculate solar intensity spectra for the quiet Sun and different active regions and combine them according to their fractional area on the solar disk, whereby their time-dependent contributions over the solar cycle lead to a variability in radiation. COSI calculates the continuum and line formation in non-local thermodynamic equilibrium (non-LTE). The applied temperature and density structures include the chromosphere and transition region, which is particularly important for the UV. The reconstructions are compared with observations. Title: Solar variability and global warming: a statistical comparison since 1850 Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004AdSpR..34..361K Altcode: The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations entering climate modelling. Given the complexity of the climate system, however, such modelling is perforce based on simplifying assumptions, which leaves it open to criticism. We take a complementary approach. We assume that the Sun has been responsible for climate change prior to 1970 and that their interrelation remained unchanged afterwards. Then, employing reconstructions and measured records of relevant solar quantities as well as of the cosmic-ray flux, we estimate statistically which fraction of the dramatic temperature rise after that date could be due to the influence of the Sun. We show that at least in the most recent past (since about 1970) the solar influence on climate cannot have been significant. Title: On the origin of solar variability, with an application to the search for extrasolar planets Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A. Bibcode: 2003ESASP.539..589S Altcode: 2003toed.conf..589S Detection of planetary transits holds great promise for the search of terrestrial planets. However, most stars are variable at the level of the signal produced by the transit of an Earth-like planet. Hence, intrinsic stellar variability can lead to "false" planet transit detections. An understanding of the origin of the stellar variability is needed to ensure reliable transit detections. We consider the Sun as the closest and best studied star and analyze its variability on timescales relevant to the transit effect, namely from an hour to several days. Total and spectral solar irradiance measurements obtained by the VIRGO instrument on board the SOHO spacecraft have been analyzed by applying Fourier and wavelet techniques. Preliminary results suggest that at the time scales of interest solar variability is driven partly by solar magnetic activity, which dominates at longer time scales, and convection, in particular solar granulation, which dominates at shorter time scales. As part of a more quantitative analysis a simple numerical model of the irradiance variations due to granulation has been constructed. Irradiance variability of stars with different surface gravity was calculated in the frequency band of relevance to transits. Title: Solar total and spectral irradiance: modelling and a possible impact on climate Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2003ESASP.535..275K Altcode: 2003iscs.symp..275K There is growing evidence that solar variability influences the Earth's climate, although the underlying mechanism is not yet understood. Variations in the solar total and spectral irradiance often play a central role within various processes that have been suggested. Whereas changes in the total irradiance can affect the overall energy balance of the Earth's atmosphere, variations in its spectral distribution, in particular in the UV, have a pronounced effect on the chemistry of the Earth's upper atmosphere. Measurements of the solar total irradiance are only available since 1978 and the spectral irradiance record is even shorter. This calls for a reconstruction of irradiance variations at earlier times with the help of models. We first outline our current understanding of the main mechanism responsible for irradiance variations and describe the efforts to reconstruct them. The reconstructed total and UV irradiance is then employed to estimate the solar contribution to global warming, with particular emphasis to the period since 1970. Title: Solar irradiance fluctuations on short timescales Authors: Solanki, S. K.; Seleznyov, A. D.; Krivova, N. A. Bibcode: 2003ESASP.535..285S Altcode: 2003iscs.symp..285S Although solar irradiance variability at time-scales of days to the solar cycle has been well studied, comparatively little is known about the causes of such variations on shorter time-scales. We present an analysis that aims to distinguish between magnetic and convective causes. It suggests that on time-scales longer than 1-2 days magnetic structures are the dominant source while for time-scales shorter than a few hours convection appears to dominate. We also present a simple granulation model that includes the various paths of granule birth and death and compare its output with VIRGO data. Title: Can solar variability explain global warming since 1970? Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2003JGRA..108.1200S Altcode: The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations and climate modeling. This approach suffers from uncertainties that are difficult to estimate. Such uncertainties are introduced because the employed models may not include important but complex processes or mechanisms or may treat these in too simplified a manner. Here we take a more empirical approach. We employ time series of the most relevant solar quantities, the total and UV irradiance between 1856 and 1999 and the cosmic rays flux between 1868 and 1999. The time series are constructed using direct measurements wherever possible and reconstructions based on models and proxies at earlier times. These time series are compared with the climate record for the period 1856 to 1970. The solar records are scaled such that statistically the solar contribution to climate is as large as possible in this period. Under this assumption we repeat the comparison but now including the period 1970-1999. This comparison shows without requiring any recourse to modeling that since roughly 1970 the solar influence on climate (through the channels considered here) cannot have been dominant. In particular, the Sun cannot have contributed more than 30% to the steep temperature increase that has taken place since then, irrespective of which of the three considered channels is the dominant one determining Sun-climate interactions: tropospheric heating caused by changes in total solar irradiance, stratospheric chemistry influenced by changes in the solar UV spectrum, or cloud coverage affected by the cosmic ray flux. Title: A stream of particles from the begin {equation}ta Pictoris disc: A possible ejection mechanism Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2003A&A...402L...5K Altcode: Recently, a stream of particles originating from the direction of begin {equation}ta Pictoris, a young main sequence star surrounded by a dust disc, has been reported (Baggaley \cite{baggaley-2000}). Standard mechanisms of particle ejection from a disc fail to reproduce the properties of this stream. We find that scattering by a giant proto-planet with properties taken from the literature is consistent with the observations. The fact that a straightforward ejection mechanism reproduces the data supports the identification of the particle stream's source with beta Pic. Our work also indicates that protoplanetary dust discs form a potentially rich source of large interstellar grains, as widely detected in the Solar System. Title: Ejection of Dust from Planetary Systems into Interstellar Space Authors: Krivov, A.; Krivova, N.; Solanki, S.; Titov, V. Bibcode: 2003EAEJA.....4264K Altcode: Extensive observational and theoretical research of the last two decades have made it clear that young exoplanetary systems comprise not only planets themselves, but also disks of small bodies and of dust-sized material these bodies replenish. In much the same way as in the young Solar System, interaction of the solids in a circumstellar disk with planets at late stages of planet formation should hang the material on the outskirts of the system, creating exosolar Kuiper belts and Oort clouds. A portion of the material is ejected by the exoplanets from the disks into interstellar space, which may represent an important source of large interstellar dust grains in the Galaxy. Here we study how the efficiency of the ejection mechanism and the involved timescales depend on the mass and orbital parameters of the planet, as well as on the `unperturbed' spatial distribution of disk particles. We also derive the distribution of the final velocities of the ejected material. The study is done semianalytically with the aid of the Oepik-Weidenschilling statistical theory of planetary encounters and numerically with direct Monte-Carlo orbital simulations. The results may have particular implications to the beta Pictoris system: they lend further support to the idea (Krivova and Solanki 2002) that a collimated stream of interstellar radiometeors detected by Baggaley (2000) originates at beta Pic and consists of grains ejected by a presumed jovian planet orbiting this star. Title: Cycles and cyclicities of the Sun Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2003ASPC..292..423S Altcode: 2003ipc..conf..423S The solar activity cycle is discussed in the broader context of solar variability. It is pointed out that the Sun exhibits periodic, cyclic, chaotic and stochastic phenomena. The origins of solar variability are mentioned and the importance of secular variations of the solar cycle for a putative influence of the Sun on the Earth's climate is briefly discussed. Title: Reconstruction of solar irradiance variations in cycle 23: Is solar surface magnetism the cause? Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M.; Unruh, Y. C. Bibcode: 2003A&A...399L...1K Altcode: A model of solar irradiance variations is presented which is based on the assumption that solar surface magnetism is responsible for all total irradiance changes on time scales of days to years. A time series of daily magnetograms and empirical models of the thermal structure of magnetic features (sunspots, faculae) are combined to reconstruct total (and spectral) irradiance from 1996 to 2002. Comparisons with observational data reveal an excellent correspondence, although the model only contains a single free parameter. This provides strong support for the hypothesis that solar irradiance variations are caused by changes in the amount and distribution of magnetic flux at the solar surface. Title: Was one sunspot cycle in the 18th century really lost? Authors: Krivova, N. A.; Solanki, S. K.; Beer, J. Bibcode: 2002A&A...396..235K Altcode: The unusually long 4th solar cycle has recently been proposed by Usoskin et al. (2001) to be composed of two cycles. They argue that a weak and short cycle might have been lost in sparse sunspot data at the end of the 18th century. Here we check this hypothesis in different ways. First, we consider the sunspot number record in greater detail and compare in a statistical sense the sunspot observations of the period in question with those at other times. In a statistical sense the sunspot numbers recorded at the time of the proposed new cycle minimum are extremely untypical for other minima in the solar cycle record, but quite usual for the declining phase of the solar cycle. We also analyse other available proxies of solar activity, such as variations of the cosmogenic nuclides 10Be and 14C as well as auroral activity. These historical records are sufficiently long and provide an independent testimony of the cyclic behaviour of solar activity at the end of the 18th century. We found no evidence for a lost cycle in any of these data sets. Finally, we compare the proposed new cycle with the other cycles in the sunspot record. This reveals that the proposed ``missing'' cycle has very unusual properties, much more so than the original, standard cycle 4. Taken together, the evidence from these various tests strongly suggests that no cycle was missed and that the official sunspot cycle numbering and parameters are correct. Title: Search for a relationship between solar cycle amplitude and length Authors: Solanki, S. K.; Krivova, N. A.; Schüssler, M.; Fligge, M. Bibcode: 2002A&A...396.1029S Altcode: The cross-correlation between time series of solar cycle length and amplitude suggests that the length precedes the amplitude. The relationship between the two is found to be more complex than a simple lag or phase shift, however. A simple empirical model is constructed which allows the amplitude of a given cycle to be predicted with relatively high accuracy from the lengths of earlier cycles. This result not only adds to the means at our disposal for predicting the amplitudes of future cycles, but also implies that the solar dynamo carries a memory of the length of one cycle over into the next. It may also have a bearing on why solar cycle length correlates better with the Earth's temperature record than cycle amplitude (Friis-Christensen & Lassen \cite{Friis-Christensen:Lassen:1991}). Thoughts on possible physical causes are presented. Title: The 1.3-year and 156-day periodicities in sunspot data: Wavelet analysis suggests a common origin Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2002A&A...394..701K Altcode: Helioseismic data have revealed a 1.3-year periodicity in the solar rotation rate near the bottom of the solar convection zone. In order to test whether these rotation rate variations have a significant impact on the solar dynamo, we search for such a periodicity in tracers of relatively freshly emerged flux at the solar surface, namely sunspots. Sunspot areas and sunspot number time series are studied with the help of the wavelet transform. Significant power at this period (1.28 years) is indeed found and is observed to vary strongly with time. This provides independent support for the presence of a 1.3 year periodicity in solar data. The power at the 154-158-day Rieger period of solar flares is seen to vary approximately in phase with the 1.28-year period. Based on this we propose that the Rieger period is the third harmonic (3x 156 days= 1.28 years) of the 1.3-year period. If the rotation rate of the Sun does vary with 1.3 years then the enhanced flaring with the Rieger period may finally be driven by the 1.3 year periodicity. However, the power in both periods is also found to approximately follow the total number of sunspots. Therefore we cannot rule out that the 1.3-year and 156-day periods are harmonics of the solar activity cycle. Finally, our analysis of a calibrated sunspot area record reveals that the 156-day period continues into the most recent cycles, in contrast to earlier results. Title: How large is the Sun's total magnetic flux? Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M. Bibcode: 2002ESASP.505..461K Altcode: 2002solm.conf..461K; 2002IAUCo.188..461K Variations in the radiative output of the Sun are directly allied to changes in the amount and distribution of solar surface magnetic field. The variability of irradiance on time-scales significantly shorter than a solar cycle can be produced by the spatial and temporal evolution of active regions, whereas its secular variations ensue from changes of the quiet-Sun magnetic flux on longer time-scales. The relative amounts of the magnetic flux in active regions and in the quiet Sun as well as their cyclic evolution have been studied by Harvey (1994) on the basis of the NSO/Kitt Peak synoptic maps. Since a single pixel of such a map is much bigger than individual small-scale magnetic elements and opposite polarities may be present within the same pixel, some magnetic flux went uncounted, so that the flux obtained by Harvey (1994) represents a lower limit to the true amount of magnetic flux present on the Sun. We use MDI full-disc and high-resolution magnetograms to estimate the total magnetic flux of the Sun. Title: Total solar magnetic flux: dependence on spatial resolution of magnetograms Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M. Bibcode: 2002ESASP.508..155K Altcode: 2002soho...11..155K The variability of the solar radiative output is tied to the evolution of the surface magnetic field. Irradiance changes on time-scales of the solar rotation are governed by the varying distribution of the magnetic features and the evolution of sunspots and individual active regions, whereas the total amount of magnetic flux in particular small-scale magnetic elements seems to dominate the irradiance variations on the time scale of the solar cycle. Possibly of even greater relevance for climate on Earth are secular variations of solar irradiance, which are at least partly caused by changes in the quiet-Sun magnetic flux on long time scales. Using NSO/Kitt Peak synoptic charts Harvey (1994) found that the total magnetic flux in active regions at activity maximum is about 3 times higher than the flux at activity minimum. The size of small-scale magnetic elements is, however, far below the resolution of currently obtainable magnetograms. At the same time, their distribution on the solar surface is highly non-uniform, with magnetic elements of opposite polarities often being grouped close together. This leads to an apparent cancellation of the flux within a relatively large pixel of a Kitt Peak synoptic chart and underestimates the total magnetic flux, mainly in the quiet Sun. Using MDI full-disc and high-resolution magnetograms and artificially reducing their spatial resolution by binning several pixels together we study the influence of the resolution on the measured total magnetic flux. Title: Solar variability and global warming Authors: Krivova, N.; Solanki, S. Bibcode: 2002cosp...34E.274K Altcode: 2002cosp.meetE.274K The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations entering climate modelling. Given the complexity of the climate system, however, such modelling is perforce based on simplifying assumptions, which leaves it open to criticism. We take a complementary approach. We assume that the Sun has been responsible for climate change prior to 1970. Then, using reconstructions and measured records of relevant solar quantities as well as of the cosmic-ray flux, we estimate which fraction of the dramatic temperature rise after that date could be due to the influence of the Sun. We show that at least in the most recent past (since 1970) the solar influence on climate cannot have been significant. Title: Dust in young solar systems (Invited) Authors: Krivova, N. A. Bibcode: 2002dsso.conf..201K Altcode: 2002IAUCo.181..201K No abstract at ADS Title: Irradiance Medels based on Magnetic Activity Authors: Solanki, S.; Krivova, N.; Unruh, Y. Bibcode: 2002cosp...34E1357S Altcode: 2002cosp.meetE1357S An introduction is given to the modelling of irradiance variations based on solar surface magnetism. We describe how a time series of daily magnetograms and empirical models of the thermal structure of magnetic features (sunspots, magnetic elements) are combined to reconstruct total and spectral irradiance on a time scale of weeks to years. Comparisons with observational data reveal an excellent correspondence. On a longer time scale of decades to centuries we first need to know how the Sun's magnetic field evolved. Hence reconstructions of the Sun's total magnetic flux are briefly introduced, before we present longer term reconstructions of the total and spectral irradiance since the Maunder minimum. Title: Size distributions of dust in circumstellar debris discs Authors: Krivov, A. V.; Mann, I.; Krivova, N. A. Bibcode: 2000A&A...362.1127K Altcode: The size distribution of particles in a dust disc is determined by, and holds the key to, sources, sinks and dynamics of grains. Here we derive the size distribution in circumstellar debris discs, exemplified by the disc of begin {equation}ta Pictoris, by modelling the dynamical evolution of the circumstellar dust, dominated by collisions. The whole disc is considered as consisting of two dust populations: larger grains moving in bound orbits (alpha -meteoroids) and smaller ones blown away by radiation pressure (begin {equation}ta-meteoroids). Although begin {equation}ta-meteoroids leave the disc in hyperbolic trajectories, they are continuously replenished by collisions, so that at any time the disc contains a substantial population of small particles. As a consequence, the fragmentation of alpha -meteoroids not only by mutual collisions, but also by impacts of begin {equation}ta-meteoroids becomes significant. This flattens the distribution of alpha -meteoroids in the size regime adjacent to the blow-out limit and shifts the cross section-dominating sizes from a few micrometres to ga 10 mum . The overall distribution shows essentially three different slopes: steeper ones for both begin {equation}ta-meteoroids and large alpha -meteoroids and a gentler one for alpha -meteoroids with sizes just above the blow-out limit. This resembles the size distribution of interplanetary dust particles in the Solar system which, however, is shaped by different mechanisms. The basic features of the modelled size distribution (the presence of a substantial population of small hyperbolic particles in the disc, the dominance of grains ~ 10 mum in size) well agree with the observational data available. Although particular calculations were made for the begin {equation}ta Pic disc, our basic qualitative conclusions directly apply to the debris discs around other Vega-type stars with low gas contents and similar or somewhat lower optical depths. Title: Porous Dust Grains in the Shells of Herbig Ae/Be Stars Authors: Il'In, V. B.; Krivova, N. A. Bibcode: 2000AstL...26..379I Altcode: The transfer of polarized radiation in inhomogeneous circumstellar shells with a spheroidal spatial distribution of porous dust particles is computed. The grains are modeled by an MRN mixture of silicate and graphite particles. The optical properties of porous particles (considered separately in the Appendix) are computed by using effective medium theory and Mie theory. The following observational characteristics have been computed for WW Vul, a typical Herbig Ae star with Algol-like minima: the spectral energy distribution from the ultraviolet to the far infrared, the color-magnitude diagrams, the wavelength dependence of linear polarization, and the shell brightness distribution. The effect of grain porosity on the results is considered. It has been found that only moderate particle porosity (the volume fraction of matter is f ~ 0.5) can explain available observational data in terms of the approach used. Since radiation pressure must rapidly sweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars, we briefly discuss how particle porosity can affect this process. Title: Size Distribution of Dust in the Disk of β Pictoris Authors: Krivova, N. A.; Krivov, A. V.; Mann, I. Bibcode: 2000ASPC..219..387K Altcode: 2000dpp..conf..387K No abstract at ADS Title: WWW database of optical constants for astronomy Authors: Henning, Th.; Il'In, V. B.; Krivova, N. A.; Michel, B.; Voshchinnikov, N. V. Bibcode: 1999A&AS..136..405H Altcode: The database we announce contains references to the papers, data files and links to the Internet resources related to measurements and calculations of the optical constants of the materials of astronomical interest: different silicates, ices, oxides, sulfides, carbides, carbonaceous species from amorphous carbon to graphite and diamonds, etc. We describe the general structure and content of the database which has now free access via Internet: http://www.astro.spbu.ru/JPDOC/entry.html\ or \ http:// www. astro.uni-jena.de/Users/database/entry.html Title: Polarization of the Zodiacal cloud and other Solar system - type debris disks Authors: Krivova, N.; Mann, I.; Krivov, A. Bibcode: 1999md98.conf..291K Altcode: No abstract at ADS Title: On the Behaviour of Interstellar CO and CN Molecules in Individual Clouds Authors: Sikorski, J.; Krelowski, J.; Gnacinski, P.; Kaczmarczyk, G.; Snow, T. P.; Krivova, N. Bibcode: 1998ESASP.413..505S Altcode: 1998uabi.conf..505S No abstract at ADS Title: Dust shells around Herbig Ae/Be stars with algol-like minima: Modeling of photometric observations Authors: Krivova, N. A.; Il'In, V. B. Bibcode: 1997AstL...23..791K Altcode: 1997PAZh...23..908K No abstract at ADS Title: Herbig Ae stars with Algol-like minima: Modeling of the spectral energy distribution and the behavior of colors at minima Authors: Krivova, N. A. Bibcode: 1997AstL...23..327K Altcode: 1997PAZh...23..371K No abstract at ADS Title: Dust shells around Herbig Ae/Be stars with Algol-like minima: results of a combined modelling of different observational data Authors: Krivova, N. A.; Il'In, V. B.; Kimura, H. Bibcode: 1997IAUJD..13E...8K Altcode: Herbig Ae/Be stars (HAeBe), young stars surrounded by dust shells, are believed to be precursors of beta Pic-like stars, and the dust around them is thought to be a possible source material for the formation of planets. A group of HAeBe stars (UX Ori-like) shows large brightness variations. The dust surrounding these stars is responsible for their excess emission in the infrared, anomalous extinction at the ultraviolet and visible wavelengths, "blueing" effect in colour-magnitude diagrams, and intrinsic polarization increasing in deep minima. An investigation of all the effects mentioned can provide a good insight into the properties of the circumstellar dust. As distinct from previous studies of the dust shells, we take an advantage of a simultaneous modelling of different kinds of the observational data that give us additional constraints on the model parameters. We use spherical and spheroidal models of the shells and different dust grain models. We also consider some effects related with the possible porosity of the grains. The data available for several UX Ori-like stars are interpreted, and the obtained parameters of the shells are compared with the results of earlier studies. Title: Dust around Herbig Ae stars: additional constraints from their photometric and polarimetric variability. Authors: Krivova, N. A.; Il'in, V. B.; Fischer, O. Bibcode: 1996NASCP3343...37K Altcode: 1996NASCP3339...37K; 1996ASPC..122...37K Using a Monte-Carlo code for polarized radiation transfer the authors compared the results obtained for different models with the observational data available. They found that the photometric and polarimeteric behaviour of the stars provided essential additional constraints on the circumstellar dust models. Title: Expected Features of Martian Dust Belts Authors: Krivov, A. V.; Krivova, N. A. Bibcode: 1996pacm.conf..235K Altcode: No abstract at ADS Title: Modelling of the IR Intensity Maps for HAEBE Stars with Algol-Like Minima Authors: Il'in, V.; Krivova, N.; Men'shchikov, A. Bibcode: 1996rdfs.conf..183I Altcode: No abstract at ADS Title: Multiwavelength Study of HAEBE Stars with Algol-Like Minima Authors: Krivova, N.; Il'in, V. Bibcode: 1996rdfs.conf..187K Altcode: No abstract at ADS