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. <BR
  /> 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. <BR
  /> 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. <BR />
  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 10<SUP>2</SUP>-10<SUP>3</SUP> in the
  upper chromosphere and by up to 10<SUP>9</SUP> near the transition
  region. <BR /> 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. <P />Movies associated to Fig. 2 are only available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141230/olm">https://www.aanda.org</A>

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. <BR /> 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. <BR
  /> 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. <BR /> 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
  T<SUB>eff</SUB>. 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. <BR /> 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 <SUB>⊙</SUB>. 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />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. <P />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 (<A
  href="https://ui.adsabs.harvard.edu/abs/2020Sci...368..518R/abstract">Reinhold
  et al. 2020</A>). 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&amp;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&amp;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&amp;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&amp;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 &amp;
    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. <P />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 &amp; 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). <P />(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<SUP>-2</SUP> to a rise of 6.3 W m<SUP>-2</SUP>. 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<SUP>-2</SUP> 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. <BR />
  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. <BR /> 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). <BR /> 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. <BR /> 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. <P
  />Full Table 2 is only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A225">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A225</A>

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. <BR />
  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. <BR />
  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. <BR /> 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 G<SUB>RP</SUB> (0.62 mmag), and Gaia G<SUB>BP</SUB> (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. <BR /> 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 G<SUB>RP</SUB>
  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. <P />Stellar fundamental parameters and rotation
  periods of the 97 periodic stars are presented. <P />(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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
  Conclusions: Our results suggest that the GPS method can successfully
  determine the rotational periods of stars with both regular and
  non-regular light curves. <P />The two movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936925/olm">https://www.aanda.org</A>

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. <BR /> 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. <BR /> 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. <BR />
  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. <BR
  /> 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. <BR
  /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <BR /> 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. <BR /> 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). <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> Aims: We investigate the underlying
  process that causes magnetic activity by studying the appearance of
  activity signatures in contemporaneous photometric and chromospheric
  time series. <BR /> 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. <BR /> 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. <BR /> 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. <P />Photometric and Mount
  Wilson data are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A21">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/621/A21</A>

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. <BR /> 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. <BR /> 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. <BR /> 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<SUB>⊙</SUB>; and (2) the tilt angles of freshly emerged bipolar
  regions show stronger variations with latitude. Polar spots can form
  at 8 Ω<SUB>⊙</SUB> by accumulation of follower-polarity flux from
  decaying bipolar regions. From 4 Ω<SUB>⊙</SUB> to 8 Ω<SUB>⊙</SUB>,
  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. <BR /> Conclusions: On Sun-like
  stars rotating at 8 Ω<SUB>⊙</SUB> (P<SUB>rot</SUB> ≃ 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. <BR /> 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. <BR /> 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. <BR /> 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 &amp;
  Guinan (2002A&amp;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). <P />(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<SUP>-2</SUP> 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. <BR /> Aims: We aim to establish a plausible
  range for the solar irradiance variability on decadal to millennial
  timescales. <BR /> 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. <BR /> 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<SUP>-2</SUP> 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 &amp;
  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 variations<SUP>1,2</SUP>,
  often referred to as solar noise<SUP>3</SUP>, is important because
  solar noise limits the detection of solar oscillations<SUP>3</SUP>,
  is one of the drivers of the Earth's climate system<SUP>4,5</SUP> and
  is a prototype of stellar variability<SUP>6,7</SUP>—an important
  limiting factor for the detection of extrasolar planets. Here,
  we model the magnetic contribution to solar brightness variability
  using high-cadence<SUP>8,9</SUP> observations from the Solar Dynamics
  Observatory (SDO) and the Spectral And Total Irradiance REconstruction
  (SATIRE)<SUP>10,11</SUP> 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)<SUP>12</SUP> 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)<SUP>13</SUP> and Kepler<SUP>14</SUP>
  missions uncovered brightness variations similar to that of the Sun,
  but with a much wider variety of patterns<SUP>15</SUP>. 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 Satellite<SUP>16</SUP> and PLAnetary Transits and Oscillations
  of stars (PLATO)<SUP>17</SUP> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> Aims:
  We estimate the relative contributions of the continuum, molecular,
  and atomic lines to the solar brightness variations on different
  timescales. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />Appendices are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201323086/olm">http://www.aanda.org</A>

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&amp;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. <BR /> 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. <BR />
  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. <BR />
  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. <P />Appendix A is available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

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. <P />We aim
  to identify possible reasons for the Sun to be currently outside of
  this relationship. <P />We employed different scenarios of solar HK
  emission and irradiance variability and compared them with available
  time series of Sun-like stars. <P />(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. <BR /><BR /> 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. <BR /><BR /> 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. <BR /><BR
  /> 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. <BR /><BR /> 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. <BR /><BR /> 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. <BR /> Aims:
  We aim to identify possible reasons for the Sun to be currently
  outside of this relationship. <BR /> Methods: We employed different
  scenarios of solar HK emission and irradiance variability and
  compared them with available time series of Sun-like stars. <BR />
  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. <P />Appendices A and B are available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>All
  time series are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A114">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/552/A114</A>

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&amp;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 (&gt;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 H<SUB>2</SUB>O 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 (H<SUB>2</SUB>O) 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 H<SUB>2</SUB>O 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 H<SUB>2</SUB>O anticorrelates (correlation coefficients up to
  -0.74) with the solar irradiance at non-zero time-lag. We found that
  the response of OH and H<SUB>2</SUB>O 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
  H<SUB>2</SUB>O 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 C<SUB>2</SUB> 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 C<SUB>2</SUB> 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. <BR
  /> Aims: We have developed a non-LTE analyzing technique for the
  C<SUB>2</SUB> 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). <BR /> 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 C<SUB>2</SUB> 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. <BR /> 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 C<SUB>2</SUB> 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 17<SUP>th</SUP> 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. <BR />
  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. <BR
  /> 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
  <SUP>10</SUP>Be 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. <BR /> 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. <P />Appendix is only available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

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. <BR /> 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 B<SUP>2</SUP>Σ - X<SUP>2</SUP>Σ
  system. Here we present an analysis of these observations. <BR />
  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. <BR /> 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 (SOCOL<SUP>i</SUP>) 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
  NO<SUB>x</SUB>, HO<SUB>x</SUB>, ClO<SUB>x</SUB> and O<SUB>3</SUB> 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 NO<SUB>x</SUB> and N<SUB>2</SUB>O, up to 70% for HO<SUB>x</SUB> and
  ClO<SUB>x</SUB> and up to 600% for HNO<SUB>3</SUB>. 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. <BR /> 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. <BR /> 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<SUP>-5</SUP>. In addition, we analyzed
  measurements taken at different limb distances. We measure signatures
  in the 5141 Å region including two C<SUB>2</SUB> 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 C<SUB>2</SUB> 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. <BR /> 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. <BR /> 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. <BR
  /> 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. <BR
  /> 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. <BR /> 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. <P />Figures 13 and 14 are only available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

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<SUP>-5</SUP> 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. <P />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. <P />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. <P />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. <P />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. <BR
  />Aims: Molecular lines are very strong and even dominate in some
  regions of the second solar spectrum. The CN B {}<SUP>2</SUP> Σ -
  X {}<SUP>2</SUP> 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 {}<SUP>2</SUP> Σ - X {}<SUP>2</SUP> Σ system
  using the Hanle effect and to obtain an estimation of the magnetic
  field strength. <BR />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. <BR />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. <P />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. <BR
  />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. <BR />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. <BR />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 {}<SUP>2</SUP> Σ - X {}<SUP>2</SUP> Σ and MgH B' {}<SUP>2</SUP>
  Σ - X {}<SUP>2</SUP> 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

