Author name code: shapiro ADS astronomy entries on 2022-09-14 author:"Shapiro, Alexander I." ------------------------------------------------------------------------ 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: Chromospheric extension of the MURaM code Authors: Przybylski, D.; Cameron, R.; Solanki, S. K.; Rempel, M.; Leenaarts, J.; Anusha, L. S.; Witzke, V.; Shapiro, A. I. Bibcode: 2022A&A...664A..91P Altcode: 2022arXiv220403126P Context. Detailed numerical models of the chromosphere and corona are required to understand the heating of the solar atmosphere. An accurate treatment of the solar chromosphere is complicated by the effects arising from non-local thermodynamic equilibrium (NLTE) radiative transfer. A small number of strong, highly scattering lines dominate the cooling and heating in the chromosphere. Additionally, the recombination times of ionised hydrogen are longer than the dynamical timescales, requiring a non-equilibrium (NE) treatment of hydrogen ionisation.
Aims: We describe a set of necessary additions to the MURaM code that allow it to handle some of the important NLTE effects. We investigate the impact on solar chromosphere models caused by NLTE and NE effects in radiation magnetohydrodynamic simulations of the solar atmosphere.
Methods: The MURaM code was extended to include the physical process required for an accurate simulation of the solar chromosphere, as implemented in the Bifrost code. This includes a time-dependent treatment of hydrogen ionisation, a scattering multi-group radiation transfer scheme, and approximations for NLTE radiative cooling.
Results: The inclusion of NE and NLTE physics has a large impact on the structure of the chromosphere; the NE treatment of hydrogen ionisation leads to a higher ionisation fraction and enhanced populations in the first excited state throughout cold inter-shock regions of the chromosphere. Additionally, this prevents hydrogen ionisation from buffering energy fluctuations, leading to hotter shocks and cooler inter-shock regions. The hydrogen populations in the ground and first excited state are enhanced by 102-103 in the upper chromosphere and by up to 109 near the transition region.
Conclusions: Including the necessary NLTE physics leads to significant differences in chromospheric structure and dynamics. The thermodynamics and hydrogen populations calculated using the extended version of the MURaM code are consistent with previous non-equilibrium simulations. The electron number and temperature calculated using the non-equilibrium treatment of the chromosphere are required to accurately synthesise chromospheric spectral lines.

Movies associated to Fig. 2 are only available at https://www.aanda.org Title: Small-scale dynamo in cool stars. I. Changes in stratification and near-surface convection for main-sequence spectral types Authors: Bhatia, Tanayveer S.; Cameron, Robert H.; Solanki, Sami K.; Peter, Hardi; Przybylski, Damien; Witzke, Veronika; Shapiro, Alexander Bibcode: 2022A&A...663A.166B Altcode: 2022arXiv220600064B Context. Some of the small-scale solar magnetic flux can be attributed to a small-scale dynamo (SSD) operating in the near-surface convection. The SSD fields have consequences for solar granular convection, basal flux, and chromospheric heating. A similar SSD mechanism is expected to be active in the near-surface convection of other cool main-sequence stars, but this has not been investigated thus far.
Aims: We aim to investigate changes in stratification and convection due to inclusion of SSD fields for F3V, G2V, K0V, and M0V spectral types in the near-surface convection.
Methods: We studied 3D magnetohydrodynamic (MHD) models of the four stellar boxes, covering the subsurface convection zone up to the lower photosphere in a small Cartesian box, based on the MURaM radiative-MHD simulation code. We compared the SSD runs against reference hydrodynamic runs.
Results: The SSD is found to efficiently produce magnetic field with energies ranging between 5% to 80% of the plasma kinetic energy at different depths. This ratio tends to be larger for larger Teff. The relative change in density and gas pressure stratification for the deeper convective layers due to SSD magnetic fields is negligible, except for the F-star. For the F-star, there is a substantial reduction in convective velocities due to Lorentz force feedback from magnetic fields, which, in turn, reduces the turbulent pressure.
Conclusions: The SSD in near-surface convection for cool main-sequence stars introduces small but significant changes in thermodynamic stratification (especially for the F-star) due to a reduction in the convective velocities. Title: Stellar limb darkening. A new MPS-ATLAS library for Kepler, TESS, CHEOPS, and PLATO passbands Authors: Kostogryz, N. M.; Witzke, V.; Shapiro, A. I.; Solanki, S. K.; Maxted, P. F. L.; Kurucz, R. L.; Gizon, L. Bibcode: 2022arXiv220606641K Altcode: The detection of the first exoplanet paved the way into the era of transit photometry space missions with a revolutionary photometric precision that aim at discovering new exoplanetary systems around different types of stars. With this high precision, it is possible to derive very accurately the radii of exoplanets which is crucial for constraining their type and composition. However, it requires an accurate description of host stars, especially their center-to-limb variation of intensities (so called limb darkening) as it affects the planet-to-star radius ratio determination. We aim at improving the accuracy of limb darkening calculations for stars with a wide range of fundamental parameters. We used the recently developed 1D MPS-ATLAS code to compute model atmosphere structures and to synthesize stellar limb darkening on a very fine grid of stellar parameters. For the computations we utilized the most accurate information on chemical element abundances and mixing length parameters including convective overshoot. The stellar limb darkening was fitted using the two most accurate limb darkening laws: the power-2 and 4-parameters non-linear laws. We present a new extensive library of stellar model atmospheric structures, the synthesized stellar limb darkening curves, and the coefficients of parameterized limb-darkening laws on a very fine grid of stellar parameters in the Kepler, TESS, CHEOPS, and PLATO passbands. The fine grid allows overcoming the sizable errors introduced by the need to interpolate. Our computations of solar limb darkening are in a good agreement with available solar measurements at different view angles and wavelengths. Our computations of stellar limb darkening agree well with available measurements of Kepler stars. A new grid of stellar model structures, limb darkening and their fitted coefficients in different broad filters is provided in CDS. Title: Making the Most of Transmission Spectra in Light of Stellar Activity: Needs Identified by ExoPAG's Study Analysis Group 21 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.; SAG 21 Committee Bibcode: 2022BAAS...54e4404R Altcode: Transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star, 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. Study Analysis Group 21 (SAG21) of NASA's Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar photospheric heterogeneity on space-based transmission spectroscopy. This SAG 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 NASA facilities like HST and JWST. Here we report on the main conclusions of this analysis, highlighting needs to be addressed and mitigation efforts underway. The analysis produced 14 findings, which fall into three Science Themes that encompass (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"). Addressing the needs identified through this large community effort will ensure that we can optimally leverage space-based transmission spectra in light of stellar activity. Title: A New Method for Calculating Solar Irradiance at Mars Authors: De Oliveira, I.; Shapiro, A. I.; Sowmya, K.; Medvedev, A.; Nèmec, N. -E.; Gizon, L. Bibcode: 2022mamo.conf.1535D Altcode: No abstract at ADS 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: 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: 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: MPS-ATLAS: A fast all-in-one code for synthesising stellar spectra Authors: Witzke, V.; Shapiro, A. I.; Cernetic, M.; Tagirov, R. V.; Kostogryz, N. M.; Anusha, L. S.; Unruh, Y. C.; Solanki, S. K.; Kurucz, R. L. Bibcode: 2021A&A...653A..65W Altcode: 2021arXiv210513611W Context. Stellar spectral synthesis is essential for various applications, ranging from determining stellar parameters to comprehensive stellar variability calculations. New observational resources as well as advanced stellar atmosphere modelling, taking three dimensional effects from radiative magnetohydrodynamics calculations into account, require a more efficient radiative transfer.
Aims: For accurate, fast and flexible calculations of opacity distribution functions (ODFs), stellar atmospheres, and stellar spectra, we developed an efficient code building on the well-established ATLAS9 code. The new code also paves the way for easy and fast access to different elemental compositions in stellar calculations.
Methods: For the generation of ODF tables, we further developed the well-established DFSYNTHE code by implementing additional functionality and a speed-up by employing a parallel computation scheme. In addition, the line lists used can be changed from Kurucz's recent lists. In particular, we implemented the VALD3 line list.
Results: A new code, the Merged Parallelised Simplified ATLAS, is presented. It combines the efficient generation of ODF, atmosphere modelling, and spectral synthesis in local thermodynamic equilibrium, therefore being an all-in-one code. This all-in-one code provides more numerical functionality and is substantially faster compared to other available codes. The fully portable MPS-ATLAS code is validated against previous ATLAS9 calculations, the PHOENIX code calculations, and high-quality observations. Title: Radiative Transfer with Opacity Distribution Functions: Application to Narrowband Filters Authors: Anusha, L. S.; Shapiro, A. I.; Witzke, V.; Cernetic, M.; Solanki, S. K.; Gizon, L. Bibcode: 2021ApJS..255....3A Altcode: 2021arXiv210413661A Modeling of stellar radiative intensities in various spectral passbands plays an important role in stellar physics. At the same time, direct calculation of the high-resolution spectrum and then integration of it over the given spectral passband is computationally demanding due to the vast number of atomic and molecular lines. This is particularly so when employing three-dimensional (3D) models of stellar atmospheres. To accelerate the calculations, one can employ approximate methods, e.g., the use of opacity distribution functions (ODFs). Generally, ODFs provide a good approximation of traditional spectral synthesis, i.e., computation of intensities through filters with strictly rectangular transmission functions. However, their performance strongly deteriorates when the filter transmission noticeably changes within its passband, which is the case for almost all filters routinely used in stellar physics. In this context, the aims of this paper are (a) to generalize the ODFs method for calculating intensities through filters with arbitrary transmission functions, and (b) to study the performance of the standard and generalized ODFs methods for calculating intensities emergent from 3D models of stellar atmospheres. For this purpose we use the newly developed MPS-ATLAS radiative transfer code to compute intensities emergent from 3D cubes simulated with the radiative magnetohydrodynamics code MURaM. The calculations are performed in the 1.5D regime, i.e., along many parallel rays passing through the simulated cube. We demonstrate that the generalized ODFs method allows accurate and fast syntheses of spectral intensities and their center-to-limb variations. 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: Small-scale Dynamo in Cool Main-Sequence Stars: Effect on Stratification, Convection and Bolometric Intensity Authors: Bhatia, T.; Cameron, R.; Solanki, S.; Peter, H.; Przybylski, D.; Witzke, V.; Shapiro, A. Bibcode: 2021AAS...23830404B Altcode: In cool main-sequence stars, the near-surface convection has an impact on the center-to-limb variation of photospheric emission, with implications for stellar lightcurves during planetary transits. In the Sun, there is strong evidence for a small-scale dynamo (SSD) maintaining the small-scale magnetic flux. This field could affect the near-surface convection in other cool main-sequence stars.

An SSD could conceivably generate equipartition magnetic fields, which could lead to non-negligible changes not only in convection and intensity characteristics, but also in stratification. We aim to investigate these changes for F, G, K and M stars. 3D MHD models of the four stellar types covering the subsurface region to lower atmosphere in a small cartesian box are studied using the MURaM rMHD simulation code. The MHD runs are compared against a reference hydrodynamic (HD) run.

The deviations in stratification for the deeper convective layers is negligible, except for the F-star, where reduction in turbulent pressure due to magnetic fields is substantial. Convective velocities are reduced by a similar percentage for all the cases due to inhibitory effect of strong magnetic fields near the bottom boundary. All four cases show small-scale brightenings in intergranular lanes, corresponding to magnetic field concentrations, but overall effects on the r.m.s contrast and spatial powerspectra are varied. 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: Irradiance Variations of the Sun and Sun-Like Stars - Overview of Topical Collection Authors: Kopp, Greg; Shapiro, Alexander Bibcode: 2021SoPh..296...60K Altcode: 2021arXiv210206913K This topical collection summarizes recent advances in observing and modeling irradiance variations of the Sun and Sun-like stars, emphasizing the links between surface magnetic fields and the resulting solar and stellar variability. In particular, the articles composing this collection summarize recent progress in i) solar-irradiance measurements; ii) modeling of solar- and stellar-irradiance variability; and iii) understanding of the effects of such variability on Earth's climate and exoplanet environments. This topical-collection overview article gives background and more details on these aspects of variability. 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: Small-scale dynamo in an F-star: effects on near-surface stratification, convection and intensity Authors: Bhatia, Tanayveer; Cameron, Robert; Solanki, Sami; Peter, Hardi; Przybylski, Damien; Witzke, Veronika; Shapiro, Alexander Bibcode: 2021csss.confE..75B Altcode: The emission from the photosphere of stars shows a systematic center-to-limb variation. In cool main-sequence stars, the near-surface convection has an impact on this variation, with implications for lightcurves of stars during planetary transits. In the Sun, there is strong evidence for a small-scale dynamo (SSD) maintaining the small-scale magnetic flux. We aim to investigate what additional effects such a field would play for other cool main-sequence stars. In our work we first concentrate on F-stars. This is because they have sonic velocities near the surface, implying a rough equipartition between internal and kinetic energies. In addition, an SSD might create a significant magnetic energy density to impact the results. We investigate the interplay between internal, kinetic and magnetic energies in 3D cartesian box MHD models of a F3V-star in the near-surface convection, using the MURaM radiative-MHD simulation code. Along with a reference hydrodynamic run, two MHD models with self-consistently generated magnetic fields with two different lower boundary conditions are considered. We find that the SSD process creates a magnetic field with energy within an order of magnitude of the internal and the kinetic energy. Compared to the hydrodynamic run, we find slight (~1-3%) but significant deviations in density, gas pressure and temperature stratification. At the surface, this corresponds to a temperature difference of ~130 K. As expected, there is a significant reduction in kinetic energy flux once the SSD is operational. The changes in intensity are more subtle, both in total intensity and granulation pattern. From this we conclude that the presence of an SSD will have a significant impact on the atmospheric structure and intensity characteristics seen at the surface. This makes it clear that it would be important to consider the spatially and temporally averaged effects of the SSD also for global stellar models. 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: Where Have All the Solar-like Stars Gone? Rotation Period Detectability at Various Inclinations and Metallicities Authors: Reinhold, Timo; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina-E.; Işık, Emre; Solanki, Sami K. Bibcode: 2021ApJ...908L..21R Altcode: 2021arXiv210111426R The plethora of photometric data collected by the Kepler space telescope has promoted the detection of tens of thousands of stellar rotation periods. However, these periods are not found to an equal extent among different spectral types. Interestingly, early G-type stars with near-solar rotation periods are strongly underrepresented among those stars with known rotation periods. In this study we investigate whether the small number of such stars can be explained by difficulties in the period determination from photometric time series. For that purpose, we generate model light curves of early G-type stars with solar rotation periods for different inclination angles, metallicities, and (magnitude-dependent) noise levels. We find that the detectability is determined by the predominant type of activity (i.e., spot or faculae domination) on the surface, which defines the degree of irregularity of the light curve, and further depends on the level of photometric noise. These two effects significantly complicate the period detection and explain the lack of solar-like stars with known near-solar rotation periods. We conclude that the rotation periods of the majority of solar-like stars with near-solar rotation periods remain undetected to date. Finally, we promote the use of new techniques to recover more periods of near-solar rotators. 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: Power spectrum of turbulent convection in the solar photosphere Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.; Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2020A&A...644A..44Y Altcode: 2020arXiv201009037Y The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two balloon-borne SUNRISE missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the photospheric overshooting flow with the help of its governing equations and simulations. We find that quiet-Sun observations and smeared simulations are consistent with each other and exhibit a power-law behavior in the subgranular range of their Doppler velocity power spectra with a power-law index of ≈ - 2. The unsmeared simulations exhibit a power law that extends over the full range between the integral and Taylor scales with a power-law index of ≈ - 2.25. The smearing, reminiscent of observational conditions, considerably reduces the extent of the power-law-like portion of the power spectra. This suggests that the limited spatial resolution in some observations might eventually result in larger uncertainties in the estimation of the power-law indices. The simulated vertical velocity power spectra as a function of height show a rapid change in the power-law index (at the subgranular range) from roughly the optical depth unity layer, that is, the solar surface, to 300 km above it. We propose that the cause of the steepening of the power-law index is the transition from a super- to a subadiabatic region, in which the dominant source of motions is overshooting convection. A scale-dependent transport of the vertical momentum occurs. At smaller scales, the vertical momentum is more efficiently transported sideways than at larger scales. This results in less vertical velocity power transported upward at small scales than at larger scales and produces a progressively steeper vertical velocity power law below 180 km. Above this height, the gravity work progressively gains importance at all relevant scales, making the atmosphere progressively more hydrostatic and resulting in a gradually less steep power law. Radiative heating and cooling of the plasma is shown to play a dominant role in the plasma energetics in this region, which is important in terms of nonadiabatic damping of the convective motions. Title: VizieR Online Data Catalog: Faculae-Spot dominance & rotation periods (Amazo-Gomez+, 2020) Authors: Amazo-Gomez, E. M.; Shapiro, A. I.; Solanki, S. K.; Kopp, G.; Oshagh, M.; Reinhold, T.; Reiners, A. Bibcode: 2020yCat..36420225A Altcode: This table contains an example of the GPS outputs, the compared rotation period values from GLS and ACF, and stellar parameters for Kepler stars.

In column 4 and 5 values of alpha-factor and its 2-sigma uncertainty are reported respectively. Prot GPS values in column 6, as result of applying Eq. 1 using the factor alpha=0.19. 2) Column 7 shows the Prot reported by Reinhold & Gizon (2015, Cat. J/A+A/583/A65). 3) Prot and variability values reported by McQuillan et al. (2014, Cat. J/ApJS/211/24) in column 8. 4) Columns 10, 11 and 12 show the logg, [Fe/H], and Teff respectively, taken from Huber et al. (2014, Cat J/ApJS/211/2).

(1 data file). 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: Inflection point in the power spectrum of stellar brightness variations. III. Facular versus spot dominance on stars with known rotation periods Authors: Amazo-Gómez, E. M.; Shapiro, A. I.; Solanki, S. K.; Kopp, G.; Oshagh, M.; Reinhold, T.; Reiners, A. Bibcode: 2020A&A...642A.225A Altcode: 2020arXiv200811492A Context. Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. Successful stellar photometric surveys stemming from the Kepler and TESS observations have led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information on the rotation periods of older and less active stars like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to, or even shorter than, stellar rotation periods, in combination with the random emergence of active regions make period determination for such stars very difficult.
Aims: We tested the performance of a new method for the determination of stellar rotation periods against stars with previously determined rotation periods. The method is based on calculating the gradient of the power spectrum (GPS) and identifying the position of the inflection point (i.e. point with the highest gradient). The GPS method is specifically aimed at determining rotation periods of low-activity stars like the Sun.
Methods: We applied the GPS method to 1047 Sun-like stars observed by the Kepler telescope. We considered two stellar samples individually: one with near-solar rotation periods (24-27.4 d) and a broad range of effective temperatures (5000-6000 K) and the other with near-solar effective temperatures (5700-5900 K) and a broad range of rotation periods (15-40 d).
Results: We show that the GPS method returns precise values for stellar rotation periods. Furthermore, it allows us to constrain the ratio between facular and spot areas of active regions at the moment of their emergence. We also show that the relative facular area decreases with the stellar rotation rate.
Conclusions: Our results suggest that the GPS method can be successfully applied to retrieve the periods of stars with both regular and non-regular light curves.

Full Table 2 is 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/642/A225 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: 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: Solar-type Stars Observed by LAMOST and Kepler Authors: Zhang, Jinghua; Shapiro, Alexander I.; Bi, Shaolan; Xiang, Maosheng; Reinhold, Timo; Sowmya, Krishnamurthy; Li, Yaguang; Li, Tanda; Yu, Jie; Du, Minghao; Zhang, Xianfei Bibcode: 2020ApJ...894L..11Z Altcode: 2020arXiv200502717Z Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar-stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (I.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could not be measured due to the low amplitude of the photometric variability and highly irregular temporal profile of light curves, 254 stars were further identified as having near-solar rotation periods. We show that stars with near-solar rotation periods have chromospheric activities that are systematically higher than stars with undetected rotation periods. Furthermore, while the solar level of photospheric and chromospheric activity appears to be typical for stars with undetected rotation periods, the Sun appears to be less active than most stars with near-solar rotation periods (both in terms of photospheric and chromospheric activity). 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: 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: FM9 - Solar Irradiance: Physics-Based Advances Authors: Kopp, Greg; Shapiro, Alexander Bibcode: 2020IAUGA..30..331K Altcode: No abstract at ADS Title: Solar disk radius measured by Solar occultation by the Moon using bolometric and photometric instruments on board the PICARD satellite Authors: Thuillier, G.; Zhu, P.; Shapiro, A. I.; Sofia, S.; Tagirov, R.; van Ruymbeke, M.; Perrin, J. -M.; Sukhodolov, T.; Schmutz, W. Bibcode: 2020IAUGA..30..361T Altcode: No abstract at ADS 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: 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: 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: 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: Chapter 3 - The Sun's Atmospher Authors: Shapiro, Alexander I.; Peter, Hardi; Solanki, Sami K. Bibcode: 2019sgsp.book...59S Altcode: The solar atmosphere covers a broad range of temperatures and densities from the solar surface, via the chromosphere and transition region, and to the corona. Although one-dimensional (1D) models of the atmospheric structure have reached a high level of maturity, high-spatial resolution observations have cast some doubt on their validity. Thus, such observations have revealed a richness of highly variable spatial structure, often reaching down to the current resolution limit of 0.1 arcsec, or roughly 70 km on the Sun, in the photosphere and chromosphere. These observational advances have led to a new generation of models that describe the solar atmosphere self-consistently using 3D magnetohydrodynamic approximation simulations, including 3D radiative energy transport for those that cover the lower atmosphere, while simplistically taking into account the complex magnetic structure and energy dissipation processes in the upper atmosphere. These models have achieved considerable success in explaining the best observations, although there are still a number of open questions. Nonetheless, thanks to modern advances, the solar atmosphere now provides an excellent setting to test models of stellar atmospheres critically. Title: Transition from spot to faculae domination. An alternate explanation for the dearth of intermediate Kepler rotation periods Authors: Reinhold, Timo; Bell, Keaton J.; Kuszlewicz, James; Hekker, Saskia; Shapiro, Alexander I. Bibcode: 2019A&A...621A..21R Altcode: 2018arXiv181011250R; 2018A&A...621A..21R Context. The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context.
Aims: We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series.
Methods: Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period.
Results: We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast-rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less-active stars are dominated by bright faculae. We find the transition from spot to faculae domination to be at the Vaughan-Preston gap, and around a Rossby number equal to one.
Conclusions: We conclude that faculae are the dominant ingredient of stellar activity cycles at ages ≳2.55 Gyr. The data further suggest that the Vaughan-Preston gap cannot explain the previously detected dearth of Kepler rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ∼800 Myr.

Photometric and Mount Wilson 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/621/A21 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: VizieR Online Data Catalog: Activity of Kepler stars (Reinhold+, 2019) Authors: Reinhold, T.; Bell, K. J.; Kuszlewicz, J.; Hekker, S.; Shapiro, A. I. Bibcode: 2018yCat..36210021R Altcode: In the current study, data from three different sources have been used. Long-term V band and Stroemgren b and y photometric time series have successfully been requested from Messina & Guinan (2002A&A...393..225M, Cat. J/A+A/393/225) and Lockwood et al. (2007ApJS..171..260L), respectively. For all stars, chromospheric emission data from the Mount Wilson survey are publicly available (ftp://solis.nso.edu/MountWilson_HK).

(2 data files). Title: Revised historical solar irradiance forcing Authors: Egorova, T.; Schmutz, W.; Rozanov, E.; Shapiro, A. I.; Usoskin, I.; Beer, J.; Tagirov, R. V.; Peter, T. Bibcode: 2018A&A...615A..85E Altcode: 2018arXiv180400287E Context. There is no consensus on the amplitude of historical solar forcing. The estimated magnitude of the total solar irradiance (TSI) difference between the Maunder minimum and the present time ranges from 0.1 to 6 W m-2 making the simulation of the past and future climate uncertain. One reason for this disagreement is the applied evolution of the quiet Sun brightness in solar irradiance reconstruction models. This work addresses the role of the quiet Sun model choice and updated solar magnetic activity proxies on the solar forcing reconstruction.
Aims: We aim to establish a plausible range for the solar irradiance variability on decadal to millennial timescales.
Methods: The spectral solar irradiance (SSI) is calculated as a weighted sum of the contributions from sunspot umbra, sunspot penumbra, faculae, and quiet Sun, which are pre-calculated with the NLTE Spectral SYnthesis code (NESSY). We introduce activity belts of the contributions from sunspots and faculae and a new structure model for the quietest state of the Sun. We assume that the brightness of the quiet Sun varies in time proportionally to the secular (22-yr smoothed) variation of the solar modulation potential.
Results: A new reconstruction of the TSI and SSI covering the period 6000 BCE - 2015 CE is presented. The model simulates solar irradiance variability during the satellite era well. The TSI change between the Maunder and recent minima ranges between 3.7 and 4.5 W m-2 depending on the applied solar modulation potential. The implementation of a new quietest Sun model reduces, by approximately a factor of two, the relative solar forcing compared to the largest previous estimation, while the application of an updated solar modulation potential increases the forcing difference between the Maunder minimum and the present by 25-40%. Title: Response of Solar Irradiance to Sunspot-area Variations Authors: Dudok de Wit, T.; Kopp, G.; Shapiro, A.; Witzke, V.; Kretzschmar, M. Bibcode: 2018ApJ...853..197D Altcode: 2018arXiv180504350D One of the important open questions in solar irradiance studies is whether long-term variability (i.e., on timescales of years and beyond) can be reconstructed by means of models that describe short-term variability (i.e., days) using solar proxies as inputs. Preminger & Walton showed that the relationship between spectral solar irradiance and proxies of magnetic-flux emergence, such as the daily sunspot area, can be described in the framework of linear system theory by means of the impulse response. We significantly refine that empirical model by removing spurious solar-rotational effects and by including an additional term that captures long-term variations. Our results show that long-term variability cannot be reconstructed from the short-term response of the spectral irradiance, which questions the extension of solar proxy models to these timescales. In addition, we find that the solar response is nonlinear in a way that cannot be corrected simply by applying a rescaling to a sunspot area. 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: 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: NESSY: NLTE spectral synthesis code for solar and stellar atmospheres Authors: Tagirov, R. V.; Shapiro, A. I.; Schmutz, W. Bibcode: 2017A&A...603A..27T Altcode: Context. Physics-based models of solar and stellar magnetically-driven variability are based on the calculation of synthetic spectra for various surface magnetic features as well as quiet regions, which are a function of their position on the solar or stellar disc. Such calculations are performed with radiative transfer codes tailored for modeling broad spectral intervals.
Aims: We aim to present the NLTE Spectral SYnthesis code (NESSY), which can be used for modeling of the entire (UV-visible-IR and radio) spectra of solar and stellar magnetic features and quiet regions.
Methods: NESSY is a further development of the COde for Solar Irradiance (COSI), in which we have implemented an accelerated Λ-iteration (ALI) scheme for co-moving frame (CMF) line radiation transfer based on a new estimate of the local approximate Λ-operator.
Results: We show that the new version of the code performs substantially faster than the previous one and yields a reliable calculation of the entire solar spectrum. This calculation is in a good agreement with the available observations. Title: Solar disc radius determined from observations made during eclipses with bolometric and photometric instruments on board the PICARD satellite Authors: Thuillier, G.; Zhu, P.; Shapiro, A. I.; Sofia, S.; Tagirov, R.; van Ruymbeke, M.; Perrin, J. -M.; Sukhodolov, T.; Schmutz, W. Bibcode: 2017A&A...603A..28T Altcode: Context. Despite the importance of having an accurate measurement of the solar disc radius, there are large uncertainties of its value due to the use of different measurement techniques and instrument calibration. An item of particular importance is to establish whether the value of the solar disc radius correlates with the solar activity level.
Aims: The main goal of this work is to measure the solar disc radius in the near-UV, visible, and near-IR regions of the solar spectrum.
Methods: Three instruments on board the PICARD spacecraft, namely the Bolometric Oscillations Sensor (BOS), the PREcision MOnitoring Sensor (PREMOS), and a solar sensor (SES), are used to derive the solar disc radius using the light curves produced when the Sun is occulted by the Moon. Nine eclipses, from 2010 to 2013, resulted in 17 occultations as viewed from the moving satellite. The calculation of the solar disc radius uses a simulation of the light curve taking into account the center-to-limb variation provided by the Non-local thermodynamic Equilibrium Spectral SYnthesis (NESSY) code.
Results: We derive individual values for the solar disc radius for each viewed eclipse. Tests for a systematic variation of the radius with the progression of the solar cycle yield no significant results during the three years of measurements within the uncertainty of our measurements. Therefore, we derive a more precise radius value by averaging these values. At one astronomical unit, we obtain 959.79 arcseconds (arcsec) from the bolometric experiment; from PREMOS measurements, we obtain 959.78 arcsec at 782 nm and 959.76 arcsec at 535 nm. We found 960.07 arcsec at 210 nm, which is a higher value than the other determinations given the photons at this wavelength originate from the upper photosphere and lower chromosphere. We also give a detailed comparison of our results with those previously published using measurements from space-based and ground-based instruments using the Moon angular radius reference, and different methods.
Conclusions: Our results, which use the Moon as an absolute calibration, clearly show the dependence of the solar disc radius with wavelength in UV, visible and near-IR. Beyond the metrological results, solar disc radius measurements will allow the accuracy of models of the solar atmosphere to be tested. Proposed systematic variations of the solar disc radius during the time of observation would be smaller than the uncertainty of our measurement, which amounts to less than 26 milliarcseconds. 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: 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: Solar spectral irradiance model validation using Solar Spectral Irradiance and Solar Radius measurements Authors: Thuillier, Gérard; Zhu, Ping; Shapiro, Alexander; Sofia, Sabatino; Tagirov, Rinat; Van Ruymbeke, Michel; Schmutz, Werner Bibcode: 2016EGUGA..18.7407T Altcode: The importance of the reliable solar spectral irradiance (SSI) data for solar and climate physics is now well acknowledged. In particular, the irradiance time series are necessary for most of the current studies concerning climate evolution. However, space instruments are vulnerable to the degradation due to the environment while ground based measurements are limited in wavelength range and need atmospheric effects corrections. This is why SSI modeling is necessary to understand the mechanism of the solar irradiance variability and to provide long and uninterrupted irradiance records to climate and Earth atmosphere scientists. Here we present COSI (COde for Solar Irradiance) model of the SSI variability. The COSI model is based on the Non local thermodynamic Equilibrium Spectral SYnthesis Code (NESSY). We validate NESSY by two independent datasets: - The SSI at solar minimum occurring in 2008, - The radius variation with wavelength and absolute values determined from PREMOS and BOS instruments onboard the PICARD spacecraft. Comparisons between modeling and measured SSI will be shown. However, since SSI measurements have an accuracy estimated between 2 to 3%, the comparison with the solar radius data provides a very important additional constrains on model. For that, 17 partial solar occultations by the Moon are used providing solar radii clearly showing the dependence of the solar radius with wavelength. These results are compared with the NESSY predictions. The agreement between NESSY and observations is within the model and measurements accuracy. 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: Solar irradiance observations with PREMOS filter radiometers on the PICARD mission: In-flight performance and data release Authors: Cessateur, G.; Schmutz, W.; Wehrli, C.; Gröbner, J.; Haberreiter, M.; Kretzschmar, M.; Rozanov, E.; Schöll, M.; Shapiro, A.; Thuillier, G.; Egorova, T.; Finsterle, W.; Fox, N.; Hochedez, J. -F.; Koller, S.; Meftah, M.; Meindl, P.; Nyeki, S.; Pfiffner, D.; Roth, H.; Rouzé, M.; Spescha, M.; Tagirov, R.; Werner, L.; Wyss, J. -U. Bibcode: 2016A&A...588A.126C Altcode: Context. The PREcision Monitoring Sensor (PREMOS) is a solar radiometer on board the French PICARD mission that was launched in June 2010 and decommissioned in April 2014.
Aims: The PREMOS radiometer obtains solar irradiance measurements in specific spectral windows in the UV, visible, and near-infrared. In this paper, the PREMOS data and calibration methods are presented.
Methods: Using back-up channels, the degradation can theoretically be assessed to correct operational channels. However, a strong degradation within all PREMOS channels requires the application of additional methods, namely using back-up channels and assessing the degradation via a proxy-based model.
Results: The corrected Level 3 PREMOS data are then used in different contexts in order to be validated. First, the signature of the p-mode are retrieved from the PREMOS data. The Venus transit allows us to empirically determine the intrinsic noise level within the PREMOS high cadence data for the visible and near-infrared channels. We then compare the PREMOS data directly to other data sets, namely from the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) and the Solar Irradiance Monitor (SIM) instruments on board the SOlar Radiation and Climate Experiment (SORCE) spacecraft. Regarding the UV channels, we found an excellent correlation over the lifetime of the PREMOS mission. The ratio between SORCE and PREMOS observations is always less than 1%. Regarding the SSI measurements in the visible and near-infrared, a comparison of short-term variations (I.e. 27-day modulation) shows a rather good correlation by taking into consideration the intrinsic noise within both SIM and PREMOS observations. Title: Solar Spectral Irradiance Observations from the PICARD/PREMOS Radiometer Authors: Cessateur, G.; Schöll, M.; Schmutz, W. K.; Wehrli, C.; Groebner, J.; Haberreiter, M.; Kretzschmar, M.; Shapiro, A.; Thuillier, G. O.; Finsterle, W.; Fox, N.; Hochedez, J. F.; Koller, S.; Meftah, M.; Nyeki, S.; Pfiffner, D.; Roth, H.; Rouze, M.; Spescha, M.; Tagirov, R.; Werner, L.; Wyss, J. Bibcode: 2015AGUFMSH32A..06C Altcode: Space weather and space climate studies require accurate Solar Spectral Irradiance (SSI) observations. The PREcision Monitoring Sensor (PREMOS) instrument aboard the PICARD satellite acquired solar irradiance measurements in specific spectral windows in the UV, visible and near infrared from October 2010 to March 2014. This contribution aims at presenting the Level 3 data, corrected for non solar features as well as for degradation. These level 3 data has been tested over different scientific cases, such as observations during the Venus transit and the presence of the p-mode signature within high-cadence data. The PREMOS Level 3 data have also been compared to others data sets, namely the SOLSTICE and SIM instruments aboard SORCE, for nearly 3 and half years. An excellent correlation has been found for the UV spectral ranges. We have also found a rather good correlation for visible and near-infrared observations for short-term variations, for which an error of about 200 ppm has been estimated within PREMOS visible and near-infrared observations. The PREMOS data could also be used to address several scientific topics, i.e. for validating semi-empirical models of the solar irradiance. We will emphasize about our new irradiance model, COSIR for Code of Solar Irradiance Reconstruction, which is successful at reproducing the solar modulation as seen in the PREMOS, SoHO/Virgo and SORCE data. Title: Erratum: Erratum to: The Infrared Solar Spectrum Measured by the SOLSPEC Spectrometer Onboard the International Space Station Authors: Thuillier, G.; Harder, J. W.; Shapiro, A.; Woods, T. N.; Perrin, J. -M.; Snow, M.; Sukhodolov, T.; Schmutz, W. Bibcode: 2015SoPh..290.3089T Altcode: 2015SoPh..tmp..150T No abstract at ADS 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: The Infrared Solar Spectrum Measured by the SOLSPEC Spectrometer Onboard the International Space Station Authors: Thuillier, G.; Harder, J. W.; Shapiro, A.; Woods, T. N.; Perrin, J. -M.; Snow, M.; Sukhodolov, T.; Schmutz, W. Bibcode: 2015SoPh..290.1581T Altcode: 2015SoPh..tmp...75T A solar spectrum extending from the extreme ultraviolet to the near-infrared is an important input for solar physics, climate research, and atmospheric physics. Ultraviolet measurements have been conducted since the beginning of the space age, but measurements throughout the contiguous visible and infrared (IR) regions are much more sparse. Ageing is a key problem throughout the entire spectral domain, but most of the effort extended to understand degradation was concentrated on the ultraviolet spectral region, and these mechanisms may not be appropriate in the IR. This problem is further complicated by the scarcity of long-term data sets. Onboard the International Space Station, the SOLSPEC spectrometer measured an IR solar spectral irradiance lower than the one given by ATLAS 3, e.g. by about 7 % at 1 700 nm. We here evaluate the consequences of the lower solar spectral irradiance measurements and present a re-analysis of the on-orbit calibration lamp and solar data trend, which lead to a revised spectrum. Title: Upgrading the Solar-Stellar Connection: News about activity in Cool Stars Authors: Gunther, H. M.; Poppenhaeger, K.; Testa, P.; Borgniet, S.; Brun, A. S.; Cegla, H. M.; Garraffo, C.; Kowalski, A.; Shapiro, A.; Shkolnik, E.; Spada, F.; Vidotto, A. A. Bibcode: 2015csss...18...25G Altcode: 2014arXiv1408.3068G In this splinter session, ten speakers presented results on solar and stellar activity and how the two fields are connected. This was followed by a lively discussion and supplemented by short, one-minute highlight talks. The talks presented new theoretical and observational results on mass accretion on the Sun, the activity rate of flare stars, the evolution of the stellar magnetic field on time scales of a single cycle and over the lifetime of a star, and two different approaches to model the radial-velocity jitter in cool stars that is due to the granulation on the surface. Talks and discussion showed how much the interpretation of stellar activity data relies on the sun and how the large number of objects available in stellar studies can extend the parameter range of activity models. 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: 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: The Solar Irradiance Spectrum at Solar Activity Minimum Between Solar Cycles 23 and 24 Authors: Thuillier, G.; Bolsée, D.; Schmidtke, G.; Foujols, T.; Nikutowski, B.; Shapiro, A. I.; Brunner, R.; Weber, M.; Erhardt, C.; Hersé, M.; Gillotay, D.; Peetermans, W.; Decuyper, W.; Pereira, N.; Haberreiter, M.; Mandel, H.; Schmutz, W. Bibcode: 2014SoPh..289.1931T Altcode: On 7 February 2008, the SOLAR payload was placed onboard the International Space Station. It is composed of three instruments, two spectrometers and a radiometer. The two spectrometers allow us to cover the 16 - 2900 nm spectral range. In this article, we first briefly present the instrumentation, its calibration and its performance in orbit. Second, the solar spectrum measured during the transition between Solar Cycles 23 to 24 at the time of the minimum is shown and compared with other data sets. Its accuracy is estimated as a function of wavelength and the solar atmosphere brightness-temperature is calculated and compared with those derived from two theoretical models. 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: The PREMOS/PICARD Radiometer: An overview after 3 years of observations Authors: Cessateur, Gaël; Schmutz, Werner; Shapiro, Alexander Bibcode: 2014cosp...40E.469C 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 for three years, which covers covers the solar spectrum from the Ultraviolet to near-infrared. We will also introduce the algorithm COCOSIS (for Combination of COSI Spectra) which allows us to retrieve the solar variability of the irradiance. Our investigation shows that COCOSIS is highly successful at reproducing solar spectral irradiance over rotational periods for most of channels from PREMOS. Title: Middle atmosphere heating rate and photolysis response to the uncertainties in spectral solar irradiance data Authors: Sukhodolov, Timofei; Schmutz, Werner; Shapiro, Alexander; Tourpali, Kleareti; Peter, Thomas; Rozanov, Eugene; Bais, Alkiviadis; Telford, Paul Bibcode: 2014cosp...40E3225S Altcode: Solar radiation is the main source of energy for the Earth’s atmosphere and in many respects defines its composition, photochemistry, temperature profile and dynamics. Therefore, the uncertainties in the magnitude and spectral composition of the spectral solar irradiance (SSI) evolution during the declining phase of 23rd solar cycle have substantial implications for the modeling of the middle atmosphere evolution, leading to a pronounced differences in the heating rates and also affecting photolysis rates. To estimate the role of SSI uncertainties we have compared the most important photolysis rates (O2, O3, and NO2) and heating rates calculated with the reference radiation code libRadtran using SSI for June 2004 and February 2009 obtained from two models (NRL, COSI) and one observation data set based on SORCE observations. We have also evaluated the ability of the several photolysis and heating rates calculation methods widely used in atmospheric models to reproduce the absolute values of the photolysis rates and their response to the implied SSI changes. Title: Assessment of the spectral solar cycle variations in the visual and near IR from VIRGI/SOHO data Authors: Schmutz, Werner; Haberreiter, Margit; Shapiro, Alexander; Cessateur, Gaël; Wehrli, Christoph; Adams, Wilnelia Bibcode: 2014cosp...40E2929S Altcode: According to Harder et al. (2009) SIM measurements onboard the SORCE satellite reveal a several times higher spectral solar variability in UV than previously estimated. In compensation, to maintain the known amplitude of the Total Solar Irradiance variations, the visual was observed to be in anti-phase. Haigh et al. (2010) has demonstrated that this different-then-thought wavelength dependence has profound implications for the response of the terrestrial climate. In Wehrli et al. (A&A 556, L3 2013) we have presented evidence that annual averages of the VIRGO filter radiometer data in the 500 nm channel are significantly positively correlated with TSI. In this presentation we present an update including the most recent SPM VIRGO radiometer data and give an estimate of the amplitudes of the spectral solar cycle variations in the VIRGO channels. The implication is that the highly significant positive correlation at 500 nm excludes the postulated anti-phase solar cycle variations at this wavelength. Harder et al. (2009) also published large UV variations and this proposal is assessed from the point of view that Total Solar Irradiance Variations are know. It is concluded that large UV amplitudes, as proposed, are not excluded even if the 500 nm variations are in-phase with the solar cycle, given the amplitudes measured in the VIRGO/SOHO channels. Title: Correlation of spectral solar irradiance with solar activity as measured by VIRGO Authors: Wehrli, C.; Schmutz, W.; Shapiro, A. I. Bibcode: 2013A&A...556L...3W Altcode: 2013arXiv1307.1285W Context. The variability of solar spectral irradiance (SSI) over the rotational period and its trend over the solar activity cycle are important for understanding the Sun-Earth connection as well as for observational constraints for solar models. Recently the Spectral Irradiance Monitor (SIM) experiment on the Solar Radiation and Climate Experiment (SORCE) has published an unexpected negative correlation with total solar irradiance (TSI) of the visible spectral range. It is compensated by a strong and positive variability of the near UV range.
Aims: We aim to verify whether the anti-correlated SIM-trend in the visible can be confirmed by independent observations of the Variability of solar IRadiance and Gravity Oscillations (VIRGO) experiment on the SOlar and Heliospheric Observatory (SOHO) satellite. The challenge of all space experiments measuring solar irradiance are sensitivity changes of their sensors due to exposure to intense UV radiation, which are difficult to assess in orbit.
Methods: We exclude the first six years prior to 2002 where one or more fast processes contributed to instrumental changes and analyse a ten-year timeseries of VIRGO sun photometer data between 2002 and 2012. The variability of SSI is correlated with the variability of the TSI, which is taken as a proxy for solar activity.
Results: Observational evidence indicates that after six years only one single long-term process governs the degradation of the backup sun photometer in VIRGO which is operated once a month. This degradation can be well approximated by a linear function over ten years. The analysis of the residuals from the linear trend yield robust positive correlations of spectral irradiance at 862, 500, and 402 nm with total irradiance. In the analysis of annual averages of these data the positive correlations change into weak negative correlations, but with little statistical significance for the 862 nm and 402 nm data. At 500 nm the annual spectral data are still positively correlated with TSI. The persisting positive correlation at 500 nm is in contradiction to the SIM results.

Appendix A is available in electronic form at http://www.aanda.org Title: The LYRA Instrument Onboard PROBA2: Description and In-Flight Performance Authors: Dominique, M.; Hochedez, J. -F.; Schmutz, W.; Dammasch, I. E.; Shapiro, A. I.; Kretzschmar, M.; Zhukov, A. N.; Gillotay, D.; Stockman, Y.; BenMoussa, A. Bibcode: 2013SoPh..286...21D Altcode: 2013arXiv1302.6525D The Large Yield Radiometer (LYRA) is an XUV-EUV-MUV (soft X-ray to mid-ultraviolet) solar radiometer onboard the European Space Agency Project for On-Board Autonomy 2 (PROBA2) mission, which was launched in November 2009. LYRA acquires solar-irradiance measurements at a high cadence (nominally 20 Hz) in four broad spectral channels, from soft X-ray to MUV, which have been chosen for their relevance to solar physics, space weather, and aeronomy. We briefly review the design of the instrument, give an overview of the data products distributed through the instrument website, and describe how the data are calibrated. We also briefly present a summary of the main fields of research currently under investigation by the LYRA consortium. Title: Impact of a potential 21st century "grand solar minimum" on surface temperatures and stratospheric ozone Authors: Anet, J. G.; Rozanov, E. V.; Muthers, S.; Peter, T.; BröNnimann, S.; Arfeuille, F.; Beer, J.; Shapiro, A. I.; Raible, C. C.; Steinhilber, F.; Schmutz, W. K. Bibcode: 2013GeoRL..40.4420A Altcode: We investigate the effects of a recently proposed 21st century Dalton minimum like decline of solar activity on the evolution of Earth's climate and ozone layer. Three sets of two member ensemble simulations, radiatively forced by a midlevel emission scenario (Intergovernmental Panel on Climate Change RCP4.5), are performed with the atmosphere-ocean chemistry-climate model AOCCM SOCOL3-MPIOM, one with constant solar activity, the other two with reduced solar activity and different strength of the solar irradiance forcing. A future grand solar minimum will reduce the global mean surface warming of 2 K between 1986-2005 and 2081-2100 by 0.2 to 0.3 K. Furthermore, the decrease in solar UV radiation leads to a significant delay of stratospheric ozone recovery by 10 years and longer. Therefore, the effects of a solar activity minimum, should it occur, may interfere with international efforts for the protection of global climate and the ozone layer. Title: Detection of Solar Rotational Variability in the Large Yield RAdiometer (LYRA) 190 - 222 nm Spectral Band Authors: Shapiro, A. V.; Shapiro, A. I.; Dominique, M.; Dammasch, I. E.; Wehrli, C.; Rozanov, E.; Schmutz, W. Bibcode: 2013SoPh..286..289S Altcode: 2012arXiv1205.2377S; 2012SoPh..tmp..121S We analyze the variability of the spectral solar irradiance during the period from 7 January 2010 until 20 January 2010 as measured by the Herzberg channel (190 - 222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this period of time, observations by the LYRA nominal unit experienced degradation and the signal produced by the Herzberg channel frequently jumped from one level to another. Both factors significantly complicate the analysis. We present the algorithm that allowed us to extract the solar variability from the LYRA data and compare the results with SORCE/SOLSTICE measurements and with modeling based on the Code for the Solar Irradiance (COSI). Title: Eclipses Observed by Large Yield RAdiometer (LYRA) - A Sensitive Tool to Test Models for the Solar Irradiance Authors: Shapiro, A. I.; Schmutz, W.; Dominique, M.; Shapiro, A. V. Bibcode: 2013SoPh..286..271S Altcode: 2012SoPh..tmp..171S; 2012arXiv1201.6546S We analyze the light curves of the recent solar eclipses measured by the Herzberg channel (200 - 220 nm) of the Large Yield RAdiometer (LYRA) onboard Project for OnBoard Autonomy (PROBA2). The measurements allow us to accurately retrieve the center-to-limb variations (CLV) of the solar brightness. The formation height of the radiation depends on the observing angle, so the examination of the CLV provide information about a broad range of heights in the solar atmosphere. We employ the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the measured light curves and corresponding CLV dependencies. The modeling is used to test and constrain the existing 1D models of the solar atmosphere, e.g. the temperature structure of the photosphere and the treatment of the pseudo-continuum opacities in the Herzberg continuum range. We show that COSI can accurately reproduce not only the irradiance from the entire solar disk, but also the measured CLV. Hence it can be used as a reliable tool for modeling the variability of the spectral solar irradiance. 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: 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: VizieR Online Data Catalog: Sun chromospheric CaII-HK emission (Shapiro+, 2013) Authors: Shapiro, A. I.; Schmutz, W.; Cessateur, G.; Rozanov, E. Bibcode: 2013yCat..35520114S Altcode: 2013yCat..35529114S Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship.

We aim to identify possible reasons for the Sun to be currently outside of this relationship.

We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars.

(6 data files). 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: 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: The place of the Sun among the Sun-like stars Authors: Shapiro, A. I.; Schmutz, W.; Cessateur, G.; Rozanov, E. Bibcode: 2013A&A...552A.114S Altcode: 2013arXiv1303.2245S Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship.
Aims: We aim to identify possible reasons for the Sun to be currently outside of this relationship.
Methods: We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars.
Results: We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.

Appendices A and B are available in electronic form at http://www.aanda.orgAll time series 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/qcat?J/A+A/552/A114 Title: Modeling the detailed Lyman-α line profile Authors: Schoell, Micha; Haberreiter, Margit; Schmutz, Werner; Shapiro, Alexander Bibcode: 2013EGUGA..1512813S Altcode: The variability of the solar spectral irradiance is an important driver in global circulation models. The Lyman-α spectral line is of particular interest as it exhibits a high variability and has strong impact on the ozone concentration and temperature in the Earth's atmosphere. For a complete understanding of the variation of the Lyman-α line it is important to be able to model its detailed line profile. We present recent work on the improvement of the modeling efforts of this important spectral line. We employ an updated version of the COde for Solar Irradiance (COSI) along with existing model atmospheres with an improved implementation of the temperature and turbulent velocity in the chromosphere and transition region. With the latest modifications we are able to reproduce high resolution observations taken with the SUMER instrument onboard of SOHO. This is an important step towards the reliable reconstruction of the irradiance in the Lyman-α spectral line. Title: The Sun among the Sun-like stars Authors: Cessateur, Gaël; Shapiro, Alexander; Schmutz, Werner; Rozanov, Eugene Bibcode: 2013EGUGA..15.8980C Altcode: Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in average activity level and age showed that there is a correlation between the stellar average chromospheric activity level and photometric variability. We aim to understand whether the Sun obeys the empirical relationship prompted by the stellar data and to identify possible reasons for the Sun to be currently outside of this relationship. Title: The 34 Year Starship Authors: Nosanov, J.; Shapiro, A.; Garrett, H. Bibcode: 2012JBIS...65..310N Altcode: In 1974, the designers and leadership of a mission then known as ``Mariner Jupiter Saturn 1977'' decided that the name was a mouthful. They briefly deliberated and decided to rename it ``Voyager'' [1]. Similarly, we believe that ``DARPA 100 Year Starship'' is unnecessarily verbose. We propose to call it ``Enterprise.'' Among the many challenges facing such a mission is that of earning public support - we should let half a century of Hollywood storytelling do that for us. This paper is composed of four parts. Part 1 will discuss the parallels between the Voyager program and a future Enterprise program. Part 2 will go into greater detail regarding the Voyager program and the challenges and opportunities that defined it. Part 3 will discuss some of the major technical challenges for Voyager and Enterprise in greater detail. Part 4 will discuss next steps and various strategies to meet DARPA's challenge of sustainable industry-funded research into interstellar flight. Title: Confronting a solar irradiance reconstruction with solar and stellar data Authors: Judge, P. G.; Lockwood, G. W.; Radick, R. R.; Henry, G. W.; Shapiro, A. I.; Schmutz, W.; Lindsey, C. Bibcode: 2012A&A...544A..88J Altcode: Context. A recent paper by Shapiro and colleagues (2011, A&A, 529, A67) reconstructs spectral and total irradiance variations of the Sun during the holocene. Aims. In this note, we comment on why their methodology leads to large (0.5%) variations in the solar TSI on century-long time scales, in stark contrast to other reconstructions which have ∼ 0.1% variations. Methods. We examine the amplitude of the irradiance variations from the point of view of both solar and stellar data. Results. Shapiro et al.'s large amplitudes arise from differences between the irradiances computed from models A and C of Fontenla and colleagues, and from their explicit assumption that the radiances of the quiet Sun vary with the cosmic ray modulation potential. We suggest that the upper photosphere, as given by model A, is too cool, and discuss relative contributions of local vs. global dynamos to the magnetism and irradiance of the quiet Sun. We compare the slow (>22 yr) components of the irradiance reconstructions with secular changes in stellar photometric data that span 20 years or less, and find that the Sun, if varying with such large amplitudes, would still lie within the distribution of stellar photometric variations measured over a 10-20 year period. However, the stellar time series are individually too short to see if the reconstructed variations will remain consistent with stellar variations when observed for several decades more. Conclusions. By adopting model A, Shapiro et al. have over-estimated quiet-Sun irradiance variations by about a factor of two, based upon a re-analysis of sub-mm data from the James Clerk Maxwell telescope. But both estimates are within bounds set by current stellar data. It is therefore vital to continue accurate photometry of solar-like stars for at least another decade, to reveal secular and cyclic variations on multi-decadal time scales of direct interest to the Sun. 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: Comparison of Solar Irradiance reconstructions with stellar data Authors: Schmutz, Werner; Shapiro, Alexander Bibcode: 2012cosp...39.1725S Altcode: 2012cosp.meet.1725S A recent paper by Shapiro and colleagues reconstructs spectral and total irradiance variations of the Sun during the holocene. They derive a total and spectral solar irradiance that was substantially lower during the Maunder minimum than the one observed today. The difference is remarkably larger than other estimations published in the recent literature. In this presentation we examine the spectral reconstructions in the light of stellar data. We compare the observed and predicted solar variability with that of stars as observed by Radick et al. (1998), Lockwood et al. (2007) and Hall et al. (2009) in the b and y filters for the last 10- 20 years. Title: Modeling of the atmospheric response to a strong decrease of the solar activity Authors: Rozanov, Eugene V.; Egorova, Tatiana A.; Shapiro, Alexander I.; Schmutz, Werner K. Bibcode: 2012IAUS..286..215R Altcode: We estimate the consequences of a potential strong decrease of the solar activity using the model simulations of the future driven by pure anthropogenic forcing as well as its combination with different solar activity related factors: total solar irradiance, spectral solar irradiance, energetic electron precipitation, solar protons and galactic cosmic rays. The comparison of the model simulations shows that introduced strong decrease of solar activity can lead to some delay of the ozone recovery and partially compensate greenhouse warming acting in the direction opposite to anthropogenic effects. The model results also show that all considered solar forcings are important in different atmospheric layers and geographical regions. However, in the global scale the solar irradiance variability can be considered as the most important solar forcing. The obtained results constitute probably the upper limit of the possible solar influence. Development of the better constrained set of future solar forcings is necessary to address the problem of future climate and ozone layer with more confidence. 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: A New Solar Spectral Irradiance Reconstruction based on MGII and Neutral Monitoring Indices for Use in Climate Modelling Authors: Thuillier, G.; Bolsée, D.; DeLand, M.; Melo, S. M. L.; Schmutz, W.; Shapiro, A. Bibcode: 2012EGUGA..14.8248T Altcode: For atmosphere and climate studies, the solar spectral irradiance may be necessary at a time where no data exist. Use of proxies is then mandatory. In order to represent the solar forcing as variable in chemistry-climate numerical models, we need consistent series of temporal solar total and spectral variability covering over the periods of interest. While measurements are available, there is currently no harmonized series with some understanding of its accuracy and precision that can be readily implemented in model simulations. In this paper we present a new method to reconstruct the solar spectrum irradiance in the Ly α-400 nm region, and its variability, based on the Mg II index and neutron monitor data. This approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg II index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg II index. Then, the set of coefficients expressing the spectral variability can be applied to a chosen reference spectrum to reconstruct the solar spectra within a given time frame or a Mg II index values. The accuracy of this method is estimated by calculating the standard deviation between the measured spectra and their reconstruction. For the second step, the relationship between the Mg II index and the neutron monitor data is searched for the 30-year of Mg II index availability. Finally, the reconstruction at a given date consists in using the neutron monitor data at that date, derive the corresponding Mg II index and use the coefficients of SSI variability to obtain the SSI at that date using a chosen reference spectrum. One major advantage is that using technology of today, we can reconstruct the solar spectral irradiance consistently from today to times when cosmogenic isotope data are available. This calibration can be re-accessed at any time, if necessary. Reconstruction for the Maunder minimum will be shown as well as variation of stratospheric components concentration inferred by solar variability. Title: Signature of the 27-day solar rotation cycle in mesospheric OH and H2O observed by the Aura Microwave Limb Sounder Authors: Shapiro, A. V.; Rozanov, E.; Shapiro, A. I.; Wang, S.; Egorova, T.; Schmutz, W.; Peter, Th. Bibcode: 2012ACP....12.3181S Altcode: 2011ACPD...1128477S The mesospheric hydroxyl radical (OH) is mainly produced by the water vapor (H2O) photolysis and could be considered as a proxy for the influence of the solar irradiance variability on the mesosphere. We analyze the tropical mean response of the mesospheric OH and H2O data as observed by the Aura Microwave Limb Sounder (MLS) to 27-day solar variability. The analysis is performed for two time periods corresponding to the different phases of the 11-yr cycle: from December 2004 to December 2005 (the period of "high activity" with a pronounced 27-day solar cycle) and from August 2008 to August 2009 ("solar minimum" period with a vague 27-day solar cycle). We demonstrate, for the first time, that in the mesosphere the daily time series of OH concentrations correlate well with the solar irradiance (correlation coefficients up to 0.79) at zero time-lag. At the same time H2O anticorrelates (correlation coefficients up to -0.74) with the solar irradiance at non-zero time-lag. We found that the response of OH and H2O to the 27-day variability of the solar irradiance is strong for the period of the high solar activity and negligible for the solar minimum conditions. It allows us to suggest that the 27-day cycle in the solar irradiance and in OH and H2O are physically connected. Title: The Solar Spectral Irradiance as a Function of the Mg II Index for Atmosphere and Climate Modelling Authors: Thuillier, G.; DeLand, M.; Shapiro, A.; Schmutz, W.; Bolsée, D.; Melo, S. M. L. Bibcode: 2012SoPh..277..245T Altcode: We present a new method to reconstruct the solar spectrum irradiance in the Ly α - 400 nm region, and its variability, based on the Mg II index and neutron-monitor measurements. Measurements of the solar spectral irradiance available in the literature have been made with different instruments at different times and different spectral ranges. However, climate studies require harmonised data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg II index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg II index. The set of coefficients expressing the spectral variability can be applied to the chosen reference spectrum to reconstruct the solar spectra within a given time frame or Mg II index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades towards Ly α. In a further step, we extend our solar spectral-irradiance reconstruction back to the Maunder Minimum introducing the relationship between the Mg II index and the neutron-monitor data. Consistent measurements of the Mg II index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg II index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this correlation can be applied to the past, we reconstruct the Mg II index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar spectral-irradiance reconstruction back to that period. As there is no direct measurement of the spectral irradiance for this period we discuss this methodology in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate the solar irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This calibration can be re-assessed at any time, if necessary. Title: Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2 molecule Authors: Kleint, L.; Shapiro, A. I.; Berdyugina, S. V.; Bianda, M. Bibcode: 2011A&A...536A..47K Altcode: Context. Scattering polarization measurements contain a wealth of information that needs a thorough interpretation. This often requires accounting for the non-local origin of photons with different frequencies and at different limb positions. Currently, modeling scattering polarization in several molecular C2 lines simultaneously is only successful for lines with similar quantum numbers. More sophisticated models are needed to understand the dependence on quantum numbers and to reliably derive the strength of the turbulent magnetic fields using the differential Hanle effect.
Aims: We have developed a non-LTE analyzing technique for the C2 lines to determine the strength of turbulent magnetic fields and have applied it to observations obtained during our synoptic program at the Istituto Ricerche SOlari Locarno (IRSOL).
Methods: The influence of magnetic fields on scattering polarization can be interpreted differentially, i.e., by comparing several spectral lines within one spectral region. Through the application of the differential Hanle effect and non-LTE 1D radiative transfer, we are able to infer a magnetic field strength from the photospheric C2 lines around 5141 Å. Compared to previous models we include the effect of collisions and investigate their dependence on the total angular momentum number J.
Results: We carry out a detailed parameter study to investigate the influence of model parameters on the resulting scattering polarization. A good fit can now be obtained for spectral lines from different C2 triplets. For the 78 measurements obtained during the solar minimum in 2007-2009 we infer a mean magnetic field strength of 7.41 G with a standard deviation of 0.76 G. 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: Scripture in the Sky: Jeremias Drexel, Julius Schiller, and the Christianizing of the Constellations Authors: Mendillo, M.; Shapiro, A. Bibcode: 2011ASPC..441..181M Altcode: From the times of early Christians up to the Enlightenment, the twelve ancient signs of the zodiac were challenged as highly inappropriate pagan images. The most concerted efforts to replace those signs with names, mottos, and images taken from Holy Scripture occurred in the early decades of the 17th century. We review the background that led to the proposed use of sacred mottos by Jeremias Drexel, and then of the names and images of the Twelve Apostles by Julius Schiller. The reaction of a leading seventeenth-century astronomer is presented to suggest why such changes were never adopted. Finally, we address issues of conflict and motivation that might have led to efforts to Christianize the Heavens. Title: A new approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing Authors: Shapiro, A. I.; Schmutz, W.; Rozanov, E.; Schoell, M.; Haberreiter, M.; Shapiro, A. V.; Nyeki, S. Bibcode: 2011A&A...529A..67S Altcode: 2011arXiv1102.4763S Context. The variable Sun is the most likely candidate for the natural forcing of past climate changes on time scales of 50 to 1000 years. Evidence for this understanding is that the terrestrial climate correlates positively with the solar activity. During the past 10 000 years, the Sun has experienced the substantial variations in activity and there have been numerous attempts to reconstruct solar irradiance. While there is general agreement on how solar forcing varied during the last several hundred years - all reconstructions are proportional to the solar activity - there is scientific controversy on the magnitude of solar forcing.
Aims: We present a reconstruction of the total and spectral solar irradiance covering 130 nm-10 μm from 1610 to the present with an annual resolution and for the Holocene with a 22-year resolution.
Methods: We assume that the minimum state of the quiet Sun in time corresponds to the observed quietest area on the present Sun. Then we use available long-term proxies of the solar activity, which are 10Be isotope concentrations in ice cores and 22-year smoothed neutron monitor data, to interpolate between the present quiet Sun and the minimum state of the quiet Sun. This determines the long-term trend in the solar variability, which is then superposed with the 11-year activity cycle calculated from the sunspot number. The time-dependent solar spectral irradiance from about 7000 BC to the present is then derived using a state-of-the-art radiation code.
Results: We derive a total and spectral solar irradiance that was substantially lower during the Maunder minimum than the one observed today. The difference is remarkably larger than other estimations published in the recent literature. The magnitude of the solar UV variability, which indirectly affects the climate, is also found to exceed previous estimates.We discuss in detail the assumptions that lead us to this conclusion.

Appendix is only available in electronic form at http://www.aanda.org Title: NLTE modeling of Stokes vector center-to-limb variations in the CN violet system Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Bianda, M.; Ramelli, R. Bibcode: 2011A&A...529A.139S Altcode: 2011arXiv1103.2524S Context. The solar surface magnetic field is connected with and even controls most of the solar activity phenomena. Zeeman effect diagnostics allow for measuring only a small fraction of the fractal-like structured magnetic field. The remaining hidden magnetic fields can only be accessed with the Hanle effect.
Aims: Molecular lines are very convenient for applying the Hanle effect diagnostics thanks to the broad range of magnetic sensitivities in a narrow spectral region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno (IRSOL), we simultaneously observed intensity and linear polarization center-to-limb variations in two spectral regions containing the (0, 0) and (1, 1) bandheads of the CN B2Σ - X2Σ system. Here we present an analysis of these observations.
Methods: We have implemented coherent scattering in molecular lines into an NLTE radiative transfer code. A two-step approach was used. First, we separately solved the statistical equilibrium equations and compute opacities and intensity while neglecting polarization. Then we used these quantities as input for calculating scattering polarization and the Hanle effect.
Results: We have found that it is impossible to fit the intensity and polarization simultaneously at different limb angles in the framework of standard 1D modeling. The atmosphere models that provide correct intensity center-to-limb variations fail to fit linear polarization center-to-limb variations due to lacking radiation-field anisotropy. We had to increase the anisotropy by means of a specially introduced free parameter. This allows us to successfully interpret our observations. We discuss possible reasons for underestimating the anisotropy in the 1D modeling. Title: The atmospheric effects of October 2003 solar proton event simulated with the chemistry-climate model SOCOL using complete and parameterized ion chemistry Authors: Egorova, T.; Rozanov, E.; Ozolin, Y.; Shapiro, A.; Calisto, M.; Peter, Th.; Schmutz, W. Bibcode: 2011JASTP..73..356E Altcode: October 2003 solar proton events (SPE) is rather well covered by the observations; therefore its studies represent a good way for model validation and intercomparison. Here we apply chemistry-climate model (CCM) SOCOL with complete (SOCOLi) and parameterized ion chemistry to evaluate the accuracy of a commonly used ion chemistry parameterization scheme. We performed ensemble experiments with and without SPE to characterize the effect of the October 2003 SPE on the NOx, HOx, ClOx and O3 in the middle atmosphere. Preliminary comparison of the simulated effects against MIPAS observations revealed rather good general agreement for most of the species. Comparison of the results obtained with complete and parameterized ion chemistry representation showed that the model with parameterized ion chemistry underestimates the effect of SPE on chemical composition of the middle atmosphere by up to 40% for NOx and N2O, up to 70% for HOx and ClOx and up to 600% for HNO3. The parameterization is more accurate for ozone, however the model with parameterized ion chemistry underestimates ozone depletion by up to 15% during the SPE in the mesosphere and by 10% 2 weeks later in the stratosphere, which can be important for the long-term effects of SPE on the ozone layer. Title: Sensitivity of the Earth’s middle atmosphere to short-term solar variability and its dependence on the choice of solar irradiance data set Authors: Shapiro, A. V.; Rozanov, E.; Egorova, T.; Shapiro, A. I.; Peter, Th.; Schmutz, W. Bibcode: 2011JASTP..73..348S Altcode: We simulate the time evolution of the neutral and charged species in the terrestrial middle atmosphere using a 1-D radiative-convective model with interactive neutral and ion chemistry driven by four different sets of daily spectral solar irradiance (SSI) available in the literature for the year 2000. Obtained daily time series of ozone, hydroxyl and electron densities are used to calculate their sensitivity to the short-term SSI variability at 205 nm. All applied SSI data sets possess 27-day solar rotation cycle; however, its amplitude and phase as well as the correlation between considered SSI time series differ among data sets leading to the different behavior of the atmospheric response. Contrary, the ozone and hydroxyl sensitivities to the SSI changes during solar rotation cycle are almost identical for all applied SSI data sets in the stratosphere. In the mesosphere, the difference in correlation between SSI in Herzberg continuum and Lyman-α line in considered SSI data sets leads to substantial scatter of the sensitivity estimates based on 205 nm. Our results show that for the sensitivity analysis in the stratosphere based on the SSI at 205 nm any considered SSI data sets can be applied. For the mesosphere, where the sensitivity strongly varies among applied SSI data sets more robust results can be obtained using the sensitivity calculations based on the SSI in Lyman-α line. Title: The Shape of the Solar Limb: Models and Observations Authors: Thuillier, G.; Claudel, J.; Djafer, D.; Haberreiter, M.; Mein, N.; Melo, S. M. L.; Schmutz, W.; Shapiro, A.; Short, C. I.; Sofia, S. Bibcode: 2011SoPh..268..125T Altcode: 2010SoPh..tmp..202T; 2010SoPh..tmp..226T In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models. Title: Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum Authors: Kleint, L.; Berdyugina, S. V.; Shapiro, A. I.; Bianda, M. Bibcode: 2010A&A...524A..37K Altcode: Context. Small-scale, weak magnetic fields are ubiquitous in the quiet solar atmosphere. Yet their properties and temporal and spatial variations are not well known.
Aims: We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle.
Methods: Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5” on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of ~10-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C2 triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C2 R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields.
Results: We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3-8 G.
Conclusions: Our results demonstrate the potential of long-term observations of small-scale magnetic fields, which may vary with the solar cycle in both mean strength and spatial distribution. This provides important constraints on the energy budget of the solar cycle. Extending this synoptic program to many spectral lines would provide a sample of heights in the solar atmosphere. Title: New SSI and TSI reconstruction suggests large value of the radiative solar forcing Authors: Shapiro, A.; Schmutz, W. K.; Thuillier, G.; Rozanov, E.; Haberreiter, M.; Schoell, M.; Shapiro, A.; Nyeki, S. Bibcode: 2010AGUFMGC21B0875S Altcode: We have developed and published the COde for Solar Irradiance (COSI) which allows us to calculate the entire solar spectrum. COSI reproduces the spectral irradiance measured by SOLSTICE (up to 320 nm) and SIM (from 320 nm onward) onboard the SORCE satellite during the 2008 solar minimum as well as SOLSPEC during the ATLAS 3 mission in 1994 with high accuracy. COSI is also used as a tool for modeling the variability of the solar irradiance. In our new reconstruction we represent the quiet Sun as a combination of two components. The first corresponds to the least active areas of the Sun as presently observed, while the second component is responsible for the magnetic activity of the quiet Sun. The time-dependent filling factors of these components are calculated based on the solar activity as derived from cosmogenic isotope concentrations in natural archieves. This allows us to obtain a time-dependent reconstructed solar spectrum from 7000 BC to the present with a temporal resolution of 22 years. From 1610 onward we have additional information from sunspot numbers, which allows the reconstruction of the spectral solar irradiance with a yearly resolution. These basic assumptions lead to a total and spectral solar irradiance that was substantially lower during the Maunder minimum than observed today. The difference is remarkably larger than other estimations published in the recent literature. Using our reconstruction as an input to a chemistry-climate model we show that the enhanced UV variability results in significant variations of stratospheric ozone and temperature. Title: NLTE solar irradiance modeling with the COSI code Authors: Shapiro, A. I.; Schmutz, W.; Schoell, M.; Haberreiter, M.; Rozanov, E. Bibcode: 2010A&A...517A..48S Altcode: 2010arXiv1004.3048S Context. The solar irradiance is known to change on time scales of minutes to decades, and it is suspected that its substantial fluctuations are partially responsible for climate variations.
Aims: We are developing a solar atmosphere code that allows the physical modeling of the entire solar spectrum composed of quiet Sun and active regions. This code is a tool for modeling the variability of the solar irradiance and understanding its influence on Earth.
Methods: We exploit further development of the radiative transfer code COSI that now incorporates the calculation of molecular lines. We validated COSI under the conditions of local thermodynamic equilibrium (LTE) against the synthetic spectra calculated with the ATLAS code. The synthetic solar spectra were also calculated in non-local thermodynamic equilibrium (NLTE) and compared to the available measured spectra. In doing so we have defined the main problems of the modeling, e.g., the lack of opacity in the UV part of the spectrum and the inconsistency in the calculations of the visible continuum level, and we describe a solution to these problems.
Results: The improved version of COSI allows us to reach good agreement between the calculated and observed solar spectra as measured by SOLSTICE and SIM onboard the SORCE satellite and ATLAS 3 mission operated from the Space Shuttle. We find that NLTE effects are very important for the modeling of the solar spectrum even in the visual part of the spectrum and for its variability over the entire solar spectrum. In addition to the strong effect on the UV part of the spectrum, NLTE effects influence the concentration of the negative ion of hydrogen, which results in a significant change of the visible continuum level and the irradiance variability.

Figures 13 and 14 are only available in electronic form at http://www.aanda.org Title: A synoptic program for large solar telescopes: Cyclic variation of turbulent magnetic fields Authors: Kleint, L.; Berdyugina, S. V.; Gisler, D.; Shapiro, A. I.; Bianda, M. Bibcode: 2010AN....331..644K Altcode: Upcoming large solar telescopes will offer the possibility of unprecedented high resolution observations. However, during periods of non-ideal seeing such measurements are impossible and alternative programs should be considered to best use the available observing time. We present a synoptic program, currently carried out at the Istituto Ricerche Solari Locarno (IRSOL), to monitor turbulent magnetic fields employing the differential Hanle effect in atomic and molecular lines. This program can be easily adapted for the use at large telescopes exploring new science goals, nowadays impossible to achieve with smaller telescopes. The current, interesting scientific results prove that such programs are worthwhile to be continued and expanded in the future. We calculate the approximately achievable spatial resolution at a large telescope like ATST for polarimetric measurements with a noise level below 5 × 10-5 and a temporal resolution which is sufficient to explore variations on the granular scale. We show that it would be important to optimize the system for maximal photon throughput and to install a high-speed camera system to be able to study turbulent magnetic fields with unprecedented accuracy. Title: Turbulent Magnetic Fields in the Quiet Sun: A Search for Cyclic Variations Authors: Kleint, L.; Berdyugina, S. V.; Shapiro, A. I.; Bianda, M. Bibcode: 2010ASPC..428..103K Altcode: 2010arXiv1003.4103K Turbulent magnetic fields fill most of the volume of the solar atmosphere. However, their spatial and temporal variations are still unknown. Since 2007, during the current solar minimum, we are periodically monitoring several wavelength regions in the solar spectrum to search for variations of the turbulent magnetic field in the quiet Sun. These fields, which are below the resolution limit, can be detected via the Hanle effect which influences the scattering polarization signatures (Q/I) in the presence of magnetic fields. We present a description of our program and first results showing that such a synoptic program is complementary to the daily SOHO magnetograms for monitoring small-scale magnetic fields. Title: Modeling the impact of the solar UV irradiance on the middle atmosphere Authors: Rozanov, Eugene; Egorova, Tatiana; Shapiro, Anna; Shapiro, Alexander; Schmutz, Werner Bibcode: 2010cosp...38.1103R Altcode: 2010cosp.meet.1103R Modeling of the middle atmosphere response to the short-term variability of the solar spec-tral irradiance (SSI) is important to understand the mechanisms responsible for the Sun-Earth connections. The analysis of the recent publications shows that there is still substantial dis-agreement between the simulated and observed responses of the middle atmosphere to the daily SSI variability. Among other problems this disagreement can be caused by not perfect quality of the available SSI data sets which are necessary to drive models and extract the response of the middle atmosphere from the observed time series. To elucidate the role of the SSI data we exploited column and full versions of the climate-chemistry-ionosphere model SOCOL, which is based on a general circulation model and includes complete representation of the chemistry of neutral and ionized species in the atmosphere from the ground up to the mesopause. We have performed a series of experiments driven by different available observed and reconstructed daily SSI data sets to estimate the middle atmosphere response to the SSI variability during the solar rotation cycle as well as to the short-time increase of the solar UV irradiance occurred in January 2004. Our results showed that the sensitivity analysis in the stratosphere gives similar results for all SSI data sets when it is based on the SSI at 205 nm, while for the mesosphere the sensitivity based on the SSI in Lyman-line is more robust. The analysis of the correlation between ozone, hydroxyl and electron densities daily time series simulated with different SSI data sets showed substantial disagreement which emphasizes the necessity of more accurate representation of SSI for the nowcasting of the middle atmosphere state. The comparison of the short-term changes in the middle atmosphere in January 2004 with the appropriate obser-vations allows the evaluation of the model performance, which will be discussed together with the perspectives of the model application for the space weather nowcasting using the solar UV irradiance data acquired by LYRA instrument onboard ESA PROBA-2 satellite and available in near-real time. Title: Modeling of the current TSI and SSI and its reconstruction to the past Authors: Shapiro, Alexander; Schmutz, Werner; Thuillier, Gerard; Schoell, Micha; Haberreiter, Margit; Rozanov, Eugene Bibcode: 2010cosp...38..134S Altcode: 2010cosp.meet..134S We have developed and published the COde for Solar Irradiance (COSI) which provides physical modeling of the entire solar spectrum as a contribution from the quiet Sun and active regions. This code allows us to reach a good agreement between the calculated and observed solar spectrum as measured by SOLSTICE and SIM onboard the SORCE satellite and ATLAS 3 mission operated from the Space Shuttle. We find that NLTE effects are very important for the modeling of the solar spectrum even in the visible part of the spectrum and for its variability over the entire solar spectrum. We employ the assumption that the quiet Sun consists of two components. The first corresponds to the average quiet network and it is responsible for all magnetic activity of the quiet Sun. The second represents the Sun that is basically free from any magnetic activity. We use the reconstructed open magnetic flux data as well as the TSI and SSI observed by SORCE during this peculiar solar minimum to find the filling factors of these components. We use the sunspot numbers to reconstruct the filling factors of the solar active components. This approach allows us to obtain an annually averaged TSI and SSI back to the Maunder Minimum. These basic assumptions lead to aTSI and SSI that was substantially different during the Maunder minimum than as it is observed today. The difference is remarkably larger than other estimations published in the recent literature. Title: The Absolute Solar Irradiance Spectrum at Solar Minimum Activity Measured by the SOLSPEC and SOL-ACES Spectrometers from 17 to 3000 nm Placed on Board the International Space Station Authors: Thuillier, Gerard; Bolsee, David; Schmidtke, Gerhard; Schmutz, Werner; Shapiro, Alexander; Nikutowski, Bernd Bibcode: 2010cosp...38...17T Altcode: 2010cosp.meet...17T Two instruments placed on the COLOMBUS laboratory on the International Space Station constitute a payload named SOLAR measuring the spectral solar irradiance from 17 to 3000 nm for solar, atmospheric and climatology physics for which the sun-climate connection also re-quires the precise and absolute knowledge of the solar spectral irradiance. Given the significant improvements in atmosphere, climate and solar modelling, accurate data are needed. SOL-SPEC primary objectives are the measurement of the Sun absolute spectral irradiance and its variability from 165 to 3080 nm. SOLSPEC has been developed by LATMOS (France), Institut d'Aéronomie Spatiale (Belgique), and Landessternwarte (Germany). It has been calibrated in the absolute scale by using the blackbody of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig. SOL-ACES primary objectives are the measurements of the Sun absolute spec-tral irradiance from 17 to 140 nm. This instrument as developed by the Fraunhofer Institute (Freiburg, Germany) uses three-current ionisation chambers repeatedly filled with different gases to re-calibrate the three spectrometers, which are changing their efficiencies e.g. by the interaction with solar radiation. We present a composite solar spectrum for the July 2008 period, at a very low solar as occurred at the end of solar cycle 23. It has been built using SOLSPEC, SOL-ACES and TIMED SEE data. Comparison with data obtained during the ATLAS 3 period (November 1994), SORCE measurements and theoretical modelling using the COSI code will be presented. Differences will be commented (difference of the two activity levels, accuracy). Title: Response of the middle atmosphere to short-term solar irradiance variability during different Quasi-Biennial Oscillation phases Authors: Shapiro, Anna; Rozanov, Eugene; Shapiro, Alexander; Egorova, Tatiana; Schmutz, Werner; Peter, Thomas Bibcode: 2010cosp...38..138S Altcode: 2010cosp.meet..138S Variability of the spectral solar irradiance (SSI) is known to cause changes in the chemistry and dynamics of the middle atmosphere. Because the analysis of the observed temperature responses to the decadal scale is difficult owing to rather short time series we aim on the study of the middle atmosphere response to the short-term SSI variability associated with the Sun rotation cycle. One of the main problems here is that the response of the atmosphere usually derived using the cross-correlation analysis depends on the atmospheric state and internal variability. In particular, the distribution and variability of the temperature and gas species in the tropical middle atmosphere depends on the phase of quasi-biennial oscillation (QBO). We have applied the chemistry-climate model SOCOL and simulated the dependence of the middle atmosphere response to the short-term solar variability during different QBO phases with a series of ensemble runs. We have found that the hydroxyl sensitivities to the SSI changes during solar rotation cycle are almost identical for the different QBO phases if we forced the model by identical photolysis while the ozone sensitivities to the SSI changes are different for some layers of the stratosphere. Title: Model studies of the solar limb shape variation with wavelenght within the PICARD project. Authors: Melo, Stella M. L.; Thuillier, Gerard; Claudel, Jennyfer; Haberreiter, Margit; Mein, Nicole; Schmutz, Werner; Shapiro, Alexander; Sofia, Sabatino; Short, Christopher I. Bibcode: 2010cosp...38.1756M Altcode: 2010cosp.meet.1756M Solar images in the visible wavelength range show that the disk centre is brighter than the limb region. This phenomenon, which is both known as "centre to limb variation (CLV)", or "limb darkening function", is know to depend on wavelength. Since the CLV is determined by the density and temperature stratification, as well as the chemical composition of the so-lar photosphere, its measurement is important to validate theoretical assumption made when building numerical models of the solar atmosphere. The definition of the solar diameter is nor-mally adopted as the separation between two inflection points at opposite ends of a line passing through the center of the solar disk. Therefore, in order to understand long term variability on the solar diameter, it is important to understand what drives the dependence of the position of the inflection point on wavelength. In this paper we use different available solar atmosphere models to study this dependence. The results presented here refer to quiet Sun conditions and encompass the visible and near infra-red spectral regions, which are the regions of interest for the PICARD Satellite Mission. In a first step we utilize the solar atmosphere parameters with a radiative transfer code. This allows for the study of the impact of different factors such as opacities, electron density and temperature from different models on the results. Then, we compare results obtained using each solar atmosphere model. Our results are compared with existent ground based measurements performed by the Pic du Midi telescope, the balloon board measurements with the Solar Disk Sextant experiment, and with the measurements by the Michelson Doppler Imager on board SoHO satellite. The model simulations show that the position of the inflection point is sensitive to the different parameters and model assumptions. Furthermore, our study shows, for the first time, that the position of the inflection point changes dramatically with and outside of Fraunhofer lines. Title: Solar Magnetic Field Diagnostics with the Molecular Hanle Effect Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V. Bibcode: 2009ASPC..405..343S Altcode: Weak entangled magnetic fields with mixed polarity occupy the main part of the quiet solar photosphere. While the Zeeman effect diagnostics fail to measure such fields due to cancellation in polarization, the Hanle effect, accessible through the second solar spectrum, provides us with a very sensitive tool for studying the distribution of weak magnetic fields on the Sun. Molecular lines are valuable for magnetic field diagnostics thanks to their broad range of magnetic sensitivities within narrow spectral regions, so that the differential Hanle effect can be employed, which greatly reduces the model dependence of deduced magnetic field strengths. Here we present our recent results on the diagnostic of solar turbulent magnetic fields with the help of CN transitions in the violet system. In addition, we have implemented modeling of coherent scattering in molecular lines into a non-LTE radiative transfer code. Together with the Hanle effect theory this provides us with a realistic model for studying turbulent magnetic fields. Title: Molecular processes and turbulent magnetic fields in the solar atmosphere Authors: Shapiro, Alexander Ilyich Bibcode: 2009PhDT.......586S Altcode: No abstract at ADS Title: Molecular processes and turbulent magnetic fields in the solar atmosphere Authors: Shapiro, A. I. Bibcode: 2008PhDT.......417S Altcode: Coherent scattering in the solar atmosphere leads to the formation of the linearly polarized solar spectrum, just like Rayleigh scattering leads to the polarization of the blue sky. One of the most prominent features of the linearly polarized solar spectrum is the CN violet system as it is also in the unpolarized spectrum. This thesis is devoted to the modeling and interpretation of this system in both spectra and developing it into a very sensitive tool for studying the magnetic fields and the temperature structure of the solar atmosphere.

The understanding of the solar magnetic field structure is very important as it is connected with and even controls most of the solar activity phenomena. Zeeman effect diagnostics allows to measure strong directed magnetic fields which only cover about 1% of the solar atmosphere. The remaining part is occupied by weak entangled magnetic fields with mixed polarity, which might significantly contribute to the overall solar magnetic energy. These fields are invisible to the Zeeman effect due to signal cancellation. Therefore the discovery of the linearly polarized solar spectrum opened a new epoch in solar physics. The polarization due to the scattering processes is modified by weak entangled magnetic fields via the Hanle effect and thus, provides us with a unique possibility to access and study such "hidden" magnetic fields.

Molecular lines are very useful for probing magnetic fields as, due to their strong temperature sensitivity, different molecules sample different, narrow layers of the solar atmosphere. Therefore the extension of the atomic Hanle effect to molecular lines can provide the 3D structure of the solar turbulent magnetic field. Moreover, due to the broad range of magnetic sensitivities within narrow spectral regions molecular lines can be used for employing the differential Hanle effect technique, which allows dramatically reduced model dependence of the obtained magnetic field. This thesis consists of two main parts which reflect the two consecutive steps in the modeling of the polarized solar spectrum. First, the physical properties of the molecular scattering process have to be understood. For the CN violet system, it implies taking into account the Paschen-Back effect on the fine structure (which results in a change of intensities and line positions) and interference effects between the fine structure components itself. Both effects influence the Hanle effect and lead to the consequences which are analyzed in detail in the first part of this thesis.

Then, to connect the coherent scattering and physical properties of the solar plasma with the emerged solar radiation, which is measured by our detectors, radiative transfer theory has to be applied. The presence of the scattering processes implies the non-equilibrium nature of the problem. Its self-consistent solution is especially complicated for molecular bands as they usually consist of a huge number of transitions which couple the numerous vibrational-rotational molecular levels. This, for example, makes the two-level approximation, often used in atomic calculations unsuitable. Moreover, there is a strong lack of information about the molecular collision rates so they basically enter the calculations as additional free parameter. In the second part of the thesis we present two radiative transfer models with different degrees of complexity and applicability. These models allow us to successfully fit the observations of the CN violet system in both polarized and unpolarized spectra and provide us with a magnetic field estimation. We discuss in detail the model-dependence of our results and general problems of the 1D solar spectrum modeling.

The enormous potential of the linearly polarized solar spectrum makes it one of the main tools for studying solar magnetic fields, which becomes very urgent nowadays, as they can affect the Earth's climate and, hence, our life. Title: Hanle effect in the CN violet system with LTE modeling Authors: Shapiro, A. I.; Berdyugina, S. V.; Fluri, D. M.; Stenflo, J. O. Bibcode: 2007A&A...475..349S Altcode: 2007arXiv0709.2515S Context: Weak entangled magnetic fields with mixed polarity occupy the main part of the quiet Sun. The Zeeman effect diagnostics fails to measure such fields because of cancellation in circular polarization. However, the Hanle effect diagnostics, accessible through the second solar spectrum, provides us with a very sensitive tool for studying the distribution of weak magnetic fields on the Sun.
Aims: Molecular lines are very strong and even dominate in some regions of the second solar spectrum. The CN B {}2 Σ - X {}2 Σ system is one of the richest and most promising systems for molecular diagnostics and well suited for the application of the differential Hanle effect method. The aim is to interpret observations of the CN B {}2 Σ - X {}2 Σ system using the Hanle effect and to obtain an estimation of the magnetic field strength.
Methods: We assume that the CN molecular layer is situated above the region where the continuum radiation is formed and employ the single-scattering approximation. Together with the Hanle effect theory this provides us with a model that can diagnose turbulent magnetic fields.
Results: We have succeeded in fitting modeled CN lines in several regions of the second solar spectrum to observations and obtained a magnetic field strength in the range from 10-30 G in the upper solar photosphere depending on the considered lines.

Tables [see full textsee full textsee full textsee full text]-[see full textsee full textsee full textsee full text] are only available in electronic form at http://www.aanda.org Title: Broad-Band Molecular Polarization in White Dwarfs Authors: Berdyugina, S. V.; Berdyugin, A. V.; Piirola, V.; Shapiro, A. Bibcode: 2007ASPC..372..177B Altcode: We present novel calculations of broad-band polarization due to the molecular Paschen--Back effect in a strong magnetic field. Based on that, we analyze new spectropolarimetric observations of the cool magnetic helium-rich white dwarf G 99-37 which shows strongly polarized molecular bands in its spectrum. Combining the polarimetric observations with our model calculations for the CH bands at 4300 Å, we deduce a magnetic field of 8 MG on this unique magnetic white dwarf. Title: Molecular Hanle effect in the Paschen-Back regime Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Stenflo, J. O. Bibcode: 2007A&A...461..339S Altcode: Context: The second solar spectrum resulting from coherent scattering is a main tool for diagnostics of turbulent magnetic fields on the Sun. Scattering on diatomic molecules plays an important role in forming this spectrum and even dominates in some spectral regions.
Aims: In a magnetic field electronic states of a molecule are often perturbed via the Paschen-Back effect. Sometimes this perturbation can completely change the spectrum, not only quantitatively, but even qualitatively. Here we calculate molecular scattering properties taking into account the Paschen-Back effect.
Methods: Starting with the Hund's case (a) wave functions as a basis we obtain with the perturbation theory wave functions of the intermediate Hund's case (a-b) in a magnetic field. Using new, perturbed values of the Landé factors and transition amplitudes we calculate the Mueller matrix for coherent scattering at diatomic molecules in the intermediate Hund's case (a-b) and look for the effects that can be caused by the Paschen-Back effect.
Results: We have found a considerable deviation from the Zeeman regime and discuss here the quantitative and qualitative effects on observed polarization signals for the CN B {}2 Σ - X {}2 Σ and MgH B' {}2 Σ - X {}2 Σ systems as examples. Title: Molecular Hanle effect in the Paschen-Back regime: theory and application Authors: Shapiro, A. I.; Berdyugina, S. V.; Fluri, D. M.; Stenflo, J. O. Bibcode: 2007msfa.conf..317S Altcode: The second solar spectrum resulting from coherent scattering is a main tool for diagnostics of turbulent magnetic fields on the Sun. Scattering on diatomic molecules plays an important role in forming this spectrum and even dominates in some spectral regions. In a magnetic field electronic states of a molecule are often perturbed via the Paschen-Back effect. Sometimes this perturbation can completely change the spectrum, not only quantitatively, but even qualitatively. Here we calculate molecular scattering properties taking into account the Paschen-Back effect. We calculate the Mueller matrix for coherent scattering at diatomic molecules in the intermediate Hund's case (a-b) and look for the effects that can be caused by the Paschen-Back effect. We have found a considerable deviation from the Zeeman regime and discuss here the quantitative and qualitative effects on observed polarization signals for the CN B 2 [Sigma] - X 2 [Sigma] system as an example. We show an application of the Hanle effect for the interpretation of observations of Title: Hanle Effect in the Paschen-Back Regime Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Stenflo, J. O. Bibcode: 2006ASPC..358..311S Altcode: The second solar spectrum resulting from coherent scattering is an important tool for the diagnostics of turbulent magnetic fields on the Sun. Molecular scattering plays an important role in forming this spectrum, and even dominates in some spectral regions. We present a theory that allows us to calculate the Mueller matrix for coherent scattering from diatomic molecules in Hund's intermediate coupling case (a-b) for arbitrary molecular transitions. We performed the calculation of the molecular Hanle effect in the Paschen-Back regime. We found significant differences from the Zeeman regime, and as an example we discuss here, both qualitatively and quantitatively, the effects on observed polarization signals for the CN violet system. Title: Multicomponent processes in crudes Authors: van Vaerenbergh, Stéfan; Shapiro, Alexander; Galliero, Guillaume; Montel, François; Legros, Jean Claude; Caltagirone, Jean Paul; Daridon, Jean-Luc; Saghir, Ziad Bibcode: 2005ESASP1290..202V Altcode: Crude oils contain billions of components distributed in a surprisingly non-homogeneous way. When performing very costly drillings, oil companies need to deduce the overall state of the reservoir from the geological data. Improving current prediction capabilties is the focus of the Diffusion and Soret Coefficients of Crude Oils MAP team. The appraoch is to determine all the transport properties of crude oils by describing their transport processes. These include the Soret effect, the tiny separation of components induced by the geothermal gradient (about 30K/km). Theoretical predictive developments will optimise the return of measurements from the experiment aboard the Foton-M2 satellite and the SoDiUM experiment. Validation by studies of g-sensivity and the crucial role of asphaltenes are the most probable extensions of the studies. Title: The Role of Epithermal Neutrons in AGB Stars: Boron Synthesis Authors: Shapiro, A. I. Bibcode: 2004AstL...30..404S Altcode: We consider the influence of the capture of epithermal neutrons on the nucleosynthesis in asymptotic giant branch stars (the Petrov-Shlyakhter effect). We show that epithermal neutrons can be captured by nitrogen through the hitherto unanalyzed channel N-14(n,alpha)B-11. Since the proton concentration in the partial mixing zone is low, this process results in an appreciable boron concentration. This boron can be brought to the stellar surface by peculiar processes. We analyze the boron concentration as a function of the assumed parameters for the partial mixing zone. Title: Line Formation in a Purely Scattering, Optically Thick Atmosphere Authors: Shapiro, A. I. Bibcode: 2002Ap.....45..215S Altcode: A model problem in the theory of line formation in an optically thick, purely scattering, stellar atmosphere is considered. The integral equation of radiation transfer at line frequencies is solved numerically for a two-level atom in the approximation of complete frequency redistribution in scattering. The numerical results are compared with those calculated from equations of the asymptotic theory. On the basis of the asymptotic theory, the positions of intensity maxima in a line are found for different absorption profiles. Title: 1998 HH49 Authors: Veillet, C.; Shapiro, A.; Williams, G. V. Bibcode: 2000MPEC....Y...39V Altcode: No abstract at ADS Title: Grazing incidence X-ray reflectivity - Studies for the AXAF observatory Authors: Slane, P.; Schwartz, D.; van Speybroeck, L.; Jones, D.; Chappell, J.; Bilbro, J.; Shapiro, A.; Dave, S.; Kidd, P.; Texter, S. Bibcode: 1992SPIE.1546...26S Altcode: 1992SPIE.2011...26S The energy bandwidth and total throughput of a grazing incidence optics system is a strong function of the X-ray reflectivity of the surface coating. In support of the Advanced X-ray Astrophysics Facility (AXAF), studies are underway to evaluate and characterize the reflectivity of potential AXAF coatings. Here we report on results obtained for Au, Ir, and Ni coatings produced by electron-beam evaporation, evaporation with ion-assist, and sputtering. Effects of coating thickness and deposition angle have been evaluated at 6.4 and 8.1 keV; the highest reflectivities are those of the thinner, about 200 A vs about 700 A, coatings. While considerable variations exist, the best Ir samples have higher reflectivity than any of the Au coatings. Data results have been compared with models for theoretical reflectivity, particularly with regard to the effective density of the coatings. Independent measurements of the coating densities have been carried out for comparison with the reflectivity results. Title: Potential of Satellite Radar Altimetry for Determination of Short Wavelength Geoidal Undulations Authors: Shapiro, A.; Yaplee, B. S. Bibcode: 1974uasg.proc..481S Altcode: No abstract at ADS Title: Radar Measurements of Lunar Surface Roughness Authors: Shapiro, A.; Uliana, E. A.; Yaplee, B. S. Bibcode: 1970sarr.conf..145S Altcode: No abstract at ADS Title: Lunar Radius from Radar Measurements Authors: Shapiro, A.; Uliana, E. A.; Yaplee, B. S.; Knowles, S. H. Bibcode: 1968mopl.book...34S Altcode: No abstract at ADS Title: The mean distance to the Moon as determined by radar Authors: Yaplee, B. S.; Knowles, S. H.; Shapiro, A.; Craig, K. J.; Brouwer, D. Bibcode: 1965IAUS...21...81Y Altcode: No abstract at ADS Title: Design and preliminary results of an air shower experiment Authors: Sitte, K.; Davies, G.; Kasha, H.; Lerman, N.; Oren, Y.; Shapiro, A.; Segal, I.; Stern, D. Bibcode: 1960ICRC....2...44S Altcode: 1960ICRC....6b..44S No abstract at ADS