krivova

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Author name code: krivova
ADS astronomy entries on 2022-09-14
author:"Krivova, Natalie A."

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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.
2022ApJ...936L..17N Altcode:
No abstract at ADS

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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.
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.

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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.
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.

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Title: Solar cyclic activity over the last millennium
reconstructedfrom annual 14C data (Corrigendum)
Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.;
Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B.
2022A&A...664C...3U Altcode:
No abstract at ADS

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Title: Reconstructions of solar activity and variability from
cosmogenic isotope data
Authors: Krivova, Natalie; Usoskin, Ilya
2022cosp...44.1046K Altcode:
Our knowledge about the activity and variability of the Sun on the
millennial time scale relies on cosmogenic isotope data. Modulated by
the solar open magnetic field, the fluctuating flux of the energetic
cosmic ray particles in the vicinity of Earth leads to the formation
of varying amounts of the radionuclides in the atmosphere. These
radionuclides are naturally stored in terrestrial archives for millennia
and can be measured in laboratories nowadays. In combination with
appropriate models, such data then allow us to go back in time and
understand how the Sun behaved in the past. We will review the latest
progress in understanding and reconstruction of the solar activity
over the last millennia.

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Title: Historical Ca II K observations for irradiance reconstructions
Authors: Chatzistergos, Theodosios; Krivova, Natalie; Ermolli, Ilaria;
Leng Yeo, Kok; Solanki, Sami; Mandal, Sudip
2022cosp...44.1551C Altcode:
Solar irradiance has been measured by various instruments
from space since 1978 and exhibits variations on all accessible
timescales. Irradiance variations prior to satellite measurements,
knowledge of which is important for climate studies, can only be
recovered with models. Models have successfully been used to reconstruct
solar irradiance variations by attributing the variability on time
scales longer than about a day to the evolution of the solar surface
magnetic field. However, to account for the different contributions of
surface magnetic features, such as sunspots and faculae, models require
appropriate input data. Unfortunately, due to the scarcity of suitable
facular data prior to the period of direct irradiance measurements,
most available irradiance reconstructions rely on sunspot data alone or
on other, indirect data of solar magnetism. Historical Ca II K archives
carry direct information on facular regions, but they have not been
used to their full potential for irradiance reconstructions. The main
reasons limiting their use were the difficulty to account for the
non-linear response of the photographic plates along with plentiful
large-scale artefacts affecting the images. We have extensively studied
Ca II K data to overcome most issues affecting them thus paving the way
for them to be used for accurate irradiance reconstructions. We will
present our latest work on employment of Ca II K data for reconstructing
past irradiance variations.

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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.
2022AJ....163..205K Altcode: 2021arXiv211206383K
Astrometry has long been a promising technique for exoplanet
detection. At the theoretical limits, astrometry would allow for
the detection of smaller planets than previously seen by current
exoplanet search methods, but stellar activity may make these
theoretical limits unreachable. Astrometric jitter of a Sun-like
star due to magnetic activity in its photosphere induces apparent
variability in the photocenter of order 0.5 mR ⊙. This
jitter creates a fundamental astrophysical noise floor preventing
detection of lower-mass planets in a single spectral band. By injecting
planet orbits into simulated solar data at five different passbands,
we investigate mitigation of this fundamental astrometric noise using
correlations across passbands. For a true solar analog and a planet at
1 au semimajor axis, the 6σ detection limit set by stellar activity for
an ideal telescope at the best single passband is 0.01 Earth masses. We
found that pairs of passbands with highly correlated astrometric jitter
due to stellar activity, but with less motion in the redder band,
enable higher-precision measurements of the common signal from the
planet. Using this method improves detectable planet masses at 1 au
by up to a factor of 10, corresponding to at best 0.005 Earth masses
for a Sun-like star with a perfect telescope. Given these results,
we recommend that future astrometry missions consider proceeding with
two or more passbands to reduce noise due to stellar activity.

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Title: Time Lag Between Cosmic-Ray and Solar Variability: Sunspot
Numbers and Open Solar Magnetic Flux
Authors: Koldobskiy, Sergey A.; Kähkönen, Riikka; Hofer, Bernhard;
Krivova, Natalie A.; Kovaltsov, Gennady A.; Usoskin, Ilya G.
2022SoPh..297...38K Altcode:
Solar magnetic activity drives the dominant 11-year cyclic variability
of different space environmental indices, but they can be delayed
with respect to the original variations due to the different physical
processes involved. Here, we analyzed the pairwise time lags between
three global solar and heliospheric indices: sunspot numbers (SSN),
representing the solar surface magnetic activity, the open solar flux
(OSF), representing the heliospheric magnetic variability, and the
galactic cosmic-ray (GCR) intensity near Earth, using the standard
cross-correlation and the more detailed wavelet-coherence methods. All
the three indices appear highly coherent at a timescale longer than
a few years with persistent high coherence at the timescale of the
11-year solar cycle. The GCR variability is delayed with respect to
the inverted SSN by about eight 27-day Bartels rotations on average,
but the delay varies greatly with the 22-year cycle, being shorter or
longer around positive A + or negative A − solar polarity epochs,
respectively. The 22-year cyclicity of the time lag is determined by
the global heliospheric drift effects, in agreement with theoretical
models. The OSF lags by about one year behind SSN, and is likely
determined by a combination of the short lifetime of active regions and
a longer (≈3 years) transport time of the surface magnetic field to
the poles. GCRs covary nearly in antiphase with the OSF, also depicting
a strong 22-year cycle in the delay, confirming that the OSF is a
good index of the heliospheric modulation of GCRs. This provides an
important observational constraint for solar and heliospheric physics.

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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.
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").

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Title: Reconstructing solar irradiance from historical Ca II K
observations. I. Method and its validation
Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli,
Ilaria; Yeo, Kok Leng; Mandal, Sudip; Solanki, Sami K.; Kopp, Greg;
Malherbe, Jean-Marie
2021A&A...656A.104C Altcode: 2021arXiv210905844C
Context. Knowledge of solar irradiance variability is critical to
Earth's climate models and understanding the solar influence on Earth's
climate. Direct solar irradiance measurements have only been available
since 1978. Reconstructions of past variability typically rely on
sunspot data. However, sunspot records provide only indirect information
on the facular and network regions, which are decisive contributors to
irradiance variability on timescales of the solar cycle and longer. Aims: Our ultimate goal is to reconstruct past solar irradiance
variations using historical full-disc Ca II K observations to describe
the facular contribution independently of sunspot observations. Here,
we develop the method and test it extensively by using modern CCD-based
(charge-coupled device) Ca II K observations. We also carry out initial
tests on two photographic archives. Methods: We employ carefully
reduced and calibrated Ca II K images from 13 datasets, including some
of the most prominent series, such as those from the Meudon, Mt Wilson,
and Rome observatories. We convert them to unsigned magnetic field
maps and then use them as input to the adapted Spectral and Total
Irradiance Reconstruction (SATIRE) model to reconstruct total solar
irradiance (TSI) variations over the period 1978-2019, for which
direct irradiance measurements are available. Results: The
reconstructed irradiance from the analysed Ca II K archives agrees well
with direct irradiance measurements and existing reconstructions. The
model also returns good results on data taken with different bandpasses
and images with low spatial resolution. Historical Ca II K archives
suffer from numerous inconsistencies, but we show that these archives
can still be used to reconstruct TSI with reasonable accuracy provided
the observations are accurately processed and the effects of changes
in instrumentation and instrumental parameters are identified and
accounted for. The reconstructions are relatively insensitive to the
TSI reference record used to fix the single free parameter of the
model. Furthermore, even employment of a series, itself reconstructed
from Ca II K data, as a reference for further reconstructions returns
nearly equally accurate results. This will enable the Ca II K archives
without an overlap with direct irradiance measurements to be used to
reconstruct past irradiance. Conclusions: By using the unsigned
magnetic maps of the Sun reconstructed from modern high-quality Ca
II K observations as input into the SATIRE model, we can reconstruct
solar irradiance variations nearly as accurately as from directly
recorded magnetograms. Historical Ca II K observations can also be
used for past irradiance reconstructions but need additional care,
for example identifying and accounting for discontinuities and changes
in the quality of the data with time.

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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.
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.

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Title: The relationship between bipolar magnetic regions and their
sunspots
Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Jiang, J.
2021A&A...654A..28Y Altcode: 2021arXiv210914313Y
Context. The relationship between bipolar magnetic regions (BMRs)
and their sunspots is an important property of the solar magnetic
field, but it is not well constrained. One consequence is that it is a
challenge for surface flux transport models (SFTMs) based on sunspot
observations to determine the details of BMR emergence, which they
require as input, from such data. Aims: We aimed to establish
the relationship between the amount of magnetic flux in newly emerged
BMRs and the area of the enclosed sunspots, and examine the results
of its application to an established SFTM. Methods: Earlier
attempts to constrain BMR magnetic flux were hindered by the fact that
there is no extensive and reliable record of the magnetic and physical
properties of newly emerged BMRs currently available. We made use of
the empirical model of the relationship between the disc-integrated
facular and network magnetic flux and the total surface coverage by
sunspots reported in a recent study. The structure of the model is such
that it enabled us to establish, from these disc-integrated quantities,
an empirical relationship between the magnetic flux and sunspot area
of individual newly emerged BMRs, circumventing the lack of any proper
BMR database. Results: Applying the constraint on BMR magnetic
flux derived here to an established SFTM retained its key features,
in particular its ability to replicate various independent datasets
and the correlation between the model output polar field at the end
of each cycle and the observed strength of the following cycle. The
SFTM output indicates that facular and network magnetic flux rises
with increasing sunspot magnetic flux at a slowing rate such that
it appears to gradually saturate. This is analogous to what earlier
studies comparing disc-integrated quantities sensitive to the amount
of faculae and network present to sunspot indices had reported. The
activity dependence of the ratio of facular and network flux to sunspot
flux is consistent with the findings of recent studies: although the
Sun is faculae-dominated (such that its brightness is mostly positively
correlated with activity), it is only marginally so as facular and
network brightening and sunspot darkening appear to be closely balanced.

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Title: On the size distribution of spots within sunspot groups
Authors: Mandal, Sudip; Krivova, Natalie A.; Cameron, Robert; Solanki,
Sami K.
2021A&A...652A...9M Altcode: 2021arXiv210403534M
The size distribution of sunspots provides key information about
the generation and emergence processes of the solar magnetic
field. Previous studies of size distribution have primarily focused
on either the whole group or individual spot areas. In this paper we
investigate the organisation of spot areas within sunspot groups. In
particular, we analysed the ratio (R) of the area of the biggest spot
(Abig_spot) inside a group, to the total area of that group
(Agroup). We used sunspot observations from Kislovodsk,
Pulkovo, and Debrecen observatories, together covering solar cycles
17-24. We find that at the time when the group area reaches its maximum,
the single biggest spot in a group typically occupies about 60% of the
group area. For half of all groups, R lies in the range between roughly
50% and 70%. We also find R to change with Agroup, such that
R reaches a maximum of about 0.65 for groups with Agroup
≈ 200 μHem and then remains at about 0.6 for larger groups. Our
findings imply a scale-invariant emergence pattern, providing an
observational constraint on the emergence process. Furthermore,
extrapolation of our results to larger sunspot groups may have a
bearing on the giant unresolved starspot features found in Doppler
images of highly active Sun-like stars. Our results suggest that such
giant features are composed of multiple spots, with the largest spot
occupying roughly 55-75% of the total group area (i.e., the area of
the giant starspots seen in Doppler images).

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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.
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.

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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.
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.

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Title: Modelling the evolution of the Sun's open and total magnetic
flux
Authors: Krivova, N. A.; Solanki, S. K.; Hofer, B.; Wu, C. -J.;
Usoskin, I. G.; Cameron, R.
2021A&A...650A..70K Altcode: 2021arXiv210315603K
Solar activity in all its varied manifestations is driven by the
magnetic field. Two global quantities are particularly important for
many purposes, the Sun's total and open magnetic flux, which can be
computed from sunspot number records using models. Such sunspot-driven
models, however, do not take into account the presence of magnetic
flux during grand minima, such as the Maunder minimum. Here we
present a major update of a widely used simple model, which now takes
into account the observation that the distribution of all magnetic
features on the Sun follows a single power law. The exponent of the
power law changes over the solar cycle. This allows for the emergence
of small-scale magnetic flux even when no sunspots have been present
for multiple decades and leads to non-zero total and open magnetic
flux also in the deepest grand minima, such as the Maunder minimum,
thus overcoming a major shortcoming of the earlier models. The results
of the updated model compare well with the available observations and
reconstructions of the solar total and open magnetic flux. This opens
up the possibility of improved reconstructions of the sunspot number
from time series of the cosmogenic isotope production rate.

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Title: Solar cyclic activity over the last millennium reconstructed
from annual 14C data
Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N. A.; Hofer, B.;
Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B.
2021A&A...649A.141U Altcode: 2021arXiv210315112U
 Aims: The 11-year solar cycle (Schwabe cycle) is the dominant
pattern of solar magnetic activity reflecting the oscillatory dynamo
mechanism in the Sun's convection zone. Solar cycles have been
directly observed since 1700, while indirect proxy data suggest their
existence over a much longer period of time, but generally without
resolving individual cycles and their continuity. Here we reconstruct
individual solar cycles for the last millennium using recently
obtained 14C data and state-of-the-art models. 
Methods: Starting with the 14C production rate determined
from the so far most precise measurements of radiocarbon content in
tree rings, solar activity was reconstructed in the following three
physics-based steps: (1) correction of the 14C production
rate for the changing geomagnetic field; (2) computation of the
open solar magnetic flux; and (3) conversion into sunspot numbers
outside of grand minima. All known uncertainties, including both
measurement and model uncertainties, were straightforwardly accounted
for by a Monte-Carlo method. Results: Cyclic solar activity is
reconstructed for the period 971-1900 (85 individual cycles) along with
its uncertainties. This more than doubles the number of solar cycles
known from direct solar observations. We found that the lengths and
strengths of well-defined cycles outside grand minima are consistent
with those obtained from the direct sunspot observations after 1750. The
validity of the Waldmeier rule (cycles with fast-rising phase tend to
be stronger) is confirmed at a highly significant level. Solar activity
is found to be in a deep grand minimum when the activity is mostly
below the sunspot formation threshold for about 250 years. Therefore,
although considerable cyclic variability in 14C is seen even
during grand minima, individual solar cycles can hardly be reliably
resolved therein. Three potential solar particle events, ca. 994,
1052, and 1279 AD, are shown to occur around the maximum phases of
solar cycles. Conclusions: A new approximately 1000-year-long
solar activity reconstruction, in the form of annual (pseudo) sunspot
numbers with the full assessment of all known uncertainties, is
presented based on new high-precision Δ14C measurements and
state-of-the-art models, more than doubling the number of individually
resolved solar cycles. This forms a solid basis for new, more detailed
studies of solar variability. Tabular data of the reconstructed
activity are only available at the CDS via anonymous ftp to <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/cat/J/A+A/649/A141">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/649/A141</A>

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Title: VizieR Online Data Catalog: 1000-year sunspot series (Usoskin+,
2021)
Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.;
Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B.
2021yCat..36490141U Altcode:
A new quantitative reconstruction of annually resolved solar activity,
in the form of SNs (at least outside grand minima) with a full
uncertainty assessment, is presented for the period 971-1900. For the
first time, individual solar cycles are presented for the whole of the
last millennium, more than doubling the existing statistics of solar
cycles. Three tabular files are presented. File 'osf.dat'
contains annual reconstructions of the opens solar flux (OSF) [Fo]
along with its 1-sigma uncertainties [sF] and the smoothed (22-yr SSA --
see text) values [&lt;Fo&gt;]. This table corresponds to Figure 8 in
the text. File 'osn.dat' contains annual reconstructions of the
sunspot number (SN) [SN] along with its 1-sigma uncertainties [sSN]
and the smoothed (22-yr SSA -- see text) values [&lt;SN&gt;]. This
table corresponds to Figure 11 in the text. File 'table1.dat'
presents a textual tabular version of Table 1 in the text and contains
the internal cycle number [n], years of minimum [Ymin] and maximum
[Ymax] of each cycle, cycle-averaged sunspot number [&lt;SN&gt;], its
1-sigma uncertainty [sS], cycle length (min-to-min) [T], and quality
flag [q]. (3 data files).

---------------------------------------------------------
Title: Amplifying variability of solar-like stars by active longitudes
and nesting
Authors: Isik, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova,
Natalie A.
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.
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: 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.
2021ApJ...908..223S Altcode: 2020arXiv201212312S
The advent of Gaia, capable of measuring stellar wobbles caused
by orbiting planets, raised interest in the astrometric detection
of exoplanets. Another source of such wobbles (often also called
jitter) is stellar magnetic activity. A quantitative assessment
of the stellar astrometric jitter is important for a more reliable
astrometric detection and characterization of exoplanets. We calculate
the displacement of the solar photocenter due to the magnetic activity
for an almost 16 yr period (1999 February 2-2014 August 1). We also
investigate how the displacement depends on the spectral passband chosen
for observations, including the wavelength range to be covered by the
upcoming Small-JASMINE mission of JAXA. This is done by extending the
SATIRE-S model for solar irradiance variability to calculating the
displacement of the solar photocenter caused by the magnetic features
on the surface of the Sun. We found that the peak-to-peak amplitude
of the solar photocenter displacement would reach 0.5 μas if the
Sun were located 10 pc away from the observer and observed in the
Gaia G filter. This is by far too small to be detected by the Gaia
mission. However, the Sun is a relatively inactive star so one can
expect significantly larger signals for younger, and, consequently,
more active stars. The model developed in this study can be combined
with the simulations of emergence and surface transport of magnetic
flux which have recently become available to model the astrometric
jitter over the broad range of magnetic activities.

---------------------------------------------------------
Title: Reconstructing solar irradiance from Ca II K observations
Authors: Chatzistergos, T.; Krivova, N.; Ermolli, I.; Yeo, K. L.;
Solanki, S. K.; Puiu, C. C.; Giorgi, F.; Mandal, S.
2020AGUFMA237...10C Altcode:
To understand the influence of the Sun on Earth's system, long and
accurate measurements of solar irradiance are a prerequisite. The
available direct measurements of solar irradiance since 1978 are
clearly not sufficient for this purpose. This stimulated development
of models used to reconstruct past solar irradiance variations from
alternative observations. The main driver of the irradiance variations
on time scales of days to millennia is the evolution of the solar
surface magnetic field in form of dark sunspots and bright faculae
and network. Therefore, models require input data describing the
contributions of these various magnetic regions on the Sun at earlier
times. Unfortunately, records that can be used to describe the facular
and network contributions are barely longer than the direct irradiance
measurements. Thus, irradiance reconstructions to earlier periods have
to rely on sunspot data alone. Data that have hardly been used for solar
irradiance reconstructions until now are full-disc solar observations in
the Ca II K line. Such data exist since 1892 from various observatories
and include all the needed information describing faculae and the
network. However, they are plagued by a bunch of various problems and
artefacts, and recovering the non-linear response of the photographic
material to the radiation is non-trivial since the required information
is usually missing, too. We have developed a method to process Ca II K
observations from various sources and demonstrated the higher accuracy
achieved by our method compared to other techniques presented in the
literature. Here we use the carefully reduced Ca II K observations
from multiple archives to reconstruct solar irradiance variations.

---------------------------------------------------------
Title: Erratum: "Amplification of Brightness Variability by
Active-region Nesting in Solar-like Stars" (2020, ApJL, 901, L12)
Authors: Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.;
Krivova, Natalie A.
2020ApJ...905L..36I Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Reconstructing solar irradiance from Ca II K observations
Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli,
Ilaria; Leng Yeo, Kok; Solanki, Sami K.; Puiu, Cosmin C.; Giorgi,
Fabrizio; Mandal, Sudip
2020AGUFM.A237..10C Altcode:
To understand the influence of the Sun on Earth's system, long and
accurate measurements of solar irradiance are a prerequisite. The
available direct measurements of solar irradiance since 1978 are
clearly not sufficient for this purpose. This stimulated development
of models used to reconstruct past solar irradiance variations from
alternative observations. The main driver of the irradiance variations
on time scales of days to millennia is the evolution of the solar
surface magnetic field in form of dark sunspots and bright faculae
and network. Therefore, models require input data describing the
contributions of these various magnetic regions on the Sun at earlier
times. Unfortunately, records that can be used to describe the facular
and network contributions are barely longer than the direct irradiance
measurements. Thus, irradiance reconstructions to earlier periods have
to rely on sunspot data alone. Data that have hardly been used for solar
irradiance reconstructions until now are full-disc solar observations in
the Ca II K line. Such data exist since 1892 from various observatories
and include all the needed information describing faculae and the
network. However, they are plagued by a bunch of various problems and
artefacts, and recovering the non-linear response of the photographic
material to the radiation is non-trivial since the required information
is usually missing, too. We have developed a method to process Ca II K
observations from various sources and demonstrated the higher accuracy
achieved by our method compared to other techniques presented in the
literature. Here we use the carefully reduced Ca II K observations
from multiple archives to reconstruct solar irradiance variations.

---------------------------------------------------------
Title: 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.
2020GeoRL..4790243Y Altcode: 2021arXiv210209487Y
How the solar electromagnetic energy entering the Earth's atmosphere
varied since preindustrial times is an important consideration in
the climate change debate. Detrimental to this debate, estimates
of the change in total solar irradiance (TSI) since the Maunder
minimum, an extended period of weak solar activity preceding the
industrial revolution, differ markedly, ranging from a drop of 0.75
W m-2 to a rise of 6.3 W m-2. Consequently, the
exact contribution by solar forcing to the rise in global temperatures
over the past centuries remains inconclusive. Adopting a novel approach
based on state-of-the-art solar imagery and numerical simulations, we
establish the TSI level of the Sun when it is in its least-active state
to be 2.0 ± 0.7 W m-2 below the 2019 level. This means TSI
could not have risen since the Maunder minimum by more than this amount,
thus restricting the possible role of solar forcing in global warming.

---------------------------------------------------------
Title: Simulating Variability due to Faculae and Spots on GKM Stars
Authors: Johnson, Luke; Unruh, Yvonne; Norris, Charlotte; Solanki,
Sami; Krivova, Natalie; Witzke, Veronika; Shapiro, Alexander
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.
2020ApJ...901L..12I Altcode: 2020arXiv200900692I
Kepler observations revealed that hundreds of stars with near-solar
fundamental parameters and rotation periods have much stronger and
more regular brightness variations than the Sun. Here we identify one
possible reason for the peculiar behavior of these stars. Inspired by
solar nests of activity, we assume that the degree of inhomogeneity
of active-region (AR) emergence on such stars is higher than on the
Sun. To test our hypothesis, we model stellar light curves by injecting
ARs consisting of spots and faculae on stellar surfaces at various
rates and nesting patterns, using solar AR properties and differential
rotation. We show that a moderate increase of the emergence frequency
from the solar value combined with the increase of the degree of nesting
can explain the full range of observed amplitudes of variability of
Sun-like stars with nearly the solar rotation period. Furthermore,
nesting in the form of active longitudes, in which ARs tend to emerge
in the vicinity of two longitudes separated by 180°, leads to highly
regular, almost sine-like variability patterns, rather similar to
those observed in a number of solar-like stars.

---------------------------------------------------------
Title: Sunspot area catalog revisited: Daily cross-calibrated areas
since 1874
Authors: Mandal, Sudip; Krivova, Natalie A.; Solanki, Sami K.; Sinha,
Nimesh; Banerjee, Dipankar
2020A&A...640A..78M Altcode: 2020arXiv200414618M
Context. Long and consistent sunspot area records are important for
understanding long-term solar activity and variability. Multiple
observatories around the globe have regularly recorded sunspot
areas, but such individual records only cover restricted periods of
time. Furthermore, there are systematic differences between these
records and require cross-calibration before they can reliably be used
for further studies. Aims: We produce a cross-calibrated and
homogeneous record of total daily sunspot areas, both projected and
corrected, covering the period between 1874 and 2019. In addition,
we generated a catalog of calibrated individual group areas for
the same period. Methods: We compared the data from nine
archives: Royal Greenwich Observatory (RGO), Kislovodsk, Pulkovo,
Debrecen, Kodaikanal, Solar Optical Observing Network (SOON), Rome,
Catania, and Yunnan Observatories, covering the period between
1874 and 2019. Cross-comparisons of the individual records were
done to produce homogeneous and inter-calibrated records of daily
projected and corrected areas. As in earlier studies, the basis
of the composite is formed by the data from RGO. After 1976, the
only datasets used are those from Kislovodsk, Pulkovo, and Debrecen
observatories. This choice was made based on the temporal coverage
and the quality of the data. While there are still 776 days missing
in the final composite, these remaining gaps could not be filled
with data from the other archives as the missing days lie either
before 1922 or after 2016 and none of the additional archives cover
these periods. Results: In contrast to the SOON data used in
previous area composites for the post-RGO period, the properties of
the data from Kislovodsk and Pulkovo are very similar to those from
the RGO series. They also directly overlap the RGO data in time, which
makes their cross-calibration with RGO much more reliable. Indeed,
comparing our area catalog with previous such composites, we find
improvements both in data quality and coverage. We also computed the
daily Photometric Sunspot Index, which is widely used, for example,
in empirical reconstructions of solar irradiance. Generated
composites are available at the CDS via anonymous ftp to <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/640/A78">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/A78</A>
and at <A
href="http://www2.mps.mpg.de/projects/sun-climate/data.html">http://www2.mps.mpg.de/projects/sun-climate/data.html</A>

---------------------------------------------------------
Title: Modelling solar irradiance from ground-based photometric
observations
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Giorgi, Fabrizio;
Krivova, Natalie A.; Puiu, Cosmin Constantin
2020JSWSC..10...45C Altcode: 2020arXiv200810735C
Total solar irradiance (TSI) has been monitored from space since 1978,
i.e. for about four solar cycles. The measurements show a prominent
variability in phase with the solar cycle, as well as fluctuations on
timescales shorter than a few days. However, the measurements were done
by multiple and usually relatively short-lived missions. The different
absolute calibrations of the individual instruments and the unaccounted
for instrumental trends make estimates of the possible long-term trend
in the TSI highly uncertain. Furthermore, both the variability and the
uncertainty are strongly wavelength-dependent. While the variability
in the UV irradiance is clearly in-phase with the solar cycle, the
phase of the variability in the visible range has been debated. In
this paper, we aim at getting an insight into the long-term trend of
TSI since 1996 and the phase of the solar irradiance variations in
the visible part of the spectrum. We use independent ground-based
full-disc photometric observations in Ca II K and continuum from
the Rome and San Fernando observatories to compute the TSI since
1996. We follow the empirical San Fernando approach based on
the photometric sum index. We find a weak declining trend in the
TSI of {-7.8}-0.8+4.9× 1{0}-3
Wm-2 y-1 between the 1996 and 2008 activity
minima, while between 2008 and 2019 the reconstructed TSI shows no
trend to a marginally decreasing (but statistically insignificant)
trend of {-0.1}-0.02+0.25× 1{0}-3
Wm-2 y-1. The reference TSI series used for the
reconstruction does not significantly affect the determined trend. The
variation in the blue continuum (409.2 nm) is rather flat, while the
variation in the red continuum (607.1 nm) is marginally in anti-phase,
although this result is extremely sensitive to the accurate assessment
of the quiet Sun level in the images. These results provide further
insights into the long-term variation of the TSI. The amplitude of the
variations in the visible is below the uncertainties of the processing,
which prevents an assessment of the phase of the variations.

---------------------------------------------------------
Title: Analysis of full-disc Ca II K spectroheliograms. III. Plage
area composite series covering 1892-2019
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova,
Natalie A.; Solanki, Sami K.; Banerjee, Dipankar; Barata, Teresa;
Belik, Marcel; Gafeira, Ricardo; Garcia, Adriana; Hanaoka, Yoichiro;
Hegde, Manjunath; Klimeš, Jan; Korokhin, Viktor V.; Lourenço, Ana;
Malherbe, Jean-Marie; Marchenko, Gennady P.; Peixinho, Nuno; Sakurai,
Takashi; Tlatov, Andrey G.
2020A&A...639A..88C Altcode: 2020arXiv200501435C
Context. Studies of long-term solar activity and variability require
knowledge of the past evolution of the solar surface magnetism. The
archives of full-disc Ca II K observations that have been performed
more or less regularly at various sites since 1892 can serve as an
important source of such information. Aims: We derive the plage
area evolution over the last 12 solar cycles by employing data from all
Ca II K archives that are publicly available in digital form, including
several as-yet-unexplored Ca II K archives. Methods: We analysed
more than 290 000 full-disc Ca II K observations from 43 datasets
spanning the period between 1892-2019. All images were consistently
processed with an automatic procedure that performs the photometric
calibration (if needed) and the limb-darkening compensation. The
processing also accounts for artefacts affecting many of the images,
including some very specific artefacts, such as bright arcs found
in Kyoto and Yerkes data. Our employed methods have previously been
tested and evaluated on synthetic data and found to be more accurate
than other methods used in the literature to treat a subset of the data
analysed here. Results: We produced a plage area time-series
from each analysed dataset. We found that the differences between the
plage areas derived from individual archives are mainly due to the
differences in the central wavelength and the bandpass used to acquire
the data at the various sites. We empirically cross-calibrated and
combined the results obtained from each dataset to produce a composite
series of plage areas. The 'backbone' approach was used to bridge
the series together. We have also shown that the selection of the
backbone series has little effect on the final composite of the plage
area. We quantified the uncertainty of determining the plage areas
with our processing due to shifts in the central wavelength and found
it to be less than 0.01 in fraction of the solar disc for the average
conditions found on historical data. We also found the variable seeing
conditions during the observations to slightly increase the plage
areas during the activity maxima. Conclusions: We provide the
most complete so far time series of plage areas based on corrected
and calibrated historical and modern Ca II K images. Consistent
plage areas are now available on 88% of all days from 1892 onwards
and on 98% from 1907 onwards. The whole series described in
the paper are only available at the CDS via anonymous ftp to <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/639/A88">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A88</A>

---------------------------------------------------------
Title: How faculae and network relate to sunspots, and the
implications for solar and stellar brightness variations
Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.
2020A&A...639A.139Y Altcode: 2020arXiv200614274Y
Context. How global faculae and network coverage relates to that
of sunspots is relevant to the brightness variations of the Sun and
Sun-like stars. Aims: We aim to extend and improve on earlier
studies that established that the facular-to-sunspot-area ratio
diminishes with total sunspot coverage. Methods: Chromospheric
indices and the total magnetic flux enclosed in network and faculae,
referred to here as "facular indices", are modulated by the amount
of facular and network present. We probed the relationship between
various facular and sunspot indices through an empirical model, taking
into account how active regions evolve and the possible non-linear
relationship between plage emission, facular magnetic flux, and sunspot
area. This model was incorporated into a model of total solar irradiance
(TSI) to elucidate the implications for solar and stellar brightness
variations. Results: The reconstruction of the facular indices
from the sunspot indices with the model presented here replicates
most of the observed variability, and is better at doing so than
earlier models. Contrary to recent studies, we found the relationship
between the facular and sunspot indices to be stable over the past four
decades. The model indicates that, like the facular-to-sunspot-area
ratio, the ratio of the variation in chromospheric emission and
total network and facular magnetic flux to sunspot area decreases
with the latter. The TSI model indicates the ratio of the TSI excess
from faculae and network to the deficit from sunspots also declines
with sunspot area, with the consequence being that TSI rises with
sunspot area more slowly than if the two quantities were linearly
proportional to one another. This explains why even though solar cycle
23 is significantly weaker than cycle 22, TSI rose to comparable levels
over both cycles. The extrapolation of the TSI model to higher activity
levels indicates that in the activity range where Sun-like stars are
observed to switch from growing brighter with increasing activity to
becoming dimmer instead, the activity-dependence of TSI exhibits a
similar transition. This happens as sunspot darkening starts to rise
more rapidly with activity than facular and network brightening. This
bolsters the interpretation of this behaviour of Sun-like stars as
the transition from a faculae-dominated to a spot-dominated regime.

---------------------------------------------------------
Title: Reply to the comment of T. Metcalfe and J. van Saders on the
Science report "The Sun is less active than other solar-like stars"
Authors: Reinhold, T.; Shapiro, A. I.; Solanki, S. K.; Montet, B. T.;
Krivova, N. A.; Cameron, R. H.; Amazo-Gómez, E. M.
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.
2020A&A...638A..56N Altcode: 2020arXiv200406974N
Context. A comparison of solar and stellar brightness variations is
hampered by the difference in spectral passbands that are used in
observations, and also by the possible difference in the inclination
of the solar and stellar rotation axes from the line of sight. 
Aims: We calculate the rotational variability of the Sun as it would
be measured in passbands used for stellar observations. In particular,
we consider the filter systems used by the CoRoT, Kepler, TESS, and
Gaia space missions. We also quantify the effect of the inclination
of the rotation axis on the solar rotational variability. 
Methods: We employed the spectral and total irradiance reconstruction
(SATIRE) model to calculate solar brightness variations in different
filter systems as observed from the ecliptic plane. We then combined
the simulations of the surface distribution of the magnetic features
at different inclinations using a surface flux transport model with
the SATIRE calculations to compute the dependence of the variability
on the inclination. Results: For an ecliptic-bound observer,
the amplitude of the solar rotational variability, as observed in
the total solar irradiance (TSI), is 0.68 mmag (averaged over solar
cycles 21-24). We obtained corresponding amplitudes in the Kepler
(0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag),
Gaia GRP (0.62 mmag), and Gaia GBP (0.86 mmag)
passbands. Decreasing the inclination of the rotation axis decreases
the rotational variability. For a sample of randomly inclined stars,
the variability is on average 15% lower in all filter systems we
considered. This almost compensates for the difference in amplitudes
of the variability in TSI and Kepler passbands, making the amplitudes
derived from the TSI records an ideal representation of the solar
rotational variability for comparison to Kepler stars with unknown
inclinations. Conclusions: The TSI appears to be a relatively
good measure of solar variability for comparisons with stellar
measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia GRP
filters. Whereas the correction factors can be used to convert the
variability amplitude from solar measurements into the values expected
for stellar missions, the inclination affects the shapes of the light
curves so that a much more sophisticated correction than simple scaling
is needed to obtain light curves out of the ecliptic for the Sun.

---------------------------------------------------------
Title: Towards a more reliable reconstruction of the historical
solar variability: a more realistic description of solar ephemeral
magnetic regions
Authors: Hofer, Bernhard; Krivova, Natalie A.; Wu, Chi-Ju; Usoskin,
Ilya A.; Cameron, Robert
2020EGUGA..2217086H Altcode:
Solar irradiance is a crucial input to climate models, but its
measurements are only available since 1978. The variability of
solar irradiance on climate-relevant time-scales is caused by
the competition between bright and dark features formed by the
magnetic fields emerging on the solar surface. Thus, models have
been developed that reconstruct past irradiance variability from
proxies of the solar magnetic activity. The longest direct proxy is
the sunspot number. The common problem of such reconstructions is,
however, that while sunspots adequately describe the evolution of
the active regions (ARs) (large bipolar regions hosting sunspots),
the evolution of their smaller counterparts, the ephemeral regions
(ERs), is not directly featured by sunspots. At the same time, these
small regions are much more numerous and are believed to be the main
source of the long-term irradiance changes, which are of special
interest to climate models. We develop an improved description of
the ephemeral region emergence taking different solar observational
constraints into account. The model builds on the SATIRE-T model, in
which the emergence of ARs is described by the sunspot number and the
emergence of the ERs is linearly linked to that of ARs. The latter,
however, implies that whenever the sunspot number drops to zero, no
magnetic field emerges in the model. In the new model, the emergence
of the ERs is no longer linked to sunspots linearly. Instead, ARs and
ERs are considered to be parts of a single power-law size distribution
of the emerging magnetic regions. This ensures that even in the absence
of ARs (e.g., during the grand minima of solar activity), the emergence
rate of ERs remains non-zero. In particular, the solar open magnetic
flux reconstructed using this approach does not drop to zero during
the Maunder minimum, in agreement with independent reconstructions
from the cosmogenic isotope data. Such an improved description of the
ERs will allow a better constraint on the maximum solar irradiance
drop during grand minima events. This, in turn, will allow a better
constraint on the potential solar forcing in the future.

---------------------------------------------------------
Title: Historical solar Ca II K observations at the Kyoto and
Sacramento Peak observatories
Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K.
2020JPhCS1548a2007C Altcode:
Archives of full-disc solar Ca II K observations covering even short
periods of time can offer important data to fill observation gaps
and to allow a better transition from historical to modern data. Two
examples of such archives are those from the Kyoto and Sacramento
Peak observatories. As most other historical Ca II K data, they suffer
from artefacts that need to be accounted for to derive accurate plage
areas. Here we present the results of our analysis of these archives
of solar Ca II K images.

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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.
2020yCat..36349009W Altcode:
The Kepler field of view was selected in order to contain a large
fraction of solar-like stars. Focusing on stars in the effective
temperature range of 5600K-5900K, it is challenging to determine their
rotational periods. Stellar fundamental parameters and rotation
periods of the 97 periodic stars are presented. (1 data file).

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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.
2020Sci...368..518R Altcode: 2020arXiv200501401R
The magnetic activity of the Sun and other stars causes their brightness
to vary. We investigated how typical the Sun’s variability is
compared with other solar-like stars, i.e., those with near-solar
effective temperatures and rotation periods. By combining 4 years
of photometric observations from the Kepler space telescope with
astrometric data from the Gaia spacecraft, we were able to measure
photometric variabilities of 369 solar-like stars. Most of those with
well-determined rotation periods showed higher variability than the Sun
and are therefore considerably more active. These stars appear nearly
identical to the Sun except for their higher variability. Therefore,
we speculate that the Sun could potentially also go through epochs of
such high variability.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Plage area composite series
(Chatzistergos+, 2020)
Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki,
S. K.; Banerjee, D.; Barata, T.; Belik, M.; Gafeira, R.; Garcia,
A.; Hanaoka, Y.; Hegde, M.; Klimes, J.; Korokhin, V. V.; Lourenco,
A.; Malherbe, J. -M.; Marchenko, G. P.; Peixinho, N.; Sakurai, T.;
Tlatov, A. G.
2020yCat..36390088C Altcode:
Annual, monthly, and daily values of the plage area composite series. The archives included are those from the Arcetri (Ar), Baikal (Ba),
Brussels (Br), Calern (CL), Catania (CT), Coimbra (Co), Kanzelhoehe
(Ka), Kenwood (Ke), Kharkiv (Kh), Kislovodsk (Ki), Kodaikanal taken with
the spectroheliograph (Ko), Kodaikanal taken with the Twin telescope
(KT), Kodaikanal taken with the White-Light Active Region Monitor (WARM)
telescope (KW), Kyoto (Ky), Manila (Ma), Mauna Loa (ML) taken with the
Precision Solar Photometric Telescope (PSPT), McMath-Hulbert (MM), Mees
(MS), Meudon taken with the spectroheliograph (MD1), Meudon taken with
an interference filter (MD2), Mitaka taken with the spectroheliograph
(Mi1), Mitaka taken with the Solar Flare Telescope with an interference
filter (Mi2), Mt Wilson (MW), Pic du Midi (PM), SOlar Diameter Imager
and Surface Mapper (SODISM) telescope on board the PICARD spacecraft
(PS), Rome taken with the equatorial bar at Monte Mario (Ro), Rome taken
with the PSPT (RP1), Rome taken with the PSPT with narrow bandwidth
(RP2), Sacramento Peak (SP), San Fernando taken with the Cartesian
Full-Disk Telescope (CFDT) 1 (SF1), San Fernando taken with the CFDT2
(SF2), Schauinsland (Sc), Teide (Te) taken with the Chromospheric
Telescope (ChroTel), Upice (UP), Valasske Mezirici (VM), Wendelstein
(WS), and Yerkes (YR) sites (3 data files).

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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.
2020A&A...636A..43N Altcode: 2020arXiv200210895N
Context. Magnetic features on the surfaces of cool stars lead to
variations in their brightness. Such variations on the surface of
the Sun have been studied extensively. Recent planet-hunting space
telescopes have made it possible to measure brightness variations
in hundred thousands of other stars. The new data may undermine
the validity of setting the sun as a typical example of a variable
star. Putting solar variability into the stellar context suffers,
however, from a bias resulting from solar observations being carried
out from its near-equatorial plane, whereas stars are generally
observed at all possible inclinations. Aims: We model solar
brightness variations at timescales from days to years as they would
be observed at different inclinations. In particular, we consider the
effect of the inclination on the power spectrum of solar brightness
variations. The variations are calculated in several passbands that are
routinely used for stellar measurements. Methods: We employ the
surface flux transport model to simulate the time-dependent spatial
distribution of magnetic features on both the near and far sides of
the Sun. This distribution is then used to calculate solar brightness
variations following the Spectral And Total Irradiance REconstruction
approach. Results: We have quantified the effect of the
inclination on solar brightness variability at timescales down to a
single day. Thus, our results allow for solar brightness records to
be made directly comparable to those obtained by planet-hunting space
telescopes. Furthermore, we decompose solar brightness variations into
components originating from the solar rotation and from the evolution
of magnetic features.

---------------------------------------------------------
Title: Inflection point in the power spectrum of stellar brightness
variations. II. The Sun
Authors: Amazo-Gómez, E. M.; Shapiro, A. I.; Solanki, S. K.; Krivova,
N. A.; Kopp, G.; Reinhold, T.; Oshagh, M.; Reiners, A.
2020A&A...636A..69A Altcode: 2020arXiv200203455A
Context. Young and active stars generally have regular, almost
sinusoidal, patterns of variability attributed to their rotation,
while the majority of older and less active stars, including the Sun,
have more complex and non-regular light curves, which do not have clear
rotational-modulation signals. Consequently, the rotation periods have
been successfully determined only for a small fraction of the Sun-like
stars (mainly the active ones) observed by transit-based planet-hunting
missions, such as CoRoT, Kepler, and TESS. This suggests that only
a small fraction of such systems have been properly identified as
solar-like analogues. Aims: We aim to apply a new method of
determining rotation periods of low-activity stars, such as the Sun. The
method is based on calculating the gradient of the power spectrum
(GPS) of stellar brightness variations and identifying a tell-tale
inflection point in the spectrum. The rotation frequency is then
proportional to the frequency of that inflection point. In this paper,
we compare this GPS method to already-available photometric records of
the Sun. Methods: We applied GPS, auto-correlation functions,
Lomb-Scargle periodograms, and wavelet analyses to the total solar
irradiance (TSI) time series obtained from the Total Irradiance Monitor
on the Solar Radiation and Climate Experiment and the Variability of
solar IRradiance and Gravity Oscillations experiment on the SOlar
and Heliospheric Observatory missions. We analysed the performance
of all methods at various levels of solar activity. Results:
We show that the GPS method returns accurate values of solar rotation
independently of the level of solar activity. In particular, it performs
well during periods of high solar activity, when TSI variability
displays an irregular pattern, and other methods fail. Furthermore,
we show that the GPS and light curve skewness can give constraints
on facular and spot contributions to brightness variability. 
Conclusions: Our results suggest that the GPS method can successfully
determine the rotational periods of stars with both regular and
non-regular light curves. The two movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936925/olm">https://www.aanda.org</A>

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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.
2020A&A...636A..83S Altcode: 2020arXiv200208806S
Context. The variability of the spectral solar irradiance (SSI) over the
course of the 11-year solar cycle is one of the manifestations of solar
magnetic activity. There is strong evidence that the SSI variability
has an effect on the Earth's atmosphere. The faster rotation of the
Sun in the past lead to a more vigorous action of solar dynamo and
thus potentially to larger amplitude of the SSI variability on the
timescale of the solar activity cycle. This could lead to a stronger
response of the Earth's atmosphere as well as other solar system
planets' atmospheres to the solar activity cycle. Aims: We
calculate the amplitude of the SSI and total solar irradiance (TSI)
variability over the course of the solar activity cycle as a function
of solar age. Methods: We employed the relationship between the
stellar magnetic activity and the age based on observations of solar
twins. Using this relation, we reconstructed solar magnetic activity
and the corresponding solar disk area coverages by magnetic features
(i.e., spots and faculae) over the last four billion years. These disk
coverages were then used to calculate the amplitude of the solar-cycle
SSI variability as a function of wavelength and solar age. 
Results: Our calculations show that the young Sun was significantly
more variable than the present Sun. The amplitude of the solar-cycle TSI
variability of the 600 Myr old Sun was about ten times larger than that
of the present Sun. Furthermore, the variability of the young Sun was
spot-dominated (the Sun being brighter at the activity minimum than
in the maximum), that is, the Sun was overall brighter at activity
minima than at maxima. The amplitude of the TSI variability decreased
with solar age until it reached a minimum value at 2.8 Gyr. After this
point, the TSI variability is faculae-dominated (the Sun is brighter
at the activity maximum) and its amplitude increases with age.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Sunspot area catalogue revisited
(1874-2019) (Mandal+, 2020)
Authors: Mandal, S.; Krivova, N.; Solanki, S. K.; Sinha, N.;
Banerjee, D.
2020yCat..36400078M Altcode:
A number of observatories around the globe have carried
out measurements of sunspot areas and positions over the last
century. RGO, the longest sunspot area database to date, started its
campaign in 1874 and after continuing for a century, stopped it in
1976. Several other observatories from different parts of the world
(e.g., Kodaikanal, Kislovodsk, Debrecen, Rome etc.) also carried out
such observing programs throughout the 20th century. Sunspot area
datasets are invaluable historical records of solar magnetic fields
and are key to understanding the solar variability and its historical
reconstructions. In this work, we analyze and compare sunspot group
areas from a total of nine observatories (RGO, Kislovodsk, Pulkovo,
Debrecen, Kodaikanal, SOON, Rome, Catania, Yunnan). It turns out that
data from only four observatories (RGO, Kislovodsk, Pulkovo, Debrecen)
are sufficient to produce cross-calibrated, up-to-date (1874-2019)
catalogs of daily total and individual group areas. The remaining
gaps (776 days in total) could not be filled with data from the other
archives as the missing days lie either before 1922 or after 2016 and
none of the other archives cover these periods. (2 data files).

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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.
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.
2020A&A...633A..32S Altcode: 2019arXiv191008351S
Context. Considerable effort has gone into using light curves observed
by such space telescopes as CoRoT, Kepler, and TESS for determining
stellar rotation periods. While rotation periods of active stars
can be reliably determined, the light curves of many older and less
active stars, such as stars that are similar to the Sun, are quite
irregular. This hampers the determination of their rotation periods. Aims: We aim to examine the factors causing these irregularities in
stellar brightness variations and to develop a method for determining
rotation periods for low-activity stars with irregular light curves. Methods: We extended the Spectral And Total Irradiance Reconstruction
approach for modeling solar brightness variations to Sun-like stars. We
calculated the power spectra of stellar brightness variations for
various combinations of parameters that define the surface configuration
and evolution of stellar magnetic features. Results: The short
lifetime of spots in comparison to the stellar rotation period, as well
as the interplay between spot and facular contributions to brightness
variations of stars with near solar activity, cause irregularities in
their light curves. The power spectra of such stars often lack a peak
associated with the rotation period. Nevertheless, the rotation period
can still be determined by measuring the period where the concavity
of the power spectrum plotted in the log-log scale changes its sign,
that is, by identifying the position of the inflection point. 
Conclusions: The inflection point of the (log-log) power spectrum is
found to be a new diagnostic for stellar rotation periods which is
shown to work even in cases where the power spectrum shows no peak at
the rotation rate.

---------------------------------------------------------
Title: Readdressing the UV solar variability with SATIRE-S: non-LTE
effects
Authors: Tagirov, R. V.; Shapiro, A. I.; Krivova, N. A.; Unruh, Y. C.;
Yeo, K. L.; Solanki, S. K.
2019A&A...631A.178T Altcode: 2019arXiv190911736T
Context. Solar spectral irradiance (SSI) variability is one of the key
inputs to models of the Earth's climate. Understanding solar irradiance
fluctuations also helps to place the Sun among other stars in terms of
their brightness variability patterns and to set detectability limits
for terrestrial exoplanets. Aims: One of the most successful
and widely used models of solar irradiance variability is Spectral
And Total Irradiance REconstruction model (SATIRE-S). It uses spectra
of the magnetic features and surrounding quiet Sun that are computed
with the ATLAS9 spectral synthesis code under the assumption of local
thermodynamic equilibrium (LTE). SATIRE-S has been at the forefront
of solar variability modelling, but due to the limitations of the LTE
approximation its output SSI has to be empirically corrected below
300 nm, which reduces the physical consistency of its results. This
shortcoming is addressed in the present paper. Methods: We
replaced the ATLAS9 spectra of all atmospheric components in SATIRE-S
with spectra that were calculated using the Non-LTE Spectral SYnthesis
(NESSY) code. To compute the spectrum of the quiet Sun and faculae,
we used the temperature and density stratification models of the FAL
set. Results: We computed non-LTE contrasts of spots and faculae
and combined them with the corresponding fractional disc coverages,
or filling factors, to calculate the total and spectral irradiance
variability during solar cycle 24. The filling factors have been derived
from solar full-disc magnetograms and continuum images recorded by
the Helioseismic and Magnetic Imager on Solar Dynamics Observatory
(SDO/HMI). Conclusions: The non-LTE contrasts yield total and
spectral solar irradiance variations that are in good agreement with
empirically corrected LTE irradiance calculations. This shows that
the empirical correction applied to the SATIRE-S total and spectral
solar irradiance is consistent with results from non-LTE computations.

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Title: Delving into the Historical Ca II K Archive from the Kodaikanal
Observatory: The Potential of the Most Recent Digitized Series
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.;
Krivova, Natalie A.; Banerjee, Dipankar; Jha, Bibhuti K.; Chatterjee,
Subhamoy
2019SoPh..294..145C Altcode: 2019arXiv190805493C
Full-disc Ca II K photographic observations of the Sun carry direct
information as regards the evolution of solar-plage regions for more
than a century and are therefore a unique dataset for solar-activity
studies. For a long time Ca II K observations were barely explored, but
recent digitizations of multiple archives have allowed their extensive
analysis. However, various studies have reported diverse results partly
due to the insufficient quality of the digitized data. Furthermore,
inhomogeneities have been identified within the individual archives,
which, at least partly, could be due to the digitization. As a result,
some of the archives, e.g. that from the Kodaikanal observatory, were
re-digitized. The results obtained by different authors who analyzed the
data from the new digitization of the Kodaikanal archive differ from
each other and from those derived from the old digitization. Since
the data were processed and analyzed using different techniques,
it is not clear, however, whether the differences are due to the
digitization or the processing of the data. To understand the reasons
for such discrepancies, we analyze here the data from the two most
recent digitizations of this archive. We use the same techniques
to consistently process the images from both archives and to derive
the plage areas from them. Some issues have been identified in both
digitizations, implying that they are intrinsic characteristics of
the data. Moreover, errors in timing of the observations plague both
digitizations. Overall, the most recent 16-bit digitization offers an
improvement over the earlier 8-bit one. It also includes considerably
more data and should be preferred.

---------------------------------------------------------
Title: Solar irradiance variability over last four billion years
Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent;
Krivova, Natalie A.; Solanki, Sami K.
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.

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Title: Cosmic-ray variability on the multi-millennial time scale:
A new multi-proxy reconstruction
Authors: Usoskin, I.; Wu, C. J.; Krivova, N.; Solanki, S. K.;
Kovaltsov, G.; Baroni, M.; Bard, E.
2019ICRC...36.1164U Altcode: 2019PoS...358.1164U
No abstract at ADS

---------------------------------------------------------
Title: Opacity distribution functions for stellar spectra synthesis
Authors: Cernetic, M.; Shapiro, A. I.; Witzke, V.; Krivova, N. A.;
Solanki, S. K.; Tagirov, R. V.
2019A&A...627A.157C Altcode: 2019arXiv190603112C
Context. Stellar spectra synthesis is essential for the characterization
of potential planetary hosts. In addition, comprehensive stellar
variability calculations with fast radiative transfer are needed
to disentangle planetary transits from stellar magnetically driven
variability. The planet-hunting space telescopes, such as CoRoT, Kepler,
and TESS, bring vast quantities of data, rekindling the interest in
fast calculations of the radiative transfer. Aims: We revisit
the opacity distribution functions (ODF) approach routinely applied to
speed up stellar spectral synthesis. To achieve a considerable speedup
relative to the state of the art, we further optimize the approach and
search for the best ODF configuration. Furthermore, we generalize the
ODF approach for fast calculations of flux in various filters often
used in stellar observations. Methods: In a parameter-sweep
fashion, we generated ODF in the spectral range from UV to IR with
different setups. The most accurate ODF configuration for each spectral
interval was determined. We adapted the wavelength grid based on the
transmission curve for calculations of the radiative fluxes through
filters before performing the normal ODF procedure. Results:
Our optimum ODF configuration allows for a three-fold speedup, compared
to the previously used ODF configurations. The ODF generalization to
calculate fluxes through filters results in a speedup of more than
two orders of magnitude.

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Title: Recovering the unsigned photospheric magnetic field from Ca
II K observations
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.;
Krivova, Natalie A.; Giorgi, Fabrizio; Yeo, Kok Leng
2019A&A...626A.114C Altcode: 2019arXiv190503453C
Context. A number of studies have aimed at defining the exact form
of the relation between magnetic field strength and Ca II H and K
core brightness. All previous studies have however been restricted
to isolated regions on the solar disc or to a limited set of
observations. Aims: We reassess the relationship between the
photospheric magnetic field strength and the Ca II K intensity for
a variety of surface features as a function of the position on the
disc and the solar activity level. This relationship can be used to
recover the unsigned photospheric magnetic field from images recorded
in the core of Ca II K line. Methods: We have analysed 131
pairs of high-quality, full-disc, near-co-temporal observations from
the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar
Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To
analytically describe the observationally determined relation, we
considered three different functions: a power law with an offset,
a logarithmic function, and a power-law function of the logarithm
of the magnetic flux density. We used the obtained relations to
reconstruct maps of the line-of-sight component of the unsigned
magnetic field (unsigned magnetograms) from Ca II K observations,
which were then compared to the original magnetograms. Results:
We find that both power-law functions represent the data well, while
the logarithmic function is good only for quiet periods. We see
no significant variation over the solar cycle or over the disc in
the derived fit parameters, independently of the function used. We
find that errors in the independent variable, which are usually
not accounted for, introduce attenuation bias. To address this, we
binned the data with respect to the magnetic field strength and Ca II
K contrast separately and derived the relation for the bisector of
the two binned curves. The reconstructed unsigned magnetograms show
good agreement with the original ones. Root mean square differences
are less than 90 G. The results were unaffected by the stray-light
correction of the SDO/HMI and Rome/PSPT data. Conclusions:
Our results imply that accurately processed and calibrated Ca II K
observations can be used to reconstruct unsigned magnetograms by using
the relations derived in our study.

---------------------------------------------------------
Title: GPS, decrypting brightness variations of the Sun and Sun-like
Authors: Amazo-Gómez, Eliana Maritza; Shapiro, Alexander I.; Solanki,
Sami K.; Kopp, Greg; Oshagh, Mahmoudreza; Reinhold, Timo; Krivova,
Natalie A.; Reiners, Ansgar
2019shin.confE.109A Altcode:
The rotation period is in general detectable in the light curves of
young and active stars. Even after successful stellar surveys stemmed
from Kepler mission, there is still a lack of information in photometric
records of rotation periods in Sun-like stars. Non-periodic light-curve
profiles, low variability contrast -therefore low modulation amplitude-
short lifetime evolution and random emergence of magnetic features
(in comparison to the rotation time-scale) are the main reasons of
unreliable determination of rotation periodicity in the Sun and its
closer analogs. This indicates that only a small fraction of solar-like
systems have been properly analyzed. We show that the rotation periods
of those stars can be reliably determined from the profile of the
gradient of the power spectra, GPS. By analysing periodic patterns
in high-accuracy measurements of the total solar irradiance, TSI,
by SORCE/TIM and SoHO/VIRGO missions, here we test and validate
GPS, linking the variability by transits of magnetic features over
the stellar surface with a clear and enhanced signal of the solar
rotation. GPS method retrieves accurate and stable values of rotation
period during different regimes of solar activity cycle and could
be applied to stars of comparable and higher activity - where other
methods underperform. Furthermore, GPS gives us constraints on the
faculae to spot driver ratio and consequently help us to interpret
the stellar surface.

---------------------------------------------------------
Title: Analysis of full disc Ca II K spectroheliograms. II. Towards
an accurate assessment of long-term variations in plage areas
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie
A.; Solanki, Sami K.
2019A&A...625A..69C Altcode: 2019arXiv190207122C
Context. Reconstructions of past irradiance variations require suitable
data on solar activity. The longest direct proxy is the sunspot number,
and it has been most widely employed for this purpose. These data,
however, only provide information on the surface magnetic field
emerging in sunspots, while a suitable proxy of the evolution of the
bright magnetic features, specifically faculae/plage and network,
is missing. This information can potentially be extracted from the
historical full-disc observations in the Ca II K line. Aims: We
use several historical archives of full-disc Ca II K observations to
derive plage areas over more than a century. Employment of different
datasets allows the identification of systematic effects in the
images, such as changes in instruments and procedures, as well as
an assessment of the uncertainties in the results. Methods:
We have analysed over 100 000 historical images from eight digitised
photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert,
Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories,
and one archive of modern observations from the Rome/PSPT. The analysed
data cover the period 1893-2018. We first performed careful photometric
calibration and compensation for the centre-to-limb variation, and
then segmented the images to identify plage regions. This has been
consistently applied to both historical and modern observations. Results: The plage series derived from different archives are
generally in good agreement with each other. However, there are also
clear deviations that most likely hint at intrinsic differences in the
data and their digitisation. We showed that accurate image processing
significantly reduces errors in the plage area estimates. Accurate
photometric calibration also allows precise plage identification on
images from different archives without the need to arbitrarily adjust
the segmentation parameters. Finally, by comparing the plage area
series from the various records, we found the conversion laws between
them. This allowed us to produce a preliminary composite of the plage
areas obtained from all the datasets studied here. This is a first
step towards an accurate assessment of the long-term variation of plage
regions. The data are only available at the CDS via anonymous ftp
to <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/625/A69">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A69</A>

---------------------------------------------------------
Title: Evolution of Large-Scale Magnetic Fields From Near-Earth
Space During the Last 11 Solar Cycles
Authors: Pick, Leonie; Korte, Monika; Thomas, Yannik; Krivova, Natalie;
Wu, Chi-Ju
2019JGRA..124.2527P Altcode:
We use hourly mean magnetic field measurements from 34 midlatitude
geomagnetic observatories between 1900 and 2015 to investigate
the long-term evolution and driving mechanism of the large-scale
external magnetic field at ground. The Hourly Magnetospheric
Currents index (HMC) is derived as a refinement of the Annual
Magnetospheric Currents index (HMC, Pick &amp; Korte, 2017, <A
href="https://doi.org/10.1093/gji/ggx367">https://doi.org/10.1093/gji/ggx367</A>).
HMC requires an extensive revision of the observatory hourly
means. It depends on three third party geomagnetic field models used to
eliminate the core, the crustal, and the ionospheric solar-quiet field
contributions. We mitigate the dependency of HMC on the core field
model by subtracting only nondipolar components of the model from the
data. The separation of the residual (dipolar) signal into internal and
external (HMC) parts is the main methodological challenge. Observatory
crustal biases are updated with respect to AMC, and the solar-quiet
field estimation is extended to the past based on a reconstruction
of solar radio flux (F10.7). We find that HMC has more power at low
frequencies (periods ≥ 1 year) than the Dcx index, especially at
periods relevant to the solar cycle. Most of the slow variations
in HMC can be explained by the open solar magnetic flux. There is a
weakly decreasing linear trend in absolute HMC from 1900 to present,
which depends sensitively on the data rejection criteria at early
years. HMC is well suited for studying long-term variations of the
geomagnetic field.

---------------------------------------------------------
Title: Intensity contrast of solar network and
faculae. II. Implications for solar irradiance modelling
Authors: Yeo, K. L.; Krivova, N. A.
2019A&A...624A.135Y Altcode: 2021arXiv210209530Y
 Aims: We aim to gain insight into the effect of network
and faculae on solar irradiance from their apparent intensity. Methods: Taking full-disc observations from the Solar Dynamics
Observatory, we examined the intensity contrast of network and faculae
in the continuum and core of the Fe I 6173 Å line and 1700 Å,
including the variation with magnetic flux density, distance from
disc centre, nearby magnetic fields, and time. Results: The
brightness of network and faculae is believed to be suppressed by nearby
magnetic fields from its effect on convection. We note that the degree
of magnetically crowding of an area also affects the magnetic flux tube
sizes and the depth at which magnetic concentrations are embedded in
intergranular lanes, such that intensity contrast can be enhanced in
magnetically crowded areas at certain flux densities and distances
from disc centre. The difference in intensity contrast between the
quiet-Sun network and active region faculae, noted by various studies,
arises because active regions are more magnetically crowded and is
not due to any fundamental physical differences between network and
faculae. These results highlight that solar irradiance models need to
include the effect of nearby magnetic fields on network and faculae
brightness. We found evidence that suggests that departures from
local thermal equilibrium (LTE) might have limited effect on intensity
contrast. This could explain why solar irradiance models that are based
on the intensity contrast of solar surface magnetic features calculated
assuming LTE reproduce the observed spectral variability even where
the LTE assumption breaks down. Certain models of solar irradiance
employ chromospheric indices as direct indications of the effect of
network and faculae on solar irradiance. Based on past studies of the
Ca II K line and on the intensity contrast measurements derived here,
we show that the fluctuations in chromospheric emission from network
and faculae are a reasonable estimate of the emission fluctuations in
the middle photosphere, but not of those in the lower photosphere. This
is due to the different physical mechanisms that underlie the magnetic
intensity enhancement in the various atmospheric regimes, and represents
a fundamental limitation of these solar irradiance models. Any time
variation in the radiant properties of network and faculae is, of
course, relevant to their effect on solar irradiance. The data set,
which extends from 2010 to 2018, indicates that their intensity contrast
was stable to about 3% in this period. Conclusions: This study
offers new insights into the radiant behaviour of network and faculae,
with practical implications for solar irradiance modelling.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Plage area composite series
(Chatzistergos+, 2019)
Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K.
2019yCat..36250069C Altcode:
Annual, monthly, and daily values of the plage area composite series
produced in the paper. (3 data files).

---------------------------------------------------------
Title: Historical solar Ca II K observations at the Rome and Catania
observatories
Authors: Chatzistergos, T.; Ermolli, I.; Falco, M.; Giorgi, F.;
Guglielmino, S. L.; Krivova, N. A.; Romano, P.; Solanki, S. K.
2019NCimC..42....5C Altcode: 2019arXiv190207483C
Here we present the little explored Ca II K archives from the Rome
and the Catania observatories and analyse the digitised images from
these archives to derive plage areas.

---------------------------------------------------------
Title: Solar total and spectral irradiance reconstruction over the
last 9000 years
Authors: Wu, C. -J.; Krivova, N. A.; Solanki, S. K.; Usoskin, I. G.
2018A&A...620A.120W Altcode: 2018arXiv181103464W
Context. Changes in solar irradiance and in its spectral distribution
are among the main natural drivers of the climate on Earth. However,
irradiance measurements are only available for less than four decades,
while assessment of solar influence on Earth requires much longer
records. Aims: The aim of this work is to provide the most
up-to-date physics-based reconstruction of the solar total and spectral
irradiance (TSI/SSI) over the last nine millennia. Methods:
The concentrations of the cosmogenic isotopes 14C and
10Be in natural archives have been converted to decadally
averaged sunspot numbers through a chain of physics-based models. TSI
and SSI are reconstructed with an updated SATIRE model. Reconstructions
are carried out for each isotope record separately, as well as
for their composite. Results: We present the first ever
SSI reconstruction over the last 9000 years from the individual
14C and 10Be records as well as from their
newest composite. The reconstruction employs physics-based models to
describe the involved processes at each step of the procedure. Conclusions: Irradiance reconstructions based on two different
cosmogenic isotope records, those of 14C and 10Be,
agree well with each other in their long-term trends despite their
different geochemical paths in the atmosphere of Earth. Over the last
9000 years, the reconstructed secular variability in TSI is of the
order of 0.11%, or 1.5 W m-2. After the Maunder minimum,
the reconstruction from the cosmogenic isotopes is consistent with
that from the direct sunspot number observation. Furthermore, over
the nineteenth century, the agreement of irradiance reconstructions
using isotope records with the reconstruction from the sunspot number
by Chatzistergos et al. (2017, A&amp;A, 602, A69) is better than that
with the reconstruction from the WDC-SILSO series (Clette et al. 2014,
Space Sci. Rev., 186, 35), with a lower χ2-value.

---------------------------------------------------------
Title: Activity variation driven by flux emergence and transport on
Sun-like stars
Authors: Işık, Emre; Solanki, Sami K.; Krivova, Natalie A.; Shapiro,
Alexander I.
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.
2018A&A...620A.177I Altcode: 2018arXiv181006728I
Context. The latitudinal distribution of starspots deviates
from the solar pattern with increasing rotation rate. Numerical
simulations of magnetic flux emergence and transport can help model
the observed stellar activity patterns and the associated brightness
variations. Aims: We set up a composite model for the processes
of flux emergence and transport on Sun-like stars to simulate stellar
brightness variations for various levels of magnetic activity and
rotation rates. Methods: Assuming that the distribution of
magnetic flux at the base of the convection zone follows solar scaling
relations, we calculate the emergence latitudes and tilt angles of
bipolar regions at the surface for various rotation rates, using
thin-flux-tube simulations. Taking these two quantities as input to a
surface flux transport (SFT) model, we simulate the diffusive-advective
evolution of the radial field at the stellar surface, including
effects of active region nesting. Results: As the rotation
rate increases, (1) magnetic flux emerges at higher latitudes and an
inactive gap opens around the equator, reaching a half-width of 20° for
8 Ω⊙; and (2) the tilt angles of freshly emerged bipolar
regions show stronger variations with latitude. Polar spots can form
at 8 Ω⊙ by accumulation of follower-polarity flux from
decaying bipolar regions. From 4 Ω⊙ to 8 Ω⊙,
the maximum spot coverage changes from 3 to 20%, respectively, compared
to 0.4% in the solar model. Nesting of activity can lead to strongly
non-axisymmetric spot distributions. Conclusions: On Sun-like
stars rotating at 8 Ω⊙ (Prot ≃ 3 days),
polar spots can form, owing to higher levels of flux emergence rate and
tilt angles. Defining spots by a threshold field strength yields global
spot coverages that are roughly consistent with stellar observations.

---------------------------------------------------------
Title: From solar to stellar brightness variations. The effect
of metallicity
Authors: Witzke, V.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.;
Schmutz, W.
2018A&A...619A.146W Altcode: 2018arXiv180904360W
Context. Comparison studies of Sun-like stars with the Sun suggest an
anomalously low photometric variability of the Sun compared to Sun-like
stars with similar magnetic activity. Comprehensive understanding
of stellar variability is needed to find a physical reason for this
observation. Aims: We investigate the effect of metallicity
and effective temperature on the photometric brightness change
of Sun-like stars seen at different inclinations. The considered
range of fundamental stellar parameters is sufficiently small so
the stars investigated here still count as Sun-like or even as solar
twins. Methods: To model the brightness change of stars with
solar magnetic activity, we extended a well-established model of
solar brightness variations based on solar spectra, Spectral And
Total Irradiance REconstruction (SATIRE), to stars with different
fundamental parameters. For this we calculated stellar spectra for
different metallicities and effective temperature using the radiative
transfer code ATLAS9. Results: We show that even a small change
(e.g. within the observational error range) of metallicity or effective
temperature significantly affects the photometric brightness change
compared to the Sun. We find that for Sun-like stars, the amplitude
of the brightness variations obtained for Strömgren (b + y)/2 reaches
a local minimum for fundamental stellar parameters close to the solar
metallicity and effective temperature. Moreover, our results show that
the effect of inclination decreases for metallicity values greater than
the solar metallicity. Overall, we find that an exact determination of
fundamental stellar parameters is crucially important for understanding
stellar brightness changes.

---------------------------------------------------------
Title: ACTReSS: ACTive Rotating Star Simulator
Authors: Johnson, Luke Jonathan; Norris, Charlotte; Unruh, Yvonne;
Solanki, Sami; Krivova, Natalie
2018csss.confE..58J Altcode:
Variability of late-type stars on timescales of days or longer is
largely due to magnetic surface features (starspots and faculae). We
present ACTReSS, a software tool to calculate rotational lightcurves
of stars. The model uses limb-dependent intensities derived from
MURaM 3-D magneto-convection simulations (Beeck et al. 2014) for
quiet stellar photospheres and bright active regions. This allows us
to investigate the dependence of the flux variation on spectral type,
inclination and wavelength.

---------------------------------------------------------
Title: Solar irradiance variability on time scales of decades to
millennia
Authors: Krivova, Natalie; Solanki, Sami K.; Leng Yeo, Kok; Wu, Chi-Ju
2018cosp...42E1847K Altcode:
Reconstructions of past solar irradiance changes over as long periodsof
time as possible are crucial to our understanding of solarinfluence on
climate. They are only possible with the help of suitablemodels, based
on detailed understanding of the mechanisms of thevariability. With
the advance of such models, also the pastreconstructions are becoming
more reliable. Nevertheless, theremaining uncertainties spread out when
extrapolating back over longperiods of time. They are further amplified
by the increasingly pooreramount and quality of the available data that
bear information on pastsolar activity. We will discuss the progress
and the uncertainties ofirradiance reconstructions on time scales of
decades to millennia.

---------------------------------------------------------
Title: Solar Activity and Irradiance Reconstruction over the Holocene
Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami
K.; Kovaltsov, Gennady A.; Baroni, Melanie; Bard, Edouard
2018cosp...42E3695W Altcode:
Direct measurements of the solar irradiance are only available since
1978. To understand the solar influence on Earth's climate, longer
records and thus reconstructions of the solar variability in the past
are needed. The directly observed sunspot number allows going back
to 1610 A.D, although with progressively increasing uncertainty. To
assess solar variability at earlier times, one has to rely on indirect
proxies of solar activity, such as concentrations of cosmogenic isotopes
10Be and 14C in terrestrial archives. They are produced mostly in
the upper atmosphere by impinging galactic cosmic rays (GCRs). The
flux of GCRs is modulated by both the heliospheric magnetic field and
the geomagnetic field. Therefore, the isotope signals retrieved from
various sites around the globe show a very high degree of similarity,
reflecting changes in the solar activity. Still, short-and mid-term
deviations can be observed due to various systematic effects, such as
different geochemical production, atmospheric distribution processes
and local climatic conditions. To account for these differences, we
have constructed a state-of-the-art consistent multi-isotope composite
from one global 14C and six regional 10Be data sets. This composite is
then used to reconstruct decadal values of the total and spectral solar
irradiance over the Holocene with the semi-empirical SATIRE-M model,
while the quasi-11 year solar cycle has been simulated statistically.

---------------------------------------------------------
Title: Solar activity over nine millennia: A consistent multi-proxy
reconstruction
Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.;
Baroni, M.; Bard, E.; Solanki, S. K.
2018A&A...615A..93W Altcode: 2018arXiv180401302W
 Aims: The solar activity in the past millennia can only
be reconstructed from cosmogenic radionuclide proxy records in
terrestrial archives. However, because of the diversity of the proxy
archives, it is difficult to build a homogeneous reconstruction. All
previous studies were based on individual, sometimes statistically
averaged, proxy datasets. Here we aim to provide a new consistent
multi-proxy reconstruction of the solar activity over the last 9000
yr, using all available long-span datasets of 10Be and
14C in terrestrial archives. Methods: A new method,
based on a Bayesian approach, was applied for the first time to solar
activity reconstruction. A Monte Carlo search (using the χ2
statistic) for the most probable value of the modulation potential was
performed to match data from different datasets for a given time. This
provides a straightforward estimate of the related uncertainties. We
used six 10Be series of different lengths (from 500-10
000 yr) from Greenland and Antarctica, and the global 14C
production series. The 10Be series were resampled to match
wiggles related to the grand minima in the 14C reference
dataset. The stability of the long data series was tested. 
Results: The Greenland Ice-core Project (GRIP) and the Antarctic EDML
(EPICA Dronning Maud Land) 10Be series diverge from each
other during the second half of the Holocene, while the 14C
series lies in between them. A likely reason for the discrepancy is the
insufficiently precise beryllium transport and deposition model for
Greenland, which leads to an undercorrection of the GRIP series for
the geomagnetic shielding effect. A slow 6-7 millennia variability
with lows at ca. 5500 BC and 1500 AD in the long-term evolution
of solar activity is found. Two components of solar activity can
be statistically distinguished: the main component, corresponding
to the "normal" moderate level, and a component corresponding to
grand minima. A possible existence of a component representing
grand maxima is indicated, but it cannot be separated from the main
component in a statistically significant manner. Conclusions:
A new consistent reconstruction of solar activity over the last nine
millennia is presented with the most probable values of decadal sunspot
numbers and their realistic uncertainties. Independent components
of solar activity corresponding to the main moderate activity
and the grand-minimum state are identified; they may be related
to different operation modes of the dynamo. A table with the
reconstructed SN series is available at the CDS via anonymous ftp to <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/615/A93">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/A93</A>

---------------------------------------------------------
Title: Reconstructing Total Solar Irradiance with the help of MHD
simulations
Authors: Leng Yeo, Kok; Solanki, Sami; Noris, Charlotte; Unruh,
Yvonne; Krivova, Natalie; Beeck, Benjamin
2018EGUGA..20.8166L Altcode:
The variation in the solar irradiance is an important input into the
climate system. Whereas it is commonly thought that solar irradiance
variability is driven by the evolving surface magnetism, verifying this
assumption has been more difficult. Making use of 3D MHD simulations
of the solar atmosphere layers and state-of-the-art solar magnetograms
from the Solar Dynamics Observatory, we present a model of total solar
irradiance (TSI) that does not require any calibration of the modeled
TSI by the measured TSI. I.e., the modeled irradiance variability
is entirely independent of the observational record, except for the
absolute level. The model replicates 95% of the observed variability
between April 2010 and July 2016, leaving little scope for alternative
drivers of solar irradiance variability, at least over the time scales
examined (days to years).

---------------------------------------------------------
Title: VizieR Online Data Catalog: Reconstructed decadal sunspot
numbers (Wu+, 2018)
Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.;
Baroni, M.; Bard, E.; Solanki, S. K.
2018yCat..36150093W Altcode:
The file contains the decadal numbers of sunspot groups including
the decade central year [Y], the low [Sl], the mean [SM]
and the upper [S_u] 1-sigma (68% bounds). (1 data file).

---------------------------------------------------------
Title: Ca II K spectroheliograms for studies of long-term changes
in solar irradiance
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie
A.; Solanki, Sami K.
2018IAUS..340..125C Altcode: 2018arXiv180503928C
We address the importance of historical full-disc Ca II K
spectroheliograms for solar activity and irradiance reconstruction
studies. We review our work on processing such data to enable them to
be used in irradiance reconstructions. We also present our preliminary
estimates of the plage areas from five of the longest available
historical Ca II K archives.

---------------------------------------------------------
Title: The potential of Ca II K observations for solar activity and
variability studies
Authors: Ermolli, Ilaria; Chatzistergos, Theodosios; Krivova, Natalie
A.; Solanki, Sami K.
2018IAUS..340..115E Altcode: 2018arXiv180504483E
Several observatories around the globe started regular full-disc imaging
of the solar atmosphere in the Ca II K line in the early decades of
the 20th century. These observations are continued today
at a few sites with either old spectroheliographs or modern telescopes
equipped with narrow-band filters. The Ca II K time series are unique
in representing long-term variations of the Sun's chromospheric magnetic
field. However, meaningful results from their analysis require accurate
processing of the available data and robust merging of the information
stored in different archives. This paper provides an overview of the
historical and modern full-disc Ca II K observations, with focus on
their quality and the main results obtained from their analysis over
the last decade.

---------------------------------------------------------
Title: The Influence of Metallicity on Stellar Differential Rotation
and Magnetic Activity
Authors: Karoff, Christoffer; Metcalfe, Travis S.; Santos, Ângela
R. G.; Montet, Benjamin T.; Isaacson, Howard; Witzke, Veronika;
Shapiro, Alexander I.; Mathur, Savita; Davies, Guy R.; Lund, Mikkel N.;
Garcia, Rafael A.; Brun, Allan S.; Salabert, David; Avelino, Pedro P.;
van Saders, Jennifer; Egeland, Ricky; Cunha, Margarida S.; Campante,
Tiago L.; Chaplin, William J.; Krivova, Natalie; Solanki, Sami K.;
Stritzinger, Maximilian; Knudsen, Mads F.
2018ApJ...852...46K Altcode: 2017arXiv171107716K
Observations of Sun-like stars over the past half-century have improved
our understanding of how magnetic dynamos, like that responsible for the
11 yr solar cycle, change with rotation, mass, and age. Here we show
for the first time how metallicity can affect a stellar dynamo. Using
the most complete set of observations of a stellar cycle ever obtained
for a Sun-like star, we show how the solar analog HD 173701 exhibits
solar-like differential rotation and a 7.4 yr activity cycle. While
the duration of the cycle is comparable to that generated by the solar
dynamo, the amplitude of the brightness variability is substantially
stronger. The only significant difference between HD 173701 and the
Sun is its metallicity, which is twice the solar value. Therefore,
this provides a unique opportunity to study the effect of the
higher metallicity on the dynamo acting in this star and to obtain a
comprehensive understanding of the physical mechanisms responsible
for the observed photometric variability. The observations can be
explained by the higher metallicity of the star, which is predicted to
foster a deeper outer convection zone and a higher facular contrast,
resulting in stronger variability.

---------------------------------------------------------
Title: Analysis of full disc Ca II K spectroheliograms. I. Photometric
calibration and centre-to-limb variation compensation
Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.;
Krivova, Natalie A.
2018A&A...609A..92C Altcode: 2017arXiv171004949C
Context. Historical Ca II K spectroheliograms (SHG) are unique in
representing long-term variations of the solar chromospheric magnetic
field. They usually suffer from numerous problems and lack photometric
calibration. Thus accurate processing of these data is required to get
meaningful results from their analysis. Aims: In this paper we
aim at developing an automatic processing and photometric calibration
method that provides precise and consistent results when applied
to historical SHG. Methods: The proposed method is based on
the assumption that the centre-to-limb variation of the intensity in
quiet Sun regions does not vary with time. We tested the accuracy of
the proposed method on various sets of synthetic images that mimic
problems encountered in historical observations. We also tested our
approach on a large sample of images randomly extracted from seven
different SHG archives. Results: The tests carried out on the
synthetic data show that the maximum relative errors of the method are
generally &lt;6.5%, while the average error is &lt;1%, even if rather
poor quality observations are considered. In the absence of strong
artefacts the method returns images that differ from the ideal ones by
&lt;2% in any pixel. The method gives consistent values for both plage
and network areas. We also show that our method returns consistent
results for images from different SHG archives. Conclusions:
Our tests show that the proposed method is more accurate than other
methods presented in the literature. Our method can also be applied
to process images from photographic archives of solar observations at
other wavelengths than Ca II K.

---------------------------------------------------------
Title: Solar Spectral Irradiance Reconstruction over 9 Millennia
from a Composite 14C and 10Be Series
Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G.;
Solanki, S. K.
2017AGUFMSH42A..03W Altcode:
The Sun is the main external energy source to the Earth and thus the
knowledge of solar variability on different time scales is important
for understanding the solar influence on the terrestrial atmosphere
and climate. The overall energy input and its spectral distribution
are described by the total (TSI) and spectral (SSI) solar irradiance,
respectively. Direct measurements of the solar irradiance provide
information on solar variability on the decadal and shorter time
scales, while the sunspot number record covers four centuries. On
yet longer time scales only indirect proxies can be used, such
as the concentrations of the cosmogenic isotopes 10Be and 14C in
terrestrial archives. These isotopes are produced in the terrestrial
atmosphere by impinging cosmic rays, whose flux is modulated by solar
activity. Therefore the isotope data retrieved from various natural
archives around the globe show a very high degree of similarity
reflecting changes in the solar activity. Nevertheless, significant
short-term deviations can be observed due to the different geochemical
production processes and local climatic conditions. We will present
the newest TSI/SSI reconstruction over the last 9000 years based
on a new consistent composite multi-isotope proxy series. The solar
irradiance reconstruction reveals the global and robust pattern of
solar variability in the past.

---------------------------------------------------------
Title: Spectral variability of photospheric radiation due to
faculae. I. The Sun and Sun-like stars
Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne C.;
Solanki, Sami K.; Krivova, Natalie A.; Yeo, Kok Leng
2017A&A...605A..45N Altcode: 2017arXiv170504455N
Context. Stellar spectral variability on timescales of a day and
longer, arising from magnetic surface features such as dark spots
and bright faculae, is an important noise source when characterising
extra-solar planets. Current 1D models of faculae do not capture the
geometric properties and fail to reproduce observed solar facular
contrasts. Magnetoconvection simulations provide facular contrasts
accounting for geometry. Aims: We calculate facular contrast
spectra from magnetoconvection models of the solar photosphere with
a view to improve (a) future parameter determinations for planets
with early G type host stars and (b) reconstructions of solar spectral
variability. Methods: Regions of a solar twin (G2, log g = 4.44)
atmosphere with a range of initial average vertical magnetic fields
(100 to 500 G) were simulated using a 3D radiation-magnetohydrodynamics
code, MURaM, and synthetic intensity spectra were calculated from
the ultraviolet (149.5 nm) to the far infrared (160 000 nm) with the
ATLAS9 radiative transfer code. Nine viewing angles were investigated
to account for facular positions across most of the stellar disc. Results: Contrasts of the radiation from simulation boxes with
different levels of magnetic flux relative to an atmosphere with no
magnetic field are a complicated function of position, wavelength
and magnetic field strength that is not reproduced by 1D facular
models. Generally, contrasts increase towards the limb, but at UV
wavelengths a saturation and decrease are observed close to the
limb. Contrasts also increase strongly from the visible to the UV;
there is a rich spectral dependence, with marked peaks in molecular
bands and strong spectral lines. At disc centre, a complex relationship
with magnetic field was found and areas of strong magnetic field
can appear either dark or bright, depending on wavelength. Spectra
calculated for a wide variety of magnetic fluxes will also serve to
improve total and spectral solar irradiance reconstructions.

---------------------------------------------------------
Title: Solar Irradiance Variability is Caused by the Magnetic Activity
on the Solar Surface
Authors: Yeo, K. L.; Solanki, S. K.; Norris, C. M.; Beeck, B.; Unruh,
Y. C.; Krivova, N. A.
2017PhRvL.119i1102Y Altcode: 2017arXiv170900920Y
The variation in the radiative output of the Sun, described in terms
of solar irradiance, is important to climatology. A common assumption
is that solar irradiance variability is driven by its surface
magnetism. Verifying this assumption has, however, been hampered by
the fact that models of solar irradiance variability based on solar
surface magnetism have to be calibrated to observed variability. Making
use of realistic three-dimensional magnetohydrodynamic simulations
of the solar atmosphere and state-of-the-art solar magnetograms from
the Solar Dynamics Observatory, we present a model of total solar
irradiance (TSI) that does not require any such calibration. In doing
so, the modeled irradiance variability is entirely independent of the
observational record. (The absolute level is calibrated to the TSI
record from the Total Irradiance Monitor.) The model replicates 95%
of the observed variability between April 2010 and July 2016, leaving
little scope for alternative drivers of solar irradiance variability
at least over the time scales examined (days to years).

---------------------------------------------------------
Title: The Solar Ultraviolet Imaging Telescope on-board Aditya-L1
Authors: Tripathi, Durgesh; Ramaprakash, A. N.; Khan, Aafaque;
Ghosh, Avyarthana; Chatterjee, Subhamoy; Banerjee, Dipankar; Chordia,
Pravin; Gandorfer, Achim; Krivova, Natalie; Nandy, Dibyendu; Rajarshi,
Chaitanya; Solanki, Sami K.
2017CSci..113..616T Altcode: 2022arXiv220407732T
The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument
onboard the Aditya-L1 mission of ISRO that will measure and monitor
the solar radiation emitted in the near-ultraviolet wavelength range
(200-400 nm). SUIT will simultaneously map the photosphere and the
chromosphere of the Sun using 11 filters sensitive to different
wavelengths and covering different heights in the solar atmosphere
and help us understand the processes involved in the transfer of
mass and energy from one layer to the other. SUIT will also allow us
to measure and monitor spatially resolved solar spectral irradiance
that governs the chemistry of oxygen and ozone in the stratosphere of
Earth's atmosphere. This is central to our understanding of the Sun
climate relationship.

---------------------------------------------------------
Title: The nature of solar brightness variations
Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Cameron,
R. H.; Yeo, K. L.; Schmutz, W. K.
2017NatAs...1..612S Altcode: 2017arXiv171104156S
Determining the sources of solar brightness variations1,2,
often referred to as solar noise3, is important because
solar noise limits the detection of solar oscillations3,
is one of the drivers of the Earth's climate system4,5 and
is a prototype of stellar variability6,7—an important
limiting factor for the detection of extrasolar planets. Here,
we model the magnetic contribution to solar brightness variability
using high-cadence8,9 observations from the Solar Dynamics
Observatory (SDO) and the Spectral And Total Irradiance REconstruction
(SATIRE)10,11 model. The brightness variations caused by
the constantly evolving cellular granulation pattern on the solar
surface were computed with the Max Planck Institute for Solar System
Research (MPS)/University of Chicago Radiative Magnetohydrodynamics
(MURaM)12 code. We found that the surface magnetic field
and granulation can together precisely explain solar noise (that
is, solar variability excluding oscillations) on timescales from
minutes to decades, accounting for all timescales that have so far
been resolved or covered by irradiance measurements. We demonstrate
that no other sources of variability are required to explain the
data. Recent measurements of Sun-like stars by the COnvection ROtation
and planetary Transits (CoRoT)13 and Kepler14
missions uncovered brightness variations similar to that of the Sun,
but with a much wider variety of patterns15. Our finding
that solar brightness variations can be replicated in detail with
just two well-known sources will greatly simplify future modelling of
existing CoRoT and Kepler as well as anticipated Transiting Exoplanet
Survey Satellite16 and PLAnetary Transits and Oscillations
of stars (PLATO)17 data.

---------------------------------------------------------
Title: The variability of magnetic activity in solar-type stars
Authors: Fabbian, D.; Simoniello, R.; Collet, R.; Criscuoli, S.;
Korhonen, H.; Krivova, N. A.; Oláh, K.; Jouve, L.; Solanki, S. K.;
Alvarado-Gómez, J. D.; Booth, R.; García, R. A.; Lehtinen, J.;
See, V.
2017AN....338..753F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: New reconstruction of the sunspot group numbers since 1739
using direct calibration and "backbone" methods
Authors: Chatzistergos, Theodosios; Usoskin, Ilya G.; Kovaltsov,
Gennady A.; Krivova, Natalie A.; Solanki, Sami K.
2017A&A...602A..69C Altcode: 2017arXiv170206183C
Context. The group sunspot number (GSN) series constitute the longest
instrumental astronomical database providing information on solar
activity. This database is a compilation of observations by many
individual observers, and their inter-calibration has usually been
performed using linear rescaling. There are multiple published series
that show different long-term trends for solar activity. Aims:
We aim at producing a GSN series, with a non-linear non-parametric
calibration. The only underlying assumptions are that the differences
between the various series are due to different acuity thresholds
of the observers, and that the threshold of each observer remains
constant throughout the observing period. Methods: We used a
daisy chain process with backbone (BB) observers and calibrated all
overlapping observers to them. We performed the calibration of each
individual observer with a probability distribution function (PDF)
matrix constructed considering all daily values for the overlapping
period with the BB. The calibration of the BBs was carried out in a
similar manner. The final series was constructed by merging different
BB series. We modelled the propagation of errors straightforwardly
with Monte Carlo simulations. A potential bias due to the selection
of BBs was investigated and the effect was shown to lie within the 1σ
interval of the produced series. The exact selection of the reference
period was shown to have a rather small effect on our calibration
as well. Results: The final series extends back to 1739 and
includes data from 314 observers. This series suggests moderate
activity during the 18th and 19th century, which is significantly
lower than the high level of solar activity predicted by other recent
reconstructions applying linear regressions. Conclusions:
The new series provides a robust reconstruction, based on modern and
non-parametric methods, of sunspot group numbers since 1739, and it
confirms the existence of the modern grand maximum of solar activity in
the second half of the 20th century. Values of the group sunspot
number series are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A69">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A69</A>

---------------------------------------------------------
Title: Assessment of different sunspot number series using the
cosmogenic isotope 44Ti in meteorites
Authors: Asvestari, Eleanna; Usoskin, Ilya G.; Kovaltsov, Gennady A.;
Owens, Mathew J.; Krivova, Natalie A.; Rubinetti, Sara; Taricco, Carla
2017MNRAS.467.1608A Altcode: 2017MNRAS.tmp..193A
Many sunspot number series exist suggesting different levels of
solar activity during the past centuries. Their reliability can be
assessed only by comparing them with alternative indirect proxies. We
test different sunspot number series against the updated record of
cosmogenic radionuclide 44Ti measured in meteorites. Two
bounding scenarios of solar activity changes have been considered:
the HH-scenario (based on the series by Svalgaard and Schatten), in
particular, predicting moderate activity during the Maunder minimum,
and the LL-scenario (based on the RG series by Lockwood et
al.) predicting moderate activity for the 18th-19th centuries and the
very low activity level for the Maunder minimum. For each scenario,
the magnetic open solar flux, the heliospheric modulation potential and
the expected production of 44Ti were computed. The calculated
production rates were compared with the corresponding measurements of
44Ti activity in stony meteorites fallen since 1766. The
analysis reveals that the LL-scenario is fully consistent with the
measured 44Ti data, in particular, recovering the observed
secular trend between the 17th century and the Modern grand maximum. On
the contrary, the HH-scenario appears significantly inconsistent with
the data, mostly due to the moderate level of activity during the
Maunder minimum. It is concluded that the HH-scenario sunspot number
reconstruction significantly overestimates solar activity prior to the
mid-18th century, especially during the Maunder minimum. The exact level
of solar activity after 1750 cannot be distinguished with this method,
since both H- and L- scenarios appear statistically consistent with
the data.

---------------------------------------------------------
Title: EMPIRE: A robust empirical reconstruction of solar irradiance
variability
Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.
2017JGRA..122.3888Y Altcode: 2017arXiv170407652Y
We present a new empirical model of total and spectral solar irradiance
(TSI and SSI) variability entitled EMPirical Irradiance REconstruction
(EMPIRE). As with existing empirical models, TSI and SSI variability
is given by the linear combination of solar activity indices. In
empirical models, UV SSI variability is usually determined by
fitting the rotational variability in activity indices to that in
measurements. Such models have to date relied on ordinary least
squares regression, which ignores the uncertainty in the activity
indices. In an advance from earlier efforts, the uncertainty in the
activity indices is accounted for in EMPIRE by the application of
an error-in-variables regression scheme, making the resultant UV SSI
variability more robust. The result is consistent with observations and
unprecedentedly, with that from other modeling approaches, resolving
the long-standing controversy between existing empirical models and
other types of models. We demonstrate that earlier empirical models,
by neglecting the uncertainty in activity indices, underestimate UV
SSI variability. The reconstruction of TSI and visible and IR SSI from
EMPIRE is also shown to be consistent with observations. The EMPIRE
reconstruction is of utility to climate studies as a more robust
alternative to earlier empirical reconstructions.

---------------------------------------------------------
Title: Climate responses to SATIRE and SIM-based spectral solar
forcing in a 3D atmosphere-ocean coupled GCM
Authors: Wen, Guoyong; Cahalan, Robert F.; Rind, David; Jonas, Jeffrey;
Pilewskie, Peter; Wu, Dong L.; Krivova, Natalie A.
2017JSWSC...7A..11W Altcode:
We apply two reconstructed spectral solar forcing scenarios, one SIM
(Spectral Irradiance Monitor) based, the other the SATIRE (Spectral
And Total Irradiance REconstruction) modeled, as inputs to the GISS
(Goddard Institute for Space Studies) GCMAM (Global Climate Middle
Atmosphere Model) to examine climate responses on decadal to centennial
time scales, focusing on quantifying the difference of climate response
between the two solar forcing scenarios. We run the GCMAM for about
400 years with present day trace gas and aerosol for the two solar
forcing inputs. We find that the SIM-based solar forcing induces much
larger long-term response and 11-year variation in global averaged
stratospheric temperature and column ozone. We find significant
decreasing trends of planetary albedo for both forcing scenarios
in the 400-year model runs. However the mechanisms for the decrease
are very different. For SATIRE solar forcing, the decreasing trend
of planetary albedo is associated with changes in cloud cover. For
SIM-based solar forcing, without significant change in cloud cover on
centennial and longer time scales, the apparent decreasing trend of
planetary albedo is mainly due to out-of-phase variation in shortwave
radiative forcing proxy (downwelling flux for wavelength &gt;330 nm)
and total solar irradiance (TSI). From the Maunder Minimum to present,
global averaged annual mean surface air temperature has a response of
0.1 °C to SATIRE solar forcing compared to 0.04 °C to SIM-based solar
forcing. For 11-year solar cycle, the global surface air temperature
response has 3-year lagged response to either forcing scenario. The
global surface air 11-year temperature response to SATIRE forcing is
about 0.12 °C, similar to recent multi-model estimates, and comparable
to the observational-based evidence. However, the global surface air
temperature response to 11-year SIM-based solar forcing is insignificant
and inconsistent with observation-based evidence.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Group sunspot number series since
1739 (Chatzistergos+, 2017)
Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G. A.; Krivova,
N. A.; Solanki, S. K.
2017yCat..36020069C Altcode:
Annual, monthly, and daily values of the Group sunspot number series
produced in the paper. (3 data files).

---------------------------------------------------------
Title: Handling Historical Ca II K Spectroheliogram Observation
Authors: Solanki, S. K.; Chatzistergos, T.; Ermolli, I.; Krivova, N.
2016AGUFMSA51B2429S Altcode:
The total solar irradiance has been continuously monitored since 1978,
but climate studies require time-series extending further back in
time. Models assuming that the irradiance variations are due to the
evolution of the solar surface magnetic field manifesting itself
as bright faculae and dark sunspots have been quite successful in
reproducing the measured changes. Unfortunately full-disk observations
of the magnetic field exist only for the last four decades. Sunspot
observations (areas or numbers) have been employed to reconstruct past
irradiance changes, however they provide information about the plage
regions only indirectly. Information about the evolution of the plage
regions can potentially be obtained from Ca II K spectroheliograms that
are available for the last century. Such observational programs have
been carried out at several observatories and some of these photographic
archives have been digitised. However, analysis of such data is plagued
by numerous problems affecting the images and the lack of photometric
calibration. Here we present a new method to process historical Ca II K
spectroheliograms in order to perform the photometric calibration and
compensate for the centre-to-limb variations and artefacts affecting
the data. The method is tested on synthetic images with known artefacts
and a sample of images from various historical archives.

---------------------------------------------------------
Title: Solar Total and Spectral Irradiance Reconstruction over Last
9000 Years
Authors: Wu, C. J.; Krivova, N.; Solanki, S. K.; Usoskin, I. G.
2016AGUFMSA51B2430W Altcode:
Although the mechanisms of solar influence on Earth climate system
are not yet fully understood, solar total and spectral irradiance are
considered to be among the main determinants. Solar total irradiance
is the total flux of solar radiative energy entering Earth's climate
system, whereas the spectral irradiance describes this energy is
distributed over the spectrum. Solar irradiance in the UV band is of
special importance since it governs chemical processes in the middle and
upper atmosphere. On timescales of the 11-year solar cycle and shorter,
solar irradiance is measured by space-based instruments while models
are needed to reconstruct solar irradiance on longer timescale. The
SATIRE-M model (Spectral And Total Irradiance Reconstruction over
millennia) is employed in this study to reconstruct solar irradiance
from decadal radionuclide isotope data such as 14C and 10Be stored in
tree rings and ice cores, respectively. A reconstruction over the last
9000 years will be presented.

---------------------------------------------------------
Title: Non-linear re-calibration of group sunspot number series back
to 1819
Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G.; Solanki,
S. K.; Krivova, N.
2016AGUFMSH31B2547C Altcode:
Sunspot numbers form the longest series of direct observations
that give information about the activity of the Sun. It consists in
numerous records from different observers with different instruments
and techniques, and unavoidably requires a calibration of individual
observers to a standard reference condition. Such calibrations pose a
challenge and are still a subject of debate. Most of the calibration
methods are based on a simple linear scaling of data from one observer
to the other, proposed in the mid-19th century by Rudolf Wolf. This
method is still used in many recent reconstructions. However, this
linear method is shown, on both real and synthetic data, to be
very rough and incorrect because of the essentially non-linearity
of the relation.Here we present a new reconstruction of the
group sunspot number series back to 1819 attempting to take into
account the non-linearity of the relation between data of different
observers. We use an approach incorporating several overlapping backbone
observers (similar to Svalgaard and Schatten, 2016) and perform the
straightforward non-linear calibration of each observer to the backbone
(and the backbones with each other) based on the actual statistics of
the common daily values.

---------------------------------------------------------
Title: The Impact of the Revised Sunspot Record on Solar Irradiance
Reconstructions
Authors: Kopp, G.; Krivova, N.; Wu, C. J.; Lean, J.
2016SoPh..291.2951K Altcode: 2016SoPh..tmp...42K; 2016arXiv160105397K
Reliable historical records of the total solar irradiance (TSI)
are needed to assess the extent to which long-term variations in the
Sun's radiant energy that is incident upon Earth may exacerbate (or
mitigate) the more dominant warming in recent centuries that is due
to increasing concentrations of greenhouse gases. We investigate the
effects that the new Sunspot Index and Long-term Solar Observations
(SILSO) sunspot-number time series may have on model reconstructions
of the TSI. In contemporary TSI records, variations on timescales
longer than about a day are dominated by the opposing effects
of sunspot darkening and facular brightening. These two surface
magnetic features, retrieved either from direct observations or from
solar-activity proxies, are combined in TSI models to reproduce the
current TSI observational record. Indices that manifest solar-surface
magnetic activity, in particular the sunspot-number record, then enable
reconstructing historical TSI. Revisions of the sunspot-number record
therefore affect the magnitude and temporal structure of TSI variability
on centennial timescales according to the model reconstruction methods
that are employed. We estimate the effects of the new SILSO record on
two widely used TSI reconstructions, namely the NRLTSI2 and the SATIRE
models. We find that the SILSO record has little effect on either model
after 1885, but leads to solar-cycle fluctuations with greater amplitude
in the TSI reconstructions prior. This suggests that many eighteenth-
and nineteenth-century cycles could be similar in amplitude to those
of the current Modern Maximum. TSI records based on the revised sunspot
data do not suggest a significant change in Maunder Minimum TSI values,
and from comparing this era to the present, we find only very small
potential differences in the estimated solar contributions to the
climate with this new sunspot record.

---------------------------------------------------------
Title: The statistical study of global properties of sunspots observed
by SoHO/MDI continuum images over solar cycle 23
Authors: Goel, Suruchi; Krivova, Natalie; Solanki, Sami K.; Mathew,
Shibu K.
2016cosp...41E.687G Altcode:
A better understanding of inter-dependency of various sunspot parameters
such as magnetic field, intensity, temperature, size etc., and also
their variation with strength of solar activity cycle is important to
understand the magneto-convection process involved in sunspot formation
and evolution and hence to develop a consistent sunspot model. We have
investigated global sunspot properties using parameters of sunspots
identified from stray-light-corrected continuum images from SoHO/MDI
spanning from years 1996 to 2011. We find that the non-linear relation
between umbral core (minimum) intensity and sunspot area is best
represented by an exponential function, which reaches an asymptotic
value at 600 MSH. For the first time we have also observed that the
core intensity depends on shape of umbrae, i.e., circular umbrae
are statistically darker compared to the elongated ones. The core
intensity increases slightly towards the limb (by value of ~0.1 from
disk center to the limb). From sunspots sampled over the complete solar
cycle 23 and during the rising phase of cycle 24, we did not find any
solar-cycle variation in umbral core intensity. The penumbra to umbra
area ratio is found to be not a constant parameter, instead it shows a
quadratic decrease with sunspot area. Leading and following sunspots
usually have different morphological features, however in this study
we did not observe significant differences in their core intensity
and penumbra-umbra area ratio relation with the sunspot area.

---------------------------------------------------------
Title: Solar total and spectral irradiance reconstruction over last
9000 years
Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami K.
2016cosp...41E2072W Altcode:
Although the mechanisms of solar influence on Earth climate system
are not yet fully understood, solar total and spectral irradiance are
considered to be among the main determinants. Solar total irradiance
is the total flux of solar radiative energy entering Earth's climate
system, whereas the spectral irradiance describes this energy is
distributed over the spectrum. Solar irradiance in the UV band is of
special importance since it governs chemical processes in the middle and
upper atmosphere. On timescales of the 11-year solar cycle and shorter,
solar irradiance is measured by space-based instruments while models
are needed to reconstruct solar irradiance on longer timescale. The
SATIRE-M model (Spectral And Total Irradiance Reconstruction over
millennia) is employed in this study to reconstruct solar irradiance
from decadal radionuclide isotope data such as 14C and 10Be stored in
tree rings and ice cores, respectively. A reconstruction over the last
9000 years will be presented.

---------------------------------------------------------
Title: The Solar Ultraviolet Imaging Telescope onboard Aditya-L1
Authors: Ghosh, Avyarthana; Chatterjee, Subhamoy; Khan, Aafaque R.;
Tripathi, Durgesh; Ramaprakash, A. N.; Banerjee, Dipankar; Chordia,
Pravin; Gandorfer, Achim M.; Krivova, Natalie; Nandy, Dibyendu;
Rajarshi, Chaitanya; Solanki, Sami K.; Sriram, S.
2016SPIE.9905E..03G Altcode:
The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard
the Aditya-L1 spacecraft, the first dedicated solar mission of the
Indian Space Research Organization (ISRO), which will be put in a
halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis
Ritchey-Chrétien configuration with a combination of 11 narrow and
broad bandpass filters which will be used for full-disk solar imaging
in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide
near simultaneous observations of lower and middle layers of the solar
atmosphere, namely the Photosphere and Chromosphere. These observations
will help to improve our understanding of coupling and dynamics of
various layers of the solar atmosphere, mechanisms responsible for
stability, dynamics and eruption of solar prominences and Coronal Mass
ejections, and possible causes of solar irradiance variability in the
Near and Middle UV regions, which is of central interest for assessing
the Sun's influence on climate.

---------------------------------------------------------
Title: Long-term solar irradiance variability: knowns and unknowns
Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria;
Leng Yeo, Kok; Wu, Chi-Ju
2016cosp...41E1066K Altcode:
Long-term reconstructions of solar irradiance variability are crucial
to our understanding of solar influence on climate. They are only
possible with the help of suitable models, which in turn require a
thorough understanding of the mechanisms of this variability. With the
advance of such models, also the past reconstructions are becoming
more reliable. Nevertheless, the remaining uncertainties spread out
when extrapolating back over long periods of time, amplified by the
increasingly poorer quality and reliability of the available data that
bear information on past solar activity. We will discuss the progress
and the reliability of irradiance reconstructions on time scales of
decades to millennia.

---------------------------------------------------------
Title: EMPIRE reconstruction of TSI and SSI since 1947
Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K.
2016cosp...41E2107L Altcode:
TSI and SSI, at least in the UV, have been monitored from space
since 1978. This is accompanied by the development of models aimed
at reproducing TSI and SSI by relating the variability to solar
magnetism. We present a reconstruction of TSI and SSI, spanning 1947
to 2016, based on the EMPirical Irradiance REconstruction (EMPIRE)
model. EMPIRE is what is termed a proxy model, based on the regression
of solar activity indices to measured solar irradiance. One of the main
open questions in our understanding of solar irradiance variability,
and consequently its influence on the Earth's climate, is the
variation in UV SSI over the solar cycle. Solar cycle variability
in UV SSI in preceding proxy models, such as NRLSSI, NRLSSI2 and
MGNM, is significantly weaker than that indicated by other types
of models longwards of around 240 nm. We demonstrate here that this
discrepancy arises from previous proxy models neglecting the influence
of measurement uncertainty on regression analyses. EMPIRE is based
on the same activity indices as NRLSSI(2) but by taking measurement
uncertainty into account, reproduces variability consistent with other
modelling approaches and measurements. As such, EMPIRE supersedes
earlier proxy models, including NRLSSI(2) and MGNM.

---------------------------------------------------------
Title: The origin of Total Solar Irradiance variability on timescales
less than a day
Authors: Shapiro, Alexander; Krivova, Natalie; Schmutz, Werner;
Solanki, Sami K.; Leng Yeo, Kok; Cameron, Robert; Beeck, Benjamin
2016cosp...41E1774S Altcode:
Total Solar Irradiance (TSI) varies on timescales from minutes to
decades. It is generally accepted that variability on timescales of
a day and longer is dominated by solar surface magnetic fields. For
shorter time scales, several additional sources of variability have
been proposed, including convection and oscillation. However, available
simplified and highly parameterised models could not accurately explain
the observed variability in high-cadence TSI records. We employed the
high-cadence solar imagery from the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory and the SATIRE (Spectral And
Total Irradiance Reconstruction) model of solar irradiance variability
to recreate the magnetic component of TSI variability. The recent 3D
simulations of solar near-surface convection with MURAM code have been
used to calculate the TSI variability caused by convection. This allowed
us to determine the threshold timescale between TSI variability caused
by the magnetic field and convection. Our model successfully replicates
the TSI measurements by the PICARD/PREMOS radiometer which span the
period of July 2010 to February 2014 at 2-minute cadence. Hence,
we demonstrate that solar magnetism and convection can account for
TSI variability at all timescale it has ever been measured (sans the
5-minute component from p-modes).

---------------------------------------------------------
Title: Modelling the Spectral Contrasts of Stellar Faculae.
Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne; Solanki,
Sami; Yeo, Kok Leng; Krivova, Natalie
2016csss.confE..63N Altcode:
Facular contrasts are an important parameter in modelling stellar
variability and exoplanet transits. The ultimate goal of this work
will be to model the contrasts of faculae for different spectral types
and thus improve the modelling of solar and stellar photospheric
variability. This is done by using a radiative transfer algorithm
(ATLAS9) on magneto-convection simulations. Starting with solar twins,
we derive facular contrasts as a function of limb angle and discuss
their wavelength dependence.

---------------------------------------------------------
Title: Reconstruction of spectral solar irradiance since 1700 from
simulated magnetograms
Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K.;
Unruh, Y. C.; Yeo, K. L.
2016A&A...590A..63D Altcode: 2016arXiv160502039D
 Aims: We present a reconstruction of the spectral solar
irradiance since 1700 using the SATIRE-T2 (Spectral And Total Irradiance
REconstructions for the Telescope era version 2) model. This model
uses as input magnetograms simulated with a surface flux transport
model fed with semi-synthetic records of emerging sunspot groups. Methods: The record of sunspot group areas and positions from the
Royal Greenwich Observatory (RGO) is only available since 1874. We
used statistical relationships between the properties of sunspot
group emergence, such as the latitude, area, and tilt angle, and the
sunspot cycle strength and phase to produce semi-synthetic sunspot
group records starting in the year 1700. The semi-synthetic records
are fed into a surface flux transport model to obtain daily simulated
magnetograms that map the distribution of the magnetic flux in active
regions (sunspots and faculae) and their decay products on the solar
surface. The magnetic flux emerging in ephemeral regions is accounted
for separately based on the concept of extended cycles whose length
and amplitude are linked to those of the sunspot cycles through the
sunspot number. The magnetic flux in each surface component (sunspots,
faculae and network, and ephemeral regions) was used to compute the
spectral and total solar irradiance (TSI) between the years 1700
and 2009. This reconstruction is aimed at timescales of months or
longer although the model returns daily values. Results: We
found that SATIRE-T2, besides reproducing other relevant observations
such as the total magnetic flux, reconstructs the TSI on timescales
of months or longer in good agreement with the PMOD composite of
observations, as well as with the reconstruction starting in 1878
based on the RGO-SOON data. The model predicts an increase in the TSI
of 1.2+0.2-0.3 Wm-2 between 1700
and the present. The spectral irradiance reconstruction is in good
agreement with the UARS/SUSIM measurements as well as the Lyman-α
composite. The complete total and spectral (115 nm-160 μm)
irradiance reconstructions since 1700 will be available from <A
href="http://www2.mps.mpg.de/projects/sun-climate/data.html">http://www2.mps.mpg.de/projects/sun-climate/data.html</A>

---------------------------------------------------------
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.
2016A&A...589A..46S Altcode: 2016arXiv160204447S
Context. Regular spaceborne measurements have revealed that
solar brightness varies on multiple timescales, variations on
timescales greater than a day being attributed to a surface magnetic
field. Independently, ground-based and spaceborne measurements suggest
that Sun-like stars show a similar, but significantly broader pattern
of photometric variability. Aims: To understand whether the
broader pattern of stellar variations is consistent with the solar
paradigm, we assess relative contributions of faculae and spots to
solar magnetically-driven brightness variability. We investigate
how the solar brightness variability and its facular and spot
contributions depend on the wavelength, timescale of variability,
and position of the observer relative to the ecliptic plane. 
Methods: We performed calculations with the SATIRE model, which
returns solar brightness with daily cadence from solar disc area
coverages of various magnetic features. We took coverages as seen by
an Earth-based observer from full-disc SoHO/MDI and SDO/HMI data and
projected them to mimic out-of-ecliptic viewing by an appropriate
transformation. Results: Moving the observer away from the
ecliptic plane increases the amplitude of 11-year variability as it
would be seen in Strömgren (b + y)/2 photometry, but decreases the
amplitude of the rotational brightness variations as it would appear
in Kepler and CoRoT passbands. The spot and facular contributions to
the 11-year solar variability in the Strömgren (b + y)/2 photometry
almost fully compensate each other so that the Sun appears anomalously
quiet with respect to its stellar cohort. Such a compensation does not
occur on the rotational timescale. Conclusions: The rotational
solar brightness variability as it would appear in the Kepler and
CoRoT passbands from the ecliptic plane is spot-dominated, but the
relative contribution of faculae increases for out-of-ecliptic viewing
so that the apparent brightness variations are faculae-dominated for
inclinations less than about I = 45°. Over the course of the 11-year
activity cycle, the solar brightness variability is faculae-dominated
shortwards of 1.2 μm independently of the inclination.

---------------------------------------------------------
Title: Semi-empirical Long-term Reconstruction of the Heliospheric
Parameters: Validation by Cosmogenic Radionuclide Records
Authors: Asvestari, E.; Usoskin, I. G.; Cameron, R. H.; Krivova, N. A.
2016ASPC..504..269A Altcode:
We have developed a semi-empirical model that describes the heliospheric
modulation of galactic cosmic rays considering different heliospheric
parameters. This model is an improvement of a previous model. The
parameters of the model are fitted using the observations and
reconstructions of the heliospheric parameters for the period 1976 -
2013, which includes the latest very weak solar minimum. The modulation
potential is computed since 1610 using different reconstructions of the
open solar magnetic flux and it is then used to compute the production
and distribution of cosmogenic isotope 14C, which was
subsequently compared with terrestrial archives in tree rings. It is
shown that the group sunspot number series by Svalgaard &amp; Schatten
(2015) is inconsistent with the data, while other series agree well.

---------------------------------------------------------
Title: Modelling Solar and Stellar Brightness Variabilities
Authors: Yeo, K. L.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K.
2016ASPC..504..273Y Altcode:
Total and spectral solar irradiance, TSI and SSI, have been measured
from space since 1978. This is accompanied by the development of
models aimed at replicating the observed variability by relating
it to solar surface magnetism. Despite significant progress,
there remains persisting controversy over the secular change and the
wavelength-dependence of the variation with impact on our understanding
of the Sun's influence on the Earth's climate. We highlight the recent
progress in TSI and SSI modelling with SATIRE. Brightness variations
have also been observed for Sun-like stars. Their analysis can profit
from knowledge of the solar case and provide additional constraints
for solar modelling. We discuss the recent effort to extend SATIRE to
Sun-like stars.

---------------------------------------------------------
Title: Exploiting Four Historical Ca II K Spectroheliogram Archives
Authors: Chatzistergos, T.; Ermolli, I.; Solanki, S. K.; Krivova, N. A.
2016ASPC..504..227C Altcode:
Here we briefly review the status of the project aimed to analyse
the potential of historical Ca II K archives for studies of long-term
(decades to a century) solar variability.

---------------------------------------------------------
Title: Commission 12: Solar Radiation and Structure
Authors: Cauzzi, Gianna; Shchukina, Nataliya; Kosovichev, Alexander;
Bianda, Michele; Brandenburg, Axel; Chou, Dean-Yi; Dasso, Sergio; Ding,
Ming-De; Jefferies, Stuart; Krivova, Natalie; Kuznetsov, Vladimir D.;
Moreno-Insertis, Fernando
2016IAUTA..29..278C Altcode:
Commission 12 of the International Astronomical Union encompasses
investigations of the internal structure and dynamics of the Sun,
the quiet solar atmosphere, solar radiation and its variability, and
the nature of relatively stable magnetic structures like sunspots,
faculae and the magnetic network. The Commission sees participation
of over 300 scientists worldwide.

---------------------------------------------------------
Title: Variation of the Mn I 539.4 nm line with the solar cycle
Authors: Danilovic, S.; Solanki, S. K.; Livingston, W.; Krivova, N.;
Vince, I.
2016A&A...587A..33D Altcode: 2015arXiv151101286D
Context. As a part of the long-term program at Kitt Peak National
Observatory (KPNO), the Mn I 539.4 nm line has been observed for
nearly three solar cycles using the McMath telescope and the 13.5 m
spectrograph in double-pass mode. These full-disk spectrophotometric
observations revealed an unusually strong change of this line's
parameters over the solar cycle. Aims: Optical pumping by the Mg
II k line was originally proposed to explain these variations. More
recent studies have proposed that this is not required and that
the magnetic variability (I.e., the changes in solar atmospheric
structure due to faculae) might explain these changes. Magnetic
variability is also the mechanism that drives the changes in total
solar irradiance variations (TSI). With this work we investigate this
proposition quantitatively by using the same model that was earlier
successfully employed to reconstruct the irradiance. Methods:
We reconstructed the changes in the line parameters using the model
SATIRE-S, which takes only variations of the daily surface distribution
of the magnetic field into account. We applied exactly the same model
atmospheres and value of the free parameter as were used in previous
solar irradiance reconstructions to now model the variation in the Mn
I 539.4 nm line profile and in neighboring Fe I lines. We compared
the results of the theoretical model with KPNO observations. 
Results: The changes in the Mn I 539.4 nm line and a neighbouring Fe
I 539.52 nm line over approximately three solar cycles are reproduced
well by the model without additionally tweaking the model parameters,
if changes made to the instrument setup are taken into account. The
model slightly overestimates the change for the strong Fe I 539.32 nm
line. Conclusions: Our result confirms that optical pumping
of the Mn II 539.4 nm line by Mg II k is not the main cause of its
solar cycle change. It also provides independent confirmation of solar
irradiance models which are based on the assumption that irradiance
variations are caused by the evolution of the solar surface magnetic
flux. The result obtained here also supports the spectral irradiance
variations computed by these models.

---------------------------------------------------------
Title: Sunspot areas and tilt angles for solar cycles 7-10
Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova,
N. A.; Solanki, S. K.
2015A&A...584A..73S Altcode: 2015arXiv150807849S
 Aims: Extending the knowledge about the properties of solar
cycles into the past is essential for understanding the solar
dynamo. This paper aims to estimate areas of sunspots observed by
Schwabe in 1825-1867 and to calculate the tilt angles of sunspot
groups. Methods: The sunspot sizes in Schwabe's drawings are
not to scale and need to be converted into physical sunspot areas. We
employed a statistical approach assuming that the area distribution
of sunspots was the same in the 19th century as it was in the 20th
century. Results: Umbral areas for about 130 000 sunspots
observed by Schwabe were obtained, as well as the tilt angles of
sunspot groups assuming them to be bipolar. There is, of course,
no polarity information in the observations. The annually averaged
sunspot areas correlate reasonably with sunspot number. We derived
an average tilt angle by attempting to exclude unipolar groups with
a minimum separation of the two alleged polarities and an outlier
rejection method which follows the evolution of each group and detects
the moment it turns unipolar at its decay. As a result, the tilt angles,
although displaying considerable scatter, average to 5̊.85 ± 0, with
the leading polarity located closer to the equator, in good agreement
with tilt angles obtained from 20th century data sets. Sources of
uncertainties in the tilt angle determination are discussed and need to
be addressed whenever different data sets are combined. The sunspot area
and tilt angle data are provided at the CDS. The sunspot area and
tilt angle data are only available at the CDS via anonymous ftp to <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/584/A73">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/584/A73</A>

---------------------------------------------------------
Title: The Impact of the Revised Sunspot Record on Solar Irradiance
Reconstructions
Authors: Kopp, G.; Krivova, N.; Lean, J.; Wu, C. J.
2015AGUFMSH23C2451K Altcode:
We describe the expected effects of the new sunspot number time series
on proxy model based reconstructions of the total solar irradiance
(TSI), which is largely explained by the opposing effects of dark
sunspots and bright faculae. Regressions of indices for facular
brightening and sunspot darkening with time series of direct TSI
observations during the recent 37-year spacecraft TSI measurement
era determine the relative contributions from each. Historical TSI
reconstructions are enabled by extending these proxy models back in time
prior to the start of the measurement record using a variety of solar
activity indices including the sunspot number time series alone prior
to 1882. Such reconstructions are critical for Earth climate research,
which requires knowledge of the incident energy from the Sun to assess
climate sensitivity to the natural influence of solar variability. Two
prominent TSI reconstructions that utilize the sunspot record starting
in 1610 are the NRLTSI and the SATIRE models. We review the indices
that each currently uses and estimate the effects the revised sunspot
record has on these reconstructions.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Sunspot areas and tilt angles
(Senthamizh Pavai+, 2015)
Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova,
N.; Solanki, S.
2015yCat..35840073S Altcode:
We present sunspot positions and areas from historical observations
of sunspots by Samuel Heinrich Schwabe from Dessau, Germany. He has
recorded his observations of sunspots from 1825-1867 as drawings in
small circles of about 5cm diameter (representing the solar disk). Even
though he has used quite a number of telescopes for his observations,
the majority of the full-disk drawings were made with a 3-1/2-foot
telescope from Fraunhofer. His observing log books are stored in the
library of the Royal Astronomical Society in London. Those drawings
were digitized photographically with a resolution of 2912x4378 pixels
per page. The sizes and positions of the sunspots were measured using
a dozen of circular mouse cursor shapes with different diameters. The
sunspot sizes in Schwabe's drawings are not to scale and need to
be converted into physical sunspot areas. We employed a statistical
approach assuming that the area distribution of sunspots was the same
in the 19th century as it was in the 20th century. Umbral areas for
about 130,000 sunspots observed by Schwabe were obtained, as well as
the tilt angles of sunspot groups assuming them to be bipolar (two
or more spots). There is, of course, no polarity information in the
observations. Both an updated sunspot database and a tilt angle
database are available at http://www.aip.de/Members/rarlt/ sunspots
for further study. (2 data files).

---------------------------------------------------------
Title: The Maunder minimum (1645-1715) was indeed a grand minimum:
A reassessment of multiple datasets
Authors: Usoskin, Ilya G.; Arlt, Rainer; Asvestari, Eleanna; Hawkins,
Ed; Käpylä, Maarit; Kovaltsov, Gennady A.; Krivova, Natalie;
Lockwood, Michael; Mursula, Kalevi; O'Reilly, Jezebel; Owens, Matthew;
Scott, Chris J.; Sokoloff, Dmitry D.; Solanki, Sami K.; Soon, Willie;
Vaquero, José M.
2015A&A...581A..95U Altcode: 2015arXiv150705191U
 Aims: Although the time of the Maunder minimum (1645-1715) is
widely known as a period of extremely low solar activity, it is still
being debated whether solar activity during that period might have
been moderate or even higher than the current solar cycle #24. We have
revisited all existing evidence and datasets, both direct and indirect,
to assess the level of solar activity during the Maunder minimum. 
Methods: We discuss the East Asian naked-eye sunspot observations, the
telescopic solar observations, the fraction of sunspot active days,
the latitudinal extent of sunspot positions, auroral sightings at
high latitudes, cosmogenic radionuclide data as well as solar eclipse
observations for that period. We also consider peculiar features of
the Sun (very strong hemispheric asymmetry of the sunspot location,
unusual differential rotation and the lack of the K-corona) that imply
a special mode of solar activity during the Maunder minimum. 
Results: The level of solar activity during the Maunder minimum is
reassessed on the basis of all available datasets. Conclusions:
We conclude that solar activity was indeed at an exceptionally low
level during the Maunder minimum. Although the exact level is still
unclear, it was definitely lower than during the Dalton minimum of
around 1800 and significantly below that of the current solar cycle
#24. Claims of a moderate-to-high level of solar activity during the
Maunder minimum are rejected with a high confidence level.

---------------------------------------------------------
Title: The role of the Fraunhofer lines in solar brightness
variability
Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Tagirov,
R. V.; Schmutz, W. K.
2015A&A...581A.116S Altcode: 2015arXiv150705437S
Context. The solar brightness varies on timescales from minutes to
decades. A clear identification of the physical processes behind such
variations is needed for developing and improving physics-based models
of solar brightness variability and reconstructing solar brightness
in the past. This is, in turn, important for better understanding
the solar-terrestrial and solar-stellar connections. Aims:
We estimate the relative contributions of the continuum, molecular,
and atomic lines to the solar brightness variations on different
timescales. Methods: Our approach is based on the assumption that
variability of the solar brightness on timescales greater than a day
is driven by the evolution of the solar surface magnetic field. We
calculated the solar brightness variations employing the solar
disc area coverage of magnetic features deduced from the MDI/SOHO
observations. The brightness contrasts of magnetic features relative
to the quiet Sun were calculated with a non-LTE radiative transfer
code as functions of disc position and wavelength. By consecutive
elimination of molecular and atomic lines from the radiative transfer
calculations, we assessed the role of these lines in producing solar
brightness variability. Results: We show that the variations
in Fraunhofer lines define the amplitude of the solar brightness
variability on timescales greater than a day and even the phase of
the total solar irradiance variability over the 11-year cycle. We also
demonstrate that molecular lines make substantial contribution to solar
brightness variability on the 11-year activity cycle and centennial
timescales. In particular, our model indicates that roughly a quarter
of the total solar irradiance variability over the 11-year cycle
originates in molecular lines. The maximum of the absolute spectral
brightness variability on timescales greater than a day is associated
with the CN violet system between 380 and 390 nm.

---------------------------------------------------------
Title: Modelling stellar brightness variations
Authors: Shapiro, Alexander; Solanki, Sami K.; Krivova, Natalie
2015IAUGA..2256741S Altcode:
We develop a model that attributes the variability of the stellar
brightness to the imbalance between starspot darkening and facular
brightening. Our approach is based on the assumption that the
photometric variability of the Sun and Sun-like stars have the same
fundamental causes so that we can describe stellar variability by
extrapolating the solar model. Our results suggest that the solar
paradigm is remarkably successful in explaining the stellar variability
on the activity cycle time-scale. In particular, the model reproduces
the observed reversal of the in-phase activity-brightness relationship
for low-activity stars to an anti-phase one for more active stars.We
simulate the solar variability as it would be measured out-of-ecliptic
by Kepler and CoRoT and discuss the relative contributions of spots
and faculae to the photometric stellar variability.

---------------------------------------------------------
Title: UV solar irradiance in observations and the NRLSSI and
SATIRE-S models
Authors: Yeo, K. L.; Ball, W. T.; Krivova, N. A.; Solanki, S. K.;
Unruh, Y. C.; Morrill, J.
2015JGRA..120.6055Y Altcode: 2015arXiv150701224Y
Total solar irradiance and UV spectral solar irradiance has been
monitored since 1978 through a succession of space missions. This is
accompanied by the development of models aimed at replicating solar
irradiance by relating the variability to solar magnetic activity. The
Naval Research Laboratory Solar Spectral Irradiance (NRLSSI) and
Spectral And Total Irradiance REconstruction for the Satellite era
(SATIRE-S) models provide the most comprehensive reconstructions of
total and spectral solar irradiance over the period of satellite
observation currently available. There is persistent controversy
between the various measurements and models in terms of the wavelength
dependence of the variation over the solar cycle, with repercussions on
our understanding of the influence of UV solar irradiance variability
on the stratosphere. We review the measurement and modeling of UV solar
irradiance variability over the period of satellite observation. The
SATIRE-S reconstruction is consistent with spectral solar irradiance
observations where they are reliable. It is also supported by an
independent, empirical reconstruction of UV spectral solar irradiance
based on Upper Atmosphere Research Satellite/Solar Ultraviolet Spectral
Irradiance Monitor measurements from an earlier study. The weaker
solar cycle variability produced by NRLSSI between 300 and 400 nm is
not evident in any available record. We show that although the method
employed to construct NRLSSI is principally sound, reconstructed solar
cycle variability is detrimentally affected by the uncertainty in the
SSI observations it draws upon in the derivation. Based on our findings,
we recommend, when choosing between the two models, the use of SATIRE-S
for climate studies.

---------------------------------------------------------
Title: The Maunder minimum: A reassessment from multiple dataset
Authors: Usoskin, Ilya; Arlt, Rainer; Asvestari, Eleanna; Kovaltsov,
Gennady; Krivova, Natalie; Lockwood, Michael; Käpylä, Maarit; Owens,
Matthew; Sokoloff, Dmitry D.; Solanki, Sami; Soon, Willie; Vaquero,
Jose; Scott, Chris
2015IAUGA..2253036U Altcode:
The Maunder minimum (MM) in 1645-1715 was a period of the lowest ever
known solar activity recorded via sunspot numbers since 1610. Since
it is the only Grand minimum of solar activity directly observed,
it forms a benchmark for the solar variability studies. Therefore,
it is crucially important to assess the level and other features
of temporal and spatial solar magnetic variability during that
time. However, because of uncertainties related mostly to ambiguity
of some historical sunspot observation records, the exact level of
solar activity during the MM is somewhat unclear, leaving room for
continuous discussions and speculations. Many of these issues have been
addressed by Jack Eddy in his cornerstone papers of 1976 and 1983,
but since then numerous new pieces of evidence and datasets have
appeared, making it possible to verify the paradigm of the Maunder
minimum with far greater certainty than before.Here we provide a full
reassessment of the Maunder minimum using all the available datasets:
augmented sunspot counts and drawings; revisited historical archives;
both well-known and newly revealed records of auroral observations;
cosmic ray variability via cosmogenic isotope records of 14C
in tree trunks, 10Be in ice cores and 44Ti in
fallen meteorites. We show that, while the exact level of the activity
is not easy to determine, the Sun indeed exhibited exceptionally low
magnetic activity during the MM, in comparison to other periods of
moderate or decreased activity, such as the Dalton minimum (ca. 1800),
the Gleissberg minimum (ca. 1900) and the present weak solar cycle #
24. We show that a scenario of moderate or strong activity during the
MM contradicts all the available datasets.Thus, we confirm, using
all the presently available datasets of different nature, that the
period of the Maunder minimum in 1645-1715 was indeed a Grand minimum,
with very low solar surface magnetic activity, low intensity of the
interplanetary magnetic field, as well as lower frequency and higher
geographical latitude of auroral occurrence. Meanwhile some indications
of the continuation, but at a very low level, of the 11-year solar
cycle can be found in the data.

---------------------------------------------------------
Title: Solar cycle variation in UV solar spectral irradiance
Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K.
2015IAUGA..2253798L Altcode:
Solar spectral irradiance, SSI, in the UV has been measured from space,
almost without interruption, since 1978. This is accompanied by the
development of models aimed at reconstructing SSI by relating its
variability to solar magnetic activity. The various satellite records
and model reconstructions differ significantly in terms of the variation
over the solar cycle, with the consequence that their application to
climate models yield qualitatively different results. Here, we highlight
the key discrepancies between available records and reconstructions,
and discuss the possible underlying causes.

---------------------------------------------------------
Title: Solar Cycle Variation in Solar Irradiance
Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.
2015sac..book..137Y Altcode:
No abstract at ADS

---------------------------------------------------------
Title: GISS GCMAM Modeled Climate Responses to Total and Spectral
Solar Forcing on Decadal and Centennial Time Scales
Authors: Wen, G.; Cahalan, R. F.; Rind, D. H.; Jonas, J.; Pilewskie,
P.; Harder, J. W.; Krivova, N.
2014AGUFMSA53A4108W Altcode:
We examine the influence of the SORCE (Solar Radiation and Climate
Experiment) SIM (Spectral Irradiance Monitor) observed spectral
solar irradiance (SSI) variations on Earth's climate. We apply two
reconstructed spectral solar forcing scenarios, one SIM based, the other
based on the SATIRE (Spectral And Total Irradiance REconstruction)
model, as inputs to the GISS (Goddard Institute for Space Studies)
GCMAM (Global Climate Middle Atmosphere Model) to examine the climate
responses on decadal and centennial time scales. We show that the
atmosphere has different temperature, ozone, and dynamic responses to
the two solar spectral forcing scenarios, even when the variations
in TSI (Total Solar Irradiance) are the same. We find that solar
variations under either scenario contribute a small fraction of the
observed temperature increase since the industrial revolution. The trend
of global averaged surface air temperature response to the SIM-based
solar forcing is 0.02 °C/century, about half of the temperature trend
to the SATIRE-based SSI. However the temporal variation of the surface
air temperature for the SIM-based solar forcing scenario is much larger
compared to its SATIRE counterpart. Further research is required to
examine TSI and SSI variations in the ascending phase of solar cycle
24, to assess their implications for the solar influence on climate.

---------------------------------------------------------
Title: Solar Spectral Irradiance Variability in November/December
2012: Comparison of Observations by Instruments on the International
Space Station and Models
Authors: Thuillier, G.; Schmidtke, G.; Erhardt, C.; Nikutowski, B.;
Shapiro, A. I.; Bolduc, C.; Lean, J.; Krivova, N.; Charbonneau, P.;
Cessateur, G.; Haberreiter, M.; Melo, S.; Delouille, V.; Mampaey,
B.; Yeo, K. L.; Schmutz, W.
2014SoPh..289.4433T Altcode: 2014SoPh..tmp..120T
Onboard the International Space Station (ISS), two instruments
are observing the solar spectral irradiance (SSI) at wavelengths
from 16 to 2900 nm. Although the ISS platform orientation generally
precludes pointing at the Sun more than 10 - 14 days per month, in
November/December 2012 a continuous period of measurements was obtained
by implementing an ISS `bridging' maneuver. This enabled observations to
be made of the solar spectral irradiance (SSI) during a complete solar
rotation. We present these measurements, which quantify the impact
of active regions on SSI, and compare them with data simultaneously
gathered from other platforms, and with models of spectral irradiance
variability. Our analysis demonstrates that the instruments onboard the
ISS have the capability to measure SSI variations consistent with other
instruments in space. A comparison among all available SSI measurements
during November-December 2012 in absolute units with reconstructions
using solar proxies and observed solar activity features is presented
and discussed in terms of accuracy.

---------------------------------------------------------
Title: Solar Cycle Variation in Solar Irradiance
Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.
2014SSRv..186..137Y Altcode: 2014arXiv1407.4249Y; 2014SSRv..tmp...25Y
The correlation between solar irradiance and the 11-year solar activity
cycle is evident in the body of measurements made from space, which
extend over the past four decades. Models relating variation in solar
irradiance to photospheric magnetism have made significant progress
in explaining most of the apparent trends in these observations. There
are, however, persistent discrepancies between different measurements
and models in terms of the absolute radiometry, secular variation and
the spectral dependence of the solar cycle variability. We present an
overview of solar irradiance measurements and models, and discuss the
key challenges in reconciling the divergence between the two.

---------------------------------------------------------
Title: The PLATO 2.0 mission
Authors: Rauer, H.; Catala, C.; Aerts, C.; Appourchaux, T.; Benz,
W.; Brandeker, A.; Christensen-Dalsgaard, J.; Deleuil, M.; Gizon,
L.; Goupil, M. -J.; Güdel, M.; Janot-Pacheco, E.; Mas-Hesse,
M.; Pagano, I.; Piotto, G.; Pollacco, D.; Santos, Ċ.; Smith, A.;
Suárez, J. -C.; Szabó, R.; Udry, S.; Adibekyan, V.; Alibert, Y.;
Almenara, J. -M.; Amaro-Seoane, P.; Eiff, M. Ammler-von; Asplund, M.;
Antonello, E.; Barnes, S.; Baudin, F.; Belkacem, K.; Bergemann, M.;
Bihain, G.; Birch, A. C.; Bonfils, X.; Boisse, I.; Bonomo, A. S.;
Borsa, F.; Brandão, I. M.; Brocato, E.; Brun, S.; Burleigh, M.;
Burston, R.; Cabrera, J.; Cassisi, S.; Chaplin, W.; Charpinet, S.;
Chiappini, C.; Church, R. P.; Csizmadia, Sz.; Cunha, M.; Damasso, M.;
Davies, M. B.; Deeg, H. J.; Díaz, R. F.; Dreizler, S.; Dreyer, C.;
Eggenberger, P.; Ehrenreich, D.; Eigmüller, P.; Erikson, A.; Farmer,
R.; Feltzing, S.; de Oliveira Fialho, F.; Figueira, P.; Forveille,
T.; Fridlund, M.; García, R. A.; Giommi, P.; Giuffrida, G.; Godolt,
M.; Gomes da Silva, J.; Granzer, T.; Grenfell, J. L.; Grotsch-Noels,
A.; Günther, E.; Haswell, C. A.; Hatzes, A. P.; Hébrard, G.; Hekker,
S.; Helled, R.; Heng, K.; Jenkins, J. M.; Johansen, A.; Khodachenko,
M. L.; Kislyakova, K. G.; Kley, W.; Kolb, U.; Krivova, N.; Kupka, F.;
Lammer, H.; Lanza, A. F.; Lebreton, Y.; Magrin, D.; Marcos-Arenal,
P.; Marrese, P. M.; Marques, J. P.; Martins, J.; Mathis, S.; Mathur,
S.; Messina, S.; Miglio, A.; Montalban, J.; Montalto, M.; Monteiro,
M. J. P. F. G.; Moradi, H.; Moravveji, E.; Mordasini, C.; Morel, T.;
Mortier, A.; Nascimbeni, V.; Nelson, R. P.; Nielsen, M. B.; Noack,
L.; Norton, A. J.; Ofir, A.; Oshagh, M.; Ouazzani, R. -M.; Pápics,
P.; Parro, V. C.; Petit, P.; Plez, B.; Poretti, E.; Quirrenbach, A.;
Ragazzoni, R.; Raimondo, G.; Rainer, M.; Reese, D. R.; Redmer, R.;
Reffert, S.; Rojas-Ayala, B.; Roxburgh, I. W.; Salmon, S.; Santerne,
A.; Schneider, J.; Schou, J.; Schuh, S.; Schunker, H.; Silva-Valio,
A.; Silvotti, R.; Skillen, I.; Snellen, I.; Sohl, F.; Sousa, S. G.;
Sozzetti, A.; Stello, D.; Strassmeier, K. G.; Švanda, M.; Szabó,
Gy. M.; Tkachenko, A.; Valencia, D.; Van Grootel, V.; Vauclair,
S. D.; Ventura, P.; Wagner, F. W.; Walton, N. A.; Weingrill, J.;
Werner, S. C.; Wheatley, P. J.; Zwintz, K.
2014ExA....38..249R Altcode: 2014ExA...tmp...41R; 2013arXiv1310.0696R
PLATO 2.0 has recently been selected for ESA's M3 launch opportunity
(2022/24). Providing accurate key planet parameters (radius, mass,
density and age) in statistical numbers, it addresses fundamental
questions such as: How do planetary systems form and evolve? Are there
other systems with planets like ours, including potentially habitable
planets? The PLATO 2.0 instrument consists of 34 small aperture
telescopes (32 with 25 s readout cadence and 2 with 2.5 s candence)
providing a wide field-of-view (2232 deg 2) and a large
photometric magnitude range (4-16 mag). It focusses on bright (4-11
mag) stars in wide fields to detect and characterize planets down to
Earth-size by photometric transits, whose masses can then be determined
by ground-based radial-velocity follow-up measurements. Asteroseismology
will be performed for these bright stars to obtain highly accurate
stellar parameters, including masses and ages. The combination of
bright targets and asteroseismology results in high accuracy for
the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii,
masses and ages, respectively. The planned baseline observing strategy
includes two long pointings (2-3 years) to detect and bulk characterize
planets reaching into the habitable zone (HZ) of solar-like stars
and an additional step-and-stare phase to cover in total about 50 %
of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect
and characterize hundreds of small planets, and thousands of planets
in the Neptune to gas giant regime out to the HZ. It will therefore
provide the first large-scale catalogue of bulk characterized planets
with accurate radii, masses, mean densities and ages. This catalogue
will include terrestrial planets at intermediate orbital distances,
where surface temperatures are moderate. Coverage of this parameter
range with statistical numbers of bulk characterized planets is unique
to PLATO 2.0. The PLATO 2.0 catalogue allows us to e.g.: - complete
our knowledge of planet diversity for low-mass objects, - correlate the
planet mean density-orbital distance distribution with predictions from
planet formation theories,- constrain the influence of planet migration
and scattering on the architecture of multiple systems, and - specify
how planet and system parameters change with host star characteristics,
such as type, metallicity and age. The catalogue will allow us to study
planets and planetary systems at different evolutionary phases. It
will further provide a census for small, low-mass planets. This will
serve to identify objects which retained their primordial hydrogen
atmosphere and in general the typical characteristics of planets
in such low-mass, low-density range. Planets detected by PLATO 2.0
will orbit bright stars and many of them will be targets for future
atmosphere spectroscopy exploring their atmosphere. Furthermore,
the mission has the potential to detect exomoons, planetary rings,
binary and Trojan planets. The planetary science possible with PLATO
2.0 is complemented by its impact on stellar and galactic science via
asteroseismology as well as light curves of all kinds of variable stars,
together with observations of stellar clusters of different ages. This
will allow us to improve stellar models and study stellar activity. A
large number of well-known ages from red giant stars will probe the
structure and evolution of our Galaxy. Asteroseismic ages of bright
stars for different phases of stellar evolution allow calibrating
stellar age-rotation relationships. Together with the results of ESA's
Gaia mission, the results of PLATO 2.0 will provide a huge legacy to
planetary, stellar and galactic science.

---------------------------------------------------------
Title: A New SATIRE-S Spectral Solar Irradiance Reconstruction for
Solar Cycles 21-23 and Its Implications for Stratospheric Ozone*
Authors: Ball, William T.; Krivova, Natalie A.; Unruh, Yvonne C.;
Haigh, Joanna D.; Solanki, Sami K.
2014JAtS...71.4086B Altcode: 2014arXiv1408.0365B
We present a revised and extended total and spectral solar irradiance
(SSI) reconstruction, which includes a wavelength-dependent uncertainty
estimate, spanning the last three solar cycles using the SATIRE-S
model. The SSI reconstruction covers wavelengths between 115 and
160,000 nm and all dates between August 1974 and October 2009. This
represents the first full-wavelength SATIRE-S reconstruction to
cover the last three solar cycles without data gaps and with an
uncertainty estimate. SATIRE-S is compared with the NRLSSI model and
SORCE/SOLSTICE ultraviolet (UV) observations. SATIRE-S displays similar
cycle behaviour to NRLSSI for wavelengths below 242 nm and almost twice
the variability between 242 and 310 nm. During the decline of last
solar cycle, between 2003 and 2008, SSI from SORCE/SOLSTICE version
12 and 10 typically displays more than three times the variability of
SATIRE-S between 200 and 300 nm. All three datasets are used to model
changes in stratospheric ozone within a 2D atmospheric model for a
decline from high solar activity to solar minimum. The different flux
changes result in different modelled ozone trends. Using NRLSSI leads
to a decline in mesospheric ozone, while SATIRE-S and SORCE/SOLSTICE
result in an increase. Recent publications have highlighted increases
in mesospheric ozone when considering version 10 SORCE/SOLSTICE
irradiances. The recalibrated SORCE/SOLSTICE version 12 irradiances
result in a much smaller mesospheric ozone response than when using
version 10 and now similar in magnitude to SATIRE-S. This shows that
current knowledge of variations in spectral irradiance is not sufficient
to warrant robust conclusions concerning the impact of solar variability
on the atmosphere and climate.

---------------------------------------------------------
Title: Modelling total solar irradiance since 1878 from simulated
magnetograms
Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K.
2014A&A...570A..23D Altcode: 2014arXiv1409.1941D
 Aims: We present a new model of total solar irradiance (TSI)
based on magnetograms simulated with a surface flux transport model
(SFTM) and the Spectral And Total Irradiance REconstructions (SATIRE)
model. Our model provides daily maps of the distribution of the
photospheric field and the TSI starting from 1878. Methods:
The modelling is done in two main steps. We first calculate the
magnetic flux on the solar surface emerging in active and ephemeral
regions. The evolution of the magnetic flux in active regions (sunspots
and faculae) is computed using a surface flux transport model fed
with the observed record of sunspot group areas and positions. The
magnetic flux in ephemeral regions is treated separately using the
concept of overlapping cycles. We then use a version of the SATIRE
model to compute the TSI. The area coverage and the distribution of
different magnetic features as a function of time, which are required by
SATIRE, are extracted from the simulated magnetograms and the modelled
ephemeral region magnetic flux. Previously computed intensity spectra
of the various types of magnetic features are employed. Results:
Our model reproduces the PMOD composite of TSI measurements starting
from 1978 at daily and rotational timescales more accurately than the
previous version of the SATIRE model computing TSI over this period of
time. The simulated magnetograms provide a more realistic representation
of the evolution of the magnetic field on the photosphere and also
allow us to make use of information on the spatial distribution
of the magnetic fields before the times when observed magnetograms
were available. We find that the secular increase in TSI since 1878
is fairly stable to modifications of the treatment of the ephemeral
region magnetic flux.

---------------------------------------------------------
Title: Reconstruction of total and spectral solar irradiance from
1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations
Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.; Glassmeier, K. H.
2014A&A...570A..85Y Altcode: 2014arXiv1408.1229Y
Context. Total and spectral solar irradiance are key parameters in the
assessment of solar influence on changes in the Earth's climate. 
Aims: We present a reconstruction of daily solar irradiance obtained
using the SATIRE-S model spanning 1974 to 2013 based on full-disc
observations from the KPVT, SoHO/MDI, and SDO/HMI. Methods:
SATIRE-S ascribes variation in solar irradiance on timescales
greater than a day to photospheric magnetism. The solar spectrum
is reconstructed from the apparent surface coverage of bright
magnetic features and sunspots in the daily data using the modelled
intensity spectra of these magnetic structures. We cross-calibrated
the various data sets, harmonizing the model input so as to yield a
single consistent time series as the output. Results: The model
replicates 92% (R2 = 0.916) of the variability in the PMOD
TSI composite including the secular decline between the 1996 and 2008
solar cycle minima. The model also reproduces most of the variability
in observed Lyman-α irradiance and the Mg II index. The ultraviolet
solar irradiance measurements from the UARS and SORCE missions are
mutually consistent up to about 180 nm before they start to exhibit
discrepant rotational and cyclical variability, indicative of unresolved
instrumental effects. As a result, the agreement between model and
measurement, while relatively good below 180 nm, starts to deteriorate
above this wavelength. As with earlier similar investigations,
the reconstruction cannot reproduce the overall trends in SORCE/SIM
SSI. We argue, from the lack of clear solar cycle modulation in the
SIM record and the inconsistency between the total flux recorded
by the instrument and TSI, that unaccounted instrumental trends are
present. Conclusions: The daily solar irradiance time series
is consistent with observations from multiple sources, demonstrating
its validity and utility for climate models. It also provides further
evidence that photospheric magnetism is the prime driver of variation
in solar irradiance on timescales greater than a day.

---------------------------------------------------------
Title: Variability of Sun-like stars: reproducing observed photometric
trends
Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Schmutz,
W. K.; Ball, W. T.; Knaack, R.; Rozanov, E. V.; Unruh, Y. C.
2014A&A...569A..38S Altcode: 2014arXiv1406.2383S
Context. The Sun and stars with low magnetic activity levels become
photometrically brighter when their activity increases. Magnetically
more active stars display the opposite behavior and become fainter
when their activity increases. Aims: We reproduce the observed
photometric trends in stellar variations with a model thattreats stars
as hypothetical suns with coverage by magnetic features different from
that of the Sun. Methods: The model attributes the variability
of stellar spectra to the imbalance between the contributions from
different components of the solar atmosphere, such as dark starspots
and bright faculae. A stellar spectrum is calculated from spectra
of the individual components by weighting them with corresponding
disk-area coverages. The latter are obtained by extrapolating
the solar dependences of spot and facular disk-area coverages
on chromospheric activity to stars with different levels of mean
chromospheric activity. Results: We find that the contribution
by starspots to the variability increases faster with chromospheric
activity than the facular contribution. This causes the transition from
faculae-dominated variability and direct activity-brightness correlation
to spot-dominated variability and inverse activity-brightness
correlation with increasing chromospheric activity level. We show that
the regime of the variability also depends on the angle between the
stellar rotation axis and the line-of-sight and on the latitudinal
distribution of active regions on the stellar surface. Our model
can be used as a tool for extrapolating the observed photometric
variability of the Sun to Sun-like stars at different activity levels,
which makes a direct comparison between solar and stellar irradiance
data possible. Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201323086/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
2014SoPh..289.1115T Altcode:
Proper numerical simulation of the Earth's climate change requires
reliable knowledge of solar irradiance and its variability on
different time scales, as well as the wavelength dependence of this
variability. As new measurements of the solar spectral irradiance have
become available, so too have new reconstructions of historical solar
irradiance variations, based on different approaches. However, these
various solar spectral irradiance reconstructions have not yet been
compared in detail to quantify differences in their absolute values,
variability, and implications for climate and atmospheric studies. In
this paper we quantitatively compare five different reconstructions
of solar spectral irradiance changes during the past four centuries,
in order to document and analyze their differences. The impact on
atmosphere and climate studies is discussed in terms of the calculation
of short wave solar heating rates.

---------------------------------------------------------
Title: Point spread function of SDO/HMI and the effects of stray
light correction on the apparent properties of solar surface phenomena
Authors: Yeo, K. L.; Feller, A.; Solanki, S. K.; Couvidat, S.;
Danilovic, S.; Krivova, N. A.
2014A&A...561A..22Y Altcode: 2013arXiv1310.4972Y
 Aims: We present a point spread function (PSF) for the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) and discuss the effects of its removal on the
apparent properties of solar surface phenomena in HMI data. 
Methods: The PSF was retrieved from observations of Venus in transit
by matching it to the convolution of a model of the Venusian disc and
solar background with a guess PSF. We described the PSF as the sum of
five Gaussian functions, the amplitudes of which vary sinusoidally with
azimuth. This relatively complex functional form was required by the
data. Observations recorded near in time to the transit of Venus were
corrected for instrumental scattered light by the deconvolution with the
PSF. We also examined the variation in the shape of the solar aureole in
daily data, as an indication of PSF changes over time. Results:
Granulation contrast in restored HMI data is greatly enhanced relative
to the original data and exhibit reasonable agreement with numerical
simulations. Image restoration enhanced the apparent intensity and
pixel averaged magnetic field strength of photospheric magnetic features
significantly. For small-scale magnetic features, restoration enhanced
intensity contrast in the continuum and core of the Fe I 6173 Å line
by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For
sunspots and pores, the enhancement varied strongly within and between
features, being more acute for smaller features. Magnetic features are
also rendered smaller, as signal smeared onto the surrounding quiet
Sun is recovered. Image restoration increased the apparent amount of
magnetic flux above the noise floor by a factor of about 1.2, most
of the gain coming from the quiet Sun. Line-of-sight velocity due to
granulation and supergranulation is enhanced by a factor of 1.4 to 2.1,
depending on position on the solar disc. The shape of the solar aureole
varied, with time and between the two CCDs. There are also indications
that the PSF varies across the FOV. However, all these variations were
found to be relatively small, such that a single PSF can be applied to
HMI data from both CCDs, over the period examined without introducing
significant error. Conclusions: Restoring HMI observations
with the PSF presented here returns a reasonable estimate of the stray
light-free intensity contrast. Image restoration affects the measured
radiant, magnetic and dynamic properties of solar surface phenomena
sufficiently to significantly impact interpretation.

---------------------------------------------------------
Title: Reconstructions of solar irradiance on centennial time scales
Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria;
Kok Leng, Yeo
2014cosp...40E1664K Altcode:
Solar irradiance is the main external source of energy to Earth's
climate system. The record of direct measurements covering less than 40
years is too short to study solar influence on Earth's climate, which
calls for reconstructions of solar irradiance into the past with the
help of appropriate models. An obvious requirement to a competitive
model is its ability to reproduce observed irradiance changes, and a
successful example of such a model is presented by the SATIRE family of
models. As most state-of-the-art models, SATIRE assumes that irradiance
changes on time scales longer than approximately a day are caused by
the evolving distribution of dark and bright magnetic features on the
solar surface. The surface coverage by such features as a function of
time is derived from solar observations. The choice of these depends
on the time scale in question. Most accurate is the version of the
model that employs full-disc spatially-resolved solar magnetograms and
reproduces over 90% of the measured irradiance variation, including
the overall decreasing trend in the total solar irradiance over
the last four cycles. Since such magnetograms are only available
for about four decades, reconstructions on time scales of centuries
have to rely on disc-integrated proxies of solar magnetic activity,
such as sunspot areas and numbers. Employing a surface flux transport
model and sunspot observations as input, we have being able to produce
synthetic magnetograms since 1700. This improves the temporal resolution
of the irradiance reconstructions on centennial time scales. The most
critical aspect of such reconstructions remains the uncertainty in
the magnitude of the secular change.

---------------------------------------------------------
Title: Total solar irradiance reconstruction since 1700 using a flux
transport model
Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.;
Jiang, Jie
2014cosp...40E.634D Altcode:
Reconstructions of solar irradiance into the past are crucial for
studies of solar influence on climate. Models based on the assumption
that irradiance changes are caused by the evolution of the photospheric
magnetic fields have been most successful in reproducing the measured
irradiance variations. Daily magnetograms, such as those from MDI and
HMI, provide the most detailed information on the changing distribution
of the photospheric magnetic fields. Since such magnetograms are
only available from 1974, we used a surface flux transport model to
describe the evolution of the magnetic fields on the solar surface
due to the effects of differential rotation, meridional circulation,
and turbulent diffusivity, before 1974. In this model, the sources of
magnetic flux are the active regions, which are introduced based on
sunspot group areas, positions, and tilt angles. The RGO record is,
however, only available since 1874. Here we present a model of solar
irradiance since 1700, which is based on a semi-synthetic sunspot
record. The semi-synthetic record was obtained using statistical
relationships between sunspot group properties (areas, positions,
tilt angles) derived from the RGO record on one hand, and the cycle
strength and phase derived from the sunspot group number (Rg) on the
other. These relationships were employed to produce daily records
of sunspot group positions, areas, and tilt angles before 1874. The
semi-synthetic records were fed into the surface flux transport model
to simulate daily magnetograms since 1700. By combining the simulated
magnetograms with a SATIRE-type model, we then reconstructed total
solar irradiance since 1700.

---------------------------------------------------------
Title: Solar Irradiance Variability and Climate
Authors: Solanki, Sami K.; Krivova, Natalie A.; Haigh, Joanna D.
2013ARA&A..51..311S Altcode: 2013arXiv1306.2770S
The brightness of the Sun varies on all timescales on which it has been
observed, and there is increasing evidence that this has an influence
on climate. The amplitudes of such variations depend on the wavelength
and possibly the timescale. Although many aspects of this variability
are well established, the exact magnitude of secular variations (going
beyond a solar cycle) and the spectral dependence of variations are
under discussion. The main drivers of solar variability are thought
to be magnetic features at the solar surface. The climate response
can be, on a global scale, largely accounted for by simple energetic
considerations, but understanding the regional climate effects is more
difficult. Promising mechanisms for such a driving have been identified,
including through the influence of UV irradiance on the stratosphere and
dynamical coupling to the surface. Here, we provide an overview of the
current state of our knowledge, as well as of the main open questions.

---------------------------------------------------------
Title: Sunspot group tilt angles and the strength of the solar cycle
(Corrigendum)
Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron,
R.; Peñuela, T.
2013A&A...556C...3D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The latitudinal dependence of the solar radiance
Authors: Finsterle, Wolfgang; Shapiro, Alexander; Schmutz, Werner;
Krivova, Natalie
2013EGUGA..1511672F Altcode:
Active regions and sunspots occur predominantly at low to mid
heliographic latitudes. Hence, it seems reasonable to assume that the
radiant output of the sun is not spherically symmetrical. Due to the
relatively small inclination (~7.25°) of the solar rotation axis this
asphericity is difficult to detect in integrated disk data taken from an
ecliptic-bound vantage point. A histogram analysis of 13 years of VIRGO
TSI data revealed a slight north-south asymmetry with maximal deviations
of ±4 parts in 10^5. Interestingly, the north-south asymmetry persists
even after subtracting the simulated TSI data by Krivova et al. (2003)
from the VIRGO TSI measurements. The Krivova time series attributes
the TSI to magnetic activity patterns as observed by MDI (sunspots,
faculae, and plage). The asymmetry thus seems to be of a different
origin, i.e. unrelated to sunspots, faculae, or plage, although smaller
magnetic structures might contribute to the asymmetry. We will also
investigate a potential asymmetry in the equator-to-pole temperature
gradient. At this point we can only speculate if the observed asymmetry
is characteristic of solar cycle 23, which is covered by the VIRGO time
series, or more fundamental. In any case it would be very interesting to
extend the TSI vs. latitude curve towards higher heliographic latitudes.

---------------------------------------------------------
Title: A Collaborative FP7 Effort towards the First European
Comprehensive SOLar Irradiance Data Exploitation (SOLID)
Authors: Haberreiter, Margit; Dasi, Maria; Delouille, Veronique; Del
Zanna, Giulio; Dudok de Wit, Thierry; Ermolli, Ilaria; Kretzschmar,
Matthieu; Krivova, Natalie; Mason, Helen; Qahwaji, Rami; Schmutz,
Werner; Solanki, Sami; Thuillier, Gerard; Tourpali, Kleareti; Unruh,
Yvonne; Verbeeck, Cis; Weber, Mark; Woods, Tom
2013EGUGA..1513079H Altcode:
Variations of solar irradiance are the most important natural factor
in the terrestrial climate and as such, the time dependent spectral
solar irradiance is a crucial input to any climate modelling. There
have been previous efforts to compile solar irradiance but it is
still uncertain by how much the spectral and total solar irradiance
changed on yearly, decadal and longer time scales. Observations of
irradiance data exist in numerous disperse data sets. Therefore, it
is important to bring together the European expertise in the field
to analyse and merge the complete set of European irradiance data,
complemented by archive data that include data from non-European
missions. We report on the initiation of a collaborative effort to unify
representatives from all European solar space experiments and European
teams specialized in multi-wavelength solar image processing. It is
intended to include the European groups involved in irradiance modelling
and reconstruction. They will work with two different state of the art
approaches to produce reconstructed spectral and total solar irradiance
data as a function of time. These results will be used to bridge
gaps in time and wavelength coverage of the observational data. This
will allow the proposing SOLID team to reduce the uncertainties in
the irradiance time series - an important requirement by the climate
community - and to provide uniform data sets of modelled and observed
solar irradiance data from the beginning of the space era to the
present including proper error and uncertainty estimates. Climate
research needs these data sets and therefore, the primary benefit
is for the climate community, but the stellar community, planetary,
lunar, and ionospheric researchers are also interested in having at
their disposition incident radiation of the Sun. The proposing team
plans to realize a wide international synergy in solar physics from
7 European countries, and collaborators from the US, complemented by
representatives from the climate community, who will accompany their
research work with wide dissemination activities.

---------------------------------------------------------
Title: Modeling the variability of Sun-like stars
Authors: Shapiro, Alexander; Knaack, Reto; Krivova, Natalie; Schmutz,
Werner; Solanki, Sami; Unruh, Yvonne
2013EGUGA..15.9981S Altcode:
We present a model which attributes the variability of the stellar
radiative energy flux to the imbalance between the contributions from
dark starspots and bright faculae. The stellar radiative energy flux
variations are modeled from the individual component's spectra, by
weighting them with corresponding filling factors. The filling factors
are deduced by extrapolating the sunspot and facular filling factors
dependencies on solar CaII S-index to stars with different levels
of the chromospheric activity. Our approach allows us to model the
stellar photometric variability vs. activity dependency and reproduce
the transition from spot-dominated to facula-dominated regimes of
variability. We show how the effect of inclination (arising due to the
random position of the Earth-bound observer relative to the directions
of stellar rotational axis) can affect these dependencies and present
the modeling of the individual stellar photometric light curves.

---------------------------------------------------------
Title: What can we learn about the Sun with PREMOS/PICARD?
Authors: Cessateur, Gaël; Shapiro, Alexander; Schmutz, Werner;
Krivova, Natalie; Solanki, Sami K.; Yeo, Kok Leng; Thuillier, Gérard
2013EGUGA..1511720C Altcode:
Total and Spectral Solar Irradiance are key input parameters to
atmospheric/oceanic and space weather models. We present here
spectral solar irradiance data from the radiometer PREMOS onboard
the PICARD satellite. This instrument convers the solar spectrum from
the Ultraviolet to near-infrared, and provides valuable information
and nourishes theoretical models. Based on redundancy strategies,
instrumental degradation has been mostly corrected, revealing
surprising behavior from the visible and near-infrared filters. We
compare these data with those from the VIRGO/SOHO and SOLSTIC/SORCE
experiments. Finally we use COSI to model the variability of the
irradiance, assuming that the latter is determined by the evolution
of the solar surface magnetic field as seen with SDO/HMI data. A
direct comparison shows a very good correlation for most of channels
from PREMOS.

---------------------------------------------------------
Title: Analysis of Ca II K images aiming to determine long-term
trends in solar irradiance variability
Authors: Kar, Anuradha; Ermolli, Ilaria; Krivova, Natalie; Solanki,
Sami
2013EGUGA..15..330K Altcode:
The change in radiative output of the Sun on time scales longer than
a day is attributed to the variability in solar surface magnetic
fields. Direct irradiance measurements are only available for less
than four decades. To reconstruct long term trends in solar total and
spectral irradiance, proxies of solar surface magnetism like sunspot,
facular and network areas are needed. Currently, sunspot records
alone are used for this purpose, from which the deduction of facular
and network areas is rather indirect. Historical records of full disk
images of the Sun taken in the Ca II K spectral line (393.3 nm) have the
potential to provide far more direct information about the distribution
and evolution of faculae and network elements. The latter appear as
bright regions in the Ca II K spectroheliograms and their intensity
is correlated with the magnetic field strength of the features on the
solar surface. Solar full disk images in the Ca II K line have been
recorded since the beginning of the 20th century at a number of solar
observatories such as at Arcetri (Italy), Mount Wilson(California,
US) and Kodaikanal (India). The images are available in digitized
archives that contain the data processed for standard instrumental
calibrations. To utilize these records for irradiance studies, the next
step is to identify the bright magnetic features from the images using
feature recognition techniques. We test different feature identification
methods which are first applied to a set of recent images from the PSPT
instrument at the Osservatorio Astronomico di Roma, taken during three
periods characterized by high, medium and low levels of activity. Then
the performance of these methods to historical images from Arcetri,
Mt. Wilson and Kodaikanal archives is tested. The results will be
presented and discussed here.

---------------------------------------------------------
Title: Recent variability of the solar spectral irradiance and its
impact on climate modelling
Authors: Ermolli, I.; Matthes, K.; Dudok de Wit, T.; Krivova,
N. A.; Tourpali, K.; Weber, M.; Unruh, Y. C.; Gray, L.; Langematz,
U.; Pilewskie, P.; Rozanov, E.; Schmutz, W.; Shapiro, A.; Solanki,
S. K.; Woods, T. N.
2013ACP....13.3945E Altcode: 2012ACPD...1224557E; 2013arXiv1303.5577E
The lack of long and reliable time series of solar spectral irradiance
(SSI) measurements makes an accurate quantification of solar
contributions to recent climate change difficult. Whereas earlier SSI
observations and models provided a qualitatively consistent picture of
the SSI variability, recent measurements by the SORCE (SOlar Radiation
and Climate Experiment) satellite suggest a significantly stronger
variability in the ultraviolet (UV) spectral range and changes in the
visible and near-infrared (NIR) bands in anti-phase with the solar
cycle. A number of recent chemistry-climate model (CCM) simulations
have shown that this might have significant implications on the
Earth's atmosphere. Motivated by these results, we summarize here
our current knowledge of SSI variability and its impact on Earth's
climate. We present a detailed overview of existing SSI
measurements and provide thorough comparison of models available to
date. SSI changes influence the Earth's atmosphere, both directly,
through changes in shortwave (SW) heating and therefore, temperature
and ozone distributions in the stratosphere, and indirectly, through
dynamical feedbacks. We investigate these direct and indirect effects
using several state-of-the art CCM simulations forced with measured
and modelled SSI changes. A unique asset of this study is the use
of a common comprehensive approach for an issue that is usually
addressed separately by different communities. We show
that the SORCE measurements are difficult to reconcile with earlier
observations and with SSI models. Of the five SSI models discussed
here, specifically NRLSSI (Naval Research Laboratory Solar Spectral
Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions
for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar
Radiation Physical Modelling), and OAR (Osservatorio Astronomico di
Roma), only one shows a behaviour of the UV and visible irradiance
qualitatively resembling that of the recent SORCE measurements. However,
the integral of the SSI computed with this model over the entire
spectral range does not reproduce the measured cyclical changes of the
total solar irradiance, which is an essential requisite for realistic
evaluations of solar effects on the Earth's climate in CCMs. We show that within the range provided by the recent SSI observations
and semi-empirical models discussed here, the NRLSSI model and SORCE
observations represent the lower and upper limits in the magnitude
of the SSI solar cycle variation. The results of the
CCM simulations, forced with the SSI solar cycle variations estimated
from the NRLSSI model and from SORCE measurements, show that the direct
solar response in the stratosphere is larger for the SORCE than for the
NRLSSI data. Correspondingly, larger UV forcing also leads to a larger
surface response. Finally, we discuss the reliability
of the available data and we propose additional coordinated work,
first to build composite SSI data sets out of scattered observations
and to refine current SSI models, and second, to run coordinated
CCM experiments.

---------------------------------------------------------
Title: Intensity contrast of solar network and faculae
Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.
2013A&A...550A..95Y Altcode: 2013arXiv1302.1442Y
 Aims: This study aims at setting observational constraints
on the continuum and line core intensity contrast of network and
faculae, specifically, their relationship with magnetic field and
disc position. Methods: Full-disc magnetograms and intensity
images by the Helioseismic and Magnetic Imager (HMI) onboard the Solar
Dynamics Observatory (SDO) were employed. Bright magnetic features,
representing network and faculae, were identified and the relationship
between their intensity contrast at continuum and line core with
magnetogram signal and heliocentric angle examined. Care was taken
to minimize the inclusion of the magnetic canopy and straylight from
sunspots and pores as network and faculae. Results: In line
with earlier studies, network features, on a per unit magnetic flux
basis, appeared brighter than facular features. Intensity contrasts
in the continuum and line core differ considerably, most notably, they
exhibit opposite centre-to-limb variations. We found this difference in
behaviour to likely be due to the different mechanisms of the formation
of the two spectral components. From a simple model based on bivariate
polynomial fits to the measured contrasts we confirmed spectral line
changes to be a significant driver of facular contribution to variation
in solar irradiance. The discrepancy between the continuum contrast
reported here and in the literature was shown to arise mainly from
differences in spatial resolution and treatment of magnetic signals
adjacent to sunspots and pores. Conclusions: HMI is a source of
accurate contrasts and low-noise magnetograms covering the full solar
disc. For irradiance studies it is important to consider not just the
contribution from the continuum but also from the spectral lines. In
order not to underestimate long-term variations in solar irradiance,
irradiance models should take the greater contrast per unit magnetic
flux associated with magnetic features with low magnetic flux into
account.

---------------------------------------------------------
Title: Climate and Weather of the Sun - Earth System
Authors: Usoskin, Ilya; Krivova, Natalie
2012aaa2.book..397U Altcode:
The Sun is a variable star whose output, including electromagnetic
radiation, magnetic fields and energetic particles varies at different
time scales, from seconds to millennia. Solar variability affects the
interplanetary medium but also planetary environments, including that of
Earth. The state of the near-Earth environment is collectively called
the Space weather, while its long-term changes make the concept of
Space Climate. This forms the field of an interdisciplinary research
focused on a wide range of topics: from solar physics, solar wind,
cosmic rays, to planetary atmospheres and climate. Special emphasis
is paid upon the processes that inter-relate solar variability
and terrestrial environment - the Sun-Earth system. In this work,
we present an overview on the state of the art in the field of the
weather and climate of the Sun-Earth system.

---------------------------------------------------------
Title: Solar Irradiance Models and Measurements: A Comparison in
the 220-240 nm wavelength band
Authors: Unruh, Yvonne C.; Ball, Will T.; Krivova, Natalie A.
2012SGeo...33..475U Altcode: 2011arXiv1111.2068U; 2011SGeo..tmp..121U
Solar irradiance models that assume solar irradiance variations
to be due to changes in the solar surface magnetic flux have been
successfully used to reconstruct total solar irradiance on rotational
as well as cyclical and secular time scales. Modelling spectral solar
irradiance is not yet as advanced, and also suffers from a lack of
comparison data, in particular on solar cycle time scales. Here,
we compare solar irradiance in the 220-240 nm band as modelled
with SATIRE-S and measured by different instruments on the UARS
and SORCE satellites. We find good agreement between the model and
measurements on rotational time scales. The long-term trends, however,
show significant differences. Both SORCE instruments, in particular,
show a much steeper gradient over the decaying part of cycle 23 than
the modelled irradiance or that measured by UARS/SUSIM.

---------------------------------------------------------
Title: A reconstruction of solar irradiance using a flux transport
model
Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.;
Jiang, Jie
2012cosp...39..418D Altcode: 2012cosp.meet..418D
Solar irradiance is one of the important drivers of the Earth's
global climate, but it has only been measured for the past 33
years. Its reconstructions are therefore crucial to study longer
term variations relevant to climate timescales. Most successful in
reproducing the measured irradiance variations have being the models
that are based on the assumption that irradiance changes are caused by
the evolution of the photospheric magnetic field. Our SATIRE-S model
is one of these, which uses solar full-disc magnetograms as an input,
and these are available for less than four decades. To reconstruct the
irradiance back to times when no observed magnetograms are available,
we combine the SATIRE-S model with synthetic magnetograms, produced
using a surface flux transport model. The model is fed with daily,
observed or modelled statistically, records of sunspot positions,
areas, and tilt angles. The concept of overlapping ephemeral region
cycles is used to describe the secular change in the irradiance.

---------------------------------------------------------
Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and
PREMOS/PICARD
Authors: Cessateur, Gaël.; Kretzschmar, Matthieu; Krivova, Natalie;
Schmutz, Werner; Solanki, Sami K.; Thuillier, Gerard; Shapiro,
Alexander; Schoell, Micha; Shapiro, Anna; Dominique, Marie; Tagirov,
Rinat; Wehrli, Christoph; Yeo, Kok Leng
2012cosp...39..287C Altcode: 2012cosp.meet..287C
No abstract at ADS

---------------------------------------------------------
Title: Long-term reconstructions of total solar irradiance
Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria
2012cosp...39..985K Altcode: 2012cosp.meet..985K
Solar irradiance is the main external driver of the Earth's climate,
although its relative contribution compared to other internal and
anthropogenic factors is not yet well determined. Variations of total
solar irradiance have being measured for over three decades and are
relatively well understood. Reconstructions of the irradiance into the
past remain, however, rather uncertain. In particular, the magnitude of
the secular change is highly debated. The reason is the lack of direct
and well-sampled proxies of solar magnetic activity on time scales
longer than a few decades. Reconstructions on time scales of centuries
rely on sunspot observations available since 1610. Reconstructions on
millennial time scales use concentrations of the cosmogenic isotopes
in terrestrial archives. We will review long-term reconstructions of
the solar irradiance using the SATIRE set of models, compare them with
other recent models and discuss the remaining uncertainties.

---------------------------------------------------------
Title: Reconstruction of total solar irradiance 1974-2009
Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.;
Wenzler, T.; Mortlock, D. J.; Jaffe, A. H.
2012A&A...541A..27B Altcode: 2012arXiv1202.3554B
Context. The study of variations in total solar irradiance (TSI)
is important for understanding how the Sun affects the Earth's
climate. Aims: Full-disk continuum images and magnetograms are
now available for three full solar cycles. We investigate how modelled
TSI compares with direct observations by building a consistent modelled
TSI dataset. The model, based only on changes in the photospheric
magnetic flux can then be tested on rotational, cyclical and secular
timescales. Methods: We use Kitt Peak and SoHO/MDI continuum
images and magnetograms in the SATIRE-S model to reconstruct TSI over
cycles 21-23. To maximise independence from TSI composites, SORCE/TIM
TSI data are used to fix the one free parameter of the model. We compare
and combine the separate data sources for the model to estimate an
uncertainty on the reconstruction and prevent any additional free
parameters entering the model. Results: The reconstruction
supports the PMOD composite as being the best historical record of
TSI observations, although on timescales of the solar rotation the
IRMB composite provides somewhat better agreement. Further to this,
the model is able to account for 92% of TSI variations from 1978
to 2009 in the PMOD composite and over 96% during cycle 23. The
reconstruction also displays an inter-cycle, secular decline of
0.20+0.12-0.09 W m-2 between cycle
23 minima, in agreement with the PMOD composite. Conclusions:
SATIRE-S is able to recreate TSI observations on all timescales of a
day and longer over 31 years from 1978. This is strong evidence that
changes in photospheric magnetic flux alone are responsible for almost
all solar irradiance variations over the last three solar cycles.

---------------------------------------------------------
Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and
PREMOS/PICARD
Authors: Cessateur, G.; Shapiro, A. I.; Dominique, M.; Kretzschmar,
M.; Krivova, N.; Shapiro, A. V.; Schmutz, W.; Schoell, M.; Solanki,
S.; Tagirov, R.; Thuillier, G.; Wehrli, C.; Yeo, K. L.
2012EGUGA..14.8254C Altcode:
Total and Spectral Solar Irradiance are key input parameters to
atmospheric/oceanic and space weather models. Both vary on time-scales
ranging from days to millennia, although a complete picture of the
solar irradiance variability is still missing. The recent launch of
two European missions PROBA-2 with LYRA radiometer onboard and PICARD
with PREMOS package onboard provides therefore valuable information and
nourishes theoretical models. Both instruments covers the solar spectrum
from the EUV to near-infrared. Here we present spectral solar irradiance
data from these two recent missions. After a proper correction of the
degradation and non-solar signatures, we compare these data with those
from the VIRGO/SOHO and SOLSTICE+SIM/SORCE experiments. Both LYRA and
PREMOS have also observed several solar eclipses, which allows us to
accurately retrieve the center-to-limb variations (CLVs) of the quiet
Sun's brightness at the wavelengths of the corresponding channels. CLVs
play indeed an important role in modelling of the solar irradiance
variability. We show that calculations with the published COde for
Solar Irradiance (COSI) yield CLVs that are in good agreement with
measurements. Finally we use COSI to model the variability of the
irradiance, assuming that the latter is determined by the evolution
of the solar surface magnetic field as seen with SDO/HMI data. These
theoretical results are compared to PREMOS and LYRA measurements.

---------------------------------------------------------
Title: The influence of spectral solar irradiance data on
stratospheric heating rates during the 11 year solar cycle
Authors: Oberländer, S.; Langematz, U.; Matthes, K.; Kunze, M.; Kubin,
A.; Harder, J.; Krivova, N. A.; Solanki, S. K.; Pagaran, J.; Weber, M.
2012GeoRL..39.1801O Altcode: 2012GeoRL..3901801O
Heating rate calculations with the FUBRad shortwave (SW) radiation
parameterization have been performed to examine the effect of
prescribed spectral solar fluxes from the NRLSSI, MPS and IUP
data sets on SW heating rates over the 11 year solar cycle 22. The
corresponding temperature response is derived from perpetual January
General Circulation Model (GCM) simulations with prescribed ozone
concentrations. The different solar flux input data sets induce clear
differences in SW heating rates at solar minimum, with the established
NRLSSI data set showing the smallest solar heating rates. The stronger
SW heating in the middle and upper stratosphere in the MPS data
warms the summer upper stratosphere by 2 K. Over the solar cycle,
SW heating rate differences vary up to 40% between the irradiance
data sets, but do not result in a significant change of the solar
temperature signal. Lower solar fluxes in the newer SIM data lead
to a significantly cooler stratosphere and mesosphere when compared
to NRLSSI data for 2007. Changes in SW heating from 2004 to 2007 are
however up to six times stronger than for the NRLSSI data.

---------------------------------------------------------
Title: Solar Irradiance Models and Measurements: A Comparison in
the 220–240 nm wavelength band
Authors: Unruh, Yvonne C.; Ball, Will T.; Krivova, Natalie A.
2012omee.book..143U Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Modeling of the Solar Spectral Irradiance as observed by
LYRA/PROBA2 and PREMOS/PICARD
Authors: Shapiro, A.; Cessateur, G.; Dominique, M.; Krivova, N. A.;
Lachat, D.; Rozanov, E.; Schmutz, W. K.; Shapiro, A. V.; Tagirov,
R. V.; Thuillier, G. O.; Wehrli, C.
2011AGUFMGC22A..07S Altcode:
Measurements and modeling of the solar irradiance have gained
an increased attention during the last few decades. Nevertheless a
complete picture of the solar variability is still missing. Therefore
a launch of every new space mission devoted to the measurements of the
spectral solar irradiance provides a crucial piece of complementary
information and nourishes the theoretical models. We present here
spectral solar irradiance data from the recent European missions PROBA-2
(launched on November 2, 2009) and PICARD (launched on June 15, 2010)
and their theoretical interpretation. The PREMOS package onboard PICARD
comprises two experiments, one observing solar irradiance in five (two
UV, one visible and two near infrared) spectral channels with filter
radiometers the other measuring TSI with absolute radiometers. LYRA
is a solar VUV radiometer onboard PROBA-2, which is a technologically
oriented ESA micro-mission, and is observing the solar irradiance
in two UV and two EUV spectral channels. The passbands of the UV
channels in the both experiments were selected on the ground of
relevance for the terrestrial ozone concentration. The PREMOS and
LYRA measurements were carefully corrected for the degradation and
cleaned for non-solar signatures. We provide a comparison with the
VIRGO/SOHO and SOLSTICE+SIM/SORCE data. Both LYRA and PREMOS have
observed several solar eclipses. The analysis of these observations
allows us to accurately retrieve the center-to-limb variations (CLV)
of the solar brightness, which play an important role in the modeling
of the solar irradiance variability on the time scale of the solar
rotation. We show that the calculations with the recently developed
and published COde for Solar Irradiance (COSI) yield the CLV which
are in a good agreement with the measurements. The irradiance in
all channels shows a clear variability on time-scale of the solar
rotation. The amplitude and the profile of the variability strongly
depend on the wavelength. We use COSI as a tool for modeling the
variability of the irradiance, assuming that the latter is determined
by the evolution of the solar surface magnetic field. The theoretical
results are compared with the PREMOS and LYRA measurements. Finally
we show that COSI and other semi-empirical and empirical models allow
to reconstruct the entire solar spectrum from the observations in a
few passbands. Hence the PREMOS and LYRA measurements can be used as
a proxies for the nowcasting of the solar irradiance (UV-visible-IR)
with the high cadence rate and spectral resolution.

---------------------------------------------------------
Title: Analyzing Solar Cycles
Authors: Solanki, Sami K.; Krivova, Natalie A.
2011Sci...334..916S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Modelling solar irradiance variability on time scales from
minutes to months
Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A.
2011A&A...532A.108S Altcode: 2013arXiv1303.1961S
We analyze and model total solar irradiance variability on time scales
from minutes to months, excluding variations due to p-mode oscillations,
using a combination of convective and magnetic components. These include
granulation, the magnetic network, faculae and sunspots. Analysis of
VIRGO data shows that on periods of a day or longer solar variability
depends on magnetic activity, but is nearly independent at shorter
periods. We assume that only granulation affects the solar irradiance
variability on time scales from minutes to hours. Granulation is
described as a large sample of bright cells and dark lanes that
evolve according to rules deduced from observations and radiation
hydrodynamic simulations. Comparison of this model combined with a high
time resolution magnetic-field based irradiance reconstruction, with
solar data reveals a good correspondence except at periods of 10 to 30
h. This suggests that the model is missing some power at these periods,
which may be due to the absence of supergranulation or insufficient
sensitivity of MDI magnetograms used for the reconstruction of the
magnetic field-based irradiance reconstructions. Our model also shows
that even for spatially unresolved data (such as those available for
stars) the Fourier or wavelet transform of time series sampled at high
cadence may allow properties of stellar granulation, in particular
granule lifetimes to be determined.

---------------------------------------------------------
Title: Evolution of the solar irradiance during the Holocene
Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I.
2011A&A...531A...6V Altcode: 2011arXiv1103.4958V
Context. Long-term records of solar radiative output are vital for
understanding solar variability and past climate change. Measurements
of solar irradiance are available for only the last three decades,
which calls for reconstructions of this quantity over longer time
scales using suitable models. Aims: We present a physically
consistent reconstruction of the total solar irradiance for the
Holocene. Methods: We extend the SATIRE (Spectral And Total
Irradiance REconstruction) models to estimate the evolution of the
total (and partly spectral) solar irradiance over the Holocene. The
basic assumption is that the variations of the solar irradiance are
due to the evolution of the dark and bright magnetic features on the
solar surface. The evolution of the decadally averaged magnetic flux
is computed from decadal values of cosmogenic isotope concentrations
recorded in natural archives employing a series of physics-based
models connecting the processes from the modulation of the cosmic
ray flux in the heliosphere to their record in natural archives. We
then compute the total solar irradiance (TSI) as a linear combination
of the jth and jth + 1 decadal values of the open magnetic flux. In
order to evaluate the uncertainties due to the evolution of the Earth's
magnetic dipole moment, we employ four reconstructions of the open flux
which are based on conceptually different paleomagnetic models. 
Results: Reconstructions of the TSI over the Holocene, each valid for
a different paleomagnetic time series, are presented. Our analysis
suggests that major sources of uncertainty in the TSI in this model
are the heritage of the uncertainty of the TSI since 1610 reconstructed
from sunspot data and the uncertainty of the evolution of the Earth's
magnetic dipole moment. The analysis of the distribution functions
of the reconstructed irradiance for the last 3000 years, which is the
period that the reconstructions overlap, indicates that the estimates
based on the virtual axial dipole moment are significantly lower at
earlier times than the reconstructions based on the virtual dipole
moment. We also present a combined reconstruction, which represents
our best estimate of total solar irradiance for any given time during
the Holocene. Conclusions: We present the first physics-based
reconstruction of the total solar irradiance over the Holocene, which
will be of interest for studies of climate change over the last 11 500
years. The reconstruction indicates that the decadally averaged total
solar irradiance ranges over approximately 1.5 W/m2 from
grand maxima to grand minima. Appendix A is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>The
TSI data is only available at the CDS via anonymous
ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A6">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A6</A>

---------------------------------------------------------
Title: Solar irradiance variability: a six-year comparison between
SORCE observations and the SATIRE model
Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.;
Harder, J. W.
2011A&A...530A..71B Altcode: 2011arXiv1104.0885B
 Aims: We investigate how well modeled solar irradiances agree
with measurements from the SORCE satellite, both for total solar
irradiance and broken down into spectral regions on timescales of
several years. Methods: We use the SATIRE model and compare
modeled total solar irradiance (TSI) with TSI measurements over the
period 25 February 2003 to 1 November 2009. Spectral solar irradiance
over 200-1630 nm is compared with the SIM instrument on SORCE over
the period 21 April 2004 to 1 November 2009. We discuss the overall
change in flux and the rotational and long-term trends during this
period of decline from moderate activity to the recent solar minimum
in ~10 nm bands and for three spectral regions of significant interest:
the UV integrated over 200-300 nm, the visible over 400-691 nm and the
IR between 972-1630 nm. Results: The model captures 97% of the
observed TSI variation. This is on the order at which TSI detectors
agree with each other during the period considered. In the spectral
comparison, rotational variability is well reproduced, especially
between 400 and 1200 nm. The magnitude of change in the long-term
trends is many times larger in SIM at almost all wavelengths while
trends in SIM oppose SATIRE in the visible between 500 and 700 nm and
again between 1000 and 1200 nm. We discuss the remaining issues with
both SIM data and the identified limits of the model, particularly
with the way facular contributions are dealt with, the limit of flux
identification in MDI magnetograms during solar minimum and the model
atmospheres in the IR employed by SATIRE. However, it is unlikely that
improvements in these areas will significantly enhance the agreement
in the long-term trends. This disagreement implies that some mechanism
other than surface magnetism is causing SSI variations, in particular
between 2004 and 2006, if the SIM data are correct. Since SATIRE
was able to reproduce UV irradiance between 1991 and 2002 from UARS,
either the solar mechanism for SSI variation fundamentally changed
around the peak of cycle 23, or there is an inconsistency between UARS
and SORCE UV measurements. We favour the second explanation.

---------------------------------------------------------
Title: Solar total irradiance in cycle 23
Authors: Krivova, N. A.; Solanki, S. K.; Schmutz, W.
2011A&A...529A..81K Altcode: 2011arXiv1102.3077K
Context. The most recent minimum of solar activity was deeper and
longer than the previous two minima as indicated by different proxies
of solar activity. This is also true for the total solar irradiance
(TSI) according to the PMOD composite. Aims: The apparently
unusual behaviour of the TSI has been interpreted as evidence against
solar surface magnetism as the main driver of the secular change
in the TSI. We test claims that the evolution of the solar surface
magnetic field does not reproduce the observed TSI in cycle 23. Methods: We use sensitive, 60-min averaged MDI magnetograms and
quasi-simultaneous continuum images as an input to our SATIRE-S model
and calculate the TSI variation over cycle 23, sampled roughly every
two weeks. The computed TSI is then compared with the PMOD composite of
TSI measurements and with the data from two individual instruments,
SORCE/TIM and UARS/ACRIM II, that monitored the TSI during the
declining phase of cycle 23 and over the previous minimum in 1996,
respectively. Results: Excellent agreement is found between the
trends shown by the model and almost all sets of measurements. The
only exception is the early, i.e. 1996 to 1998, PMOD data. Whereas
the agreement between the model and the PMOD composite over the
period 1999-2009 is almost perfect, the modelled TSI shows a steeper
increase between 1996 and 1999 than implied by the PMOD composite. On
the other hand, the steeper trend in the model agrees remarkably well
with the ACRIM II data. A closer look at the VIRGO data, which are the
basis of the PMOD composite after 1996, reveals that only one of the
two VIRGO instruments, the PMO6V, shows the shallower trend present
in the composite, whereas the DIARAD measurements indicate a steeper
trend. Conclusions: Based on these results, we conclude that (1)
the sensitivity changes of the PMO6V radiometers within VIRGO during
the first two years have very likely not been correctly evaluated;
and that (2) the TSI variations over cycle 23 and the change in the
TSI levels between the minima in 1996 and 2008 are consistent with
the solar surface magnetism mechanism.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Evolution of solar irradiance
during Holocene (Vieira+, 2011)
Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I.
2011yCat..35310006V Altcode: 2011yCat..35319006V
This is a composite total solar irradiance (TSI) time series for 9495BC
to 2007AD constructed as described in Sect. 3.3 of the paper. Since
the TSI is the main external heat input into the Earth's climate
system, a consistent record covering as long period as possible
is needed for climate models. This was our main motivation for
constructing this composite TSI time series. In order to
produce a representative time series, we divided the Holocene into
four periods according to the available data for each period. Table
4 (see below) summarizes the periods considered and the models
available for each period. After the end of the Maunder Minimum we
compute daily values, while prior to the end of the Maunder Minimum
we compute 10-year averages. For the period for which both solar disk
magnetograms and continuum images are available (period 1) we employ
the SATIRE-S reconstruction (Krivova et al. 2003A&amp;A...399L...1K;
Wenzler et al. 2006A&amp;A...460..583W). SATIRE-T (Krivova et
al. 2010JGRA..11512112K) reconstruction is used from the beginning of
the Maunder Minimum (approximately 1640AD) to 1977AD. Prior to 1640AD
reconstructions are based on cosmogenic isotopes (this paper). Different
models of the Earth's geomagnetic field are available before and after
approximately 5000BC. Therefore we treat periods 3 and 4 (before and
after 5000BC) separately. Further details can be found in the paper. We
emphasize that the reconstructions based on different proxies have
different time resolutions. (1 data file).

---------------------------------------------------------
Title: Towards a long-term record of solar total and spectral
irradiance
Authors: Krivova, N. A.; Solanki, S. K.; Unruh, Y. C.
2011JASTP..73..223K Altcode: 2009arXiv0911.4002K
The variation of total solar irradiance (TSI) has been measured since
1978 and that of the spectral irradiance for an even shorter amount of
time. Semi-empirical models are now available that reproduce over 80%
of the measured irradiance variations. An extension of these models into
the more distant past is needed in order to serve as input to climate
simulations. Here we review our most recent efforts to model solar
total and spectral irradiance on time scales from days to centuries and
even longer. Solar spectral irradiance has been reconstructed since
1947. Reconstruction of solar total irradiance goes back to 1610 and
suggests a value of about 1-1.5W/m2 for the increase in
the cycle-averaged TSI since the end of the Maunder minimum, which
is significantly lower than previously assumed but agrees with other
modern models. First steps have also been made towards reconstructions
of solar total and spectral irradiance on time scales of millennia.

---------------------------------------------------------
Title: Reconstruction of solar spectral irradiance since the Maunder
minimum
Authors: Krivova, N. A.; Vieira, L. E. A.; Solanki, S. K.
2010JGRA..11512112K Altcode:
Solar irradiance is the main external driver of the Earth's
climate. Whereas the total solar irradiance is the main source of
energy input into the climate system, solar UV irradiance exerts
control over chemical and physical processes in the Earth's upper
atmosphere. The time series of accurate irradiance measurements are,
however, relatively short and limit the assessment of the solar
contribution to the climate change. Here we reconstruct solar total
and spectral irradiance in the range 115-160,000 nm since 1610. The
evolution of the solar photospheric magnetic flux, which is a central
input to the model, is appraised from the historical record of the
sunspot number using a simple but consistent physical model. The model
predicts an increase of 1.25 W/m2, or about 0.09%, in the
11-year averaged solar total irradiance since the Maunder minimum. Also,
irradiance in individual spectral intervals has generally increased
during the past four centuries, the magnitude of the trend being higher
toward shorter wavelengths. In particular, the 11-year averaged Ly-α
irradiance has increased by almost 50%. An exception is the spectral
interval between about 1500 and 2500 nm, where irradiance has slightly
decreased (by about 0.02%).

---------------------------------------------------------
Title: Reply to comment by P. Foukal on “A homogeneous database
of sunspot areas covering more than 130 years”
Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S.
2010JGRA..115.9103B Altcode: 2010JGRA..11509103B
<A href="/journals/ja/ja1009/2010JA015375/">Abstract Available</A>
from <A href="http://www.agu.org">http://www.agu.org</A>

---------------------------------------------------------
Title: Sunspot group tilt angles and the strength of the solar cycle
Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron,
R.; Peñuela, T.
2010A&A...518A...7D Altcode: 2010arXiv1005.1774D
Context. It is well known that the tilt angles of active regions
increase with their latitude (Joy's law). It has never been checked
before, however, whether the average tilt angles change from one cycle
to the next. Flux transport models show the importance of tilt angles
for the reversal and build up of magnetic flux at the poles, which is in
turn correlated to the strength of the next cycle. Aims: Here we
analyse time series of tilt angle measurements and look for a possible
relationship of the tilt angles with other solar cycle parameters,
in order to glean information on the solar dynamo and to estimate
their potential for predicting solar activity. Methods: We
employed tilt angle data from Mount Wilson and Kodaikanal observatories
covering solar cycles 15 to 21. We analyse the latitudinal distribution
of the tilt angles (Joy's law), their variation from cycle to cycle,
and their relationship to other solar cycle parameters, such as the
strength (or total area covered by sunspots in a cycle), amplitude,
and length. Results: The two main results of our analysis
follow. 1. We find an anti-correlation between the mean normalised
tilt angle of a given cycle and the strength (or amplitude) of that
cycle, with a correlation coefficient of rc = -0.95 (99.9%
confidence level) and rc = -0.93 (99.76% confidence level)
for Mount Wilson and Kodaikanal data, respectively. 2. The product
of the cycle's averaged tilt angle and the strength of the same cycle
displays a significant correlation with the strength of the next cycle
(rc = 0.65 at 89% confidence level and rc =
0.70 at 92% confidence level for Mount Wilson and Kodaikanal data,
respectively). An even better correlation is obtained between the
source term of the poloidal flux in Babcock-Leighton-type dynamos (which
contains the tilt angle) and the amplitude of the next cycle. Further we
confirm the linear relationship (Joy's law) between the tilt angle and
latitude with slopes of 0.26 and 0.28 for Mount Wilson and Kodaikanal
data, respectively. In addition, we obtain good positive correlations
between the normalised-area-weighted tilt angle and the length of the
following cycle, whereas the strength or the amplitude of the next cycle
does not appear to be correlated to the tilt angles of the current cycle
alone. Conclusions: The results of this study indicate that,
in combination with the cycle strength, the active region tilt angles
play an important role in building up the polar fields at cycle minimum.

---------------------------------------------------------
Title: Evolution of the solar radiative forcing on climate during
the Holocene
Authors: Vieira, Luis Eduardo; Solanki, Sami K.; Krivova, Natalie
2010cosp...38.1785V Altcode: 2010cosp.meet.1785V
The main external heating source of the Earth's coupled atmosphere-ocean
system is the solar radiative energy input. The variability of
this energy source produces corresponding changes on the coupled
system. However, there is still significant uncertainty on the level of
changes. One way to distinguish the influence of the Sun on the climate
from other sources is to search for its influence in the pre-industrial
period, when the influence of human activities on the atmosphere
composition and Earth's surface properties can be neglected. Such
studies require long time series of solar and geophysical parameters,
ideally covering the whole Holocene. Here, we compute the total and
spectral irradiance for the Holocene employing the reconstructions of
the open flux and sunspot number obtained from the cosmogenic isotope
14C. The model employed in this study is identical to the spectral
and total irradiance reconstruction (SATIRE) models employed to study
these parameters on time scales from days to centuries, but adapted
to work with decadal averaged data. The model is tested by comparing
to the total and spectral solar irradiance reconstructions from the
sunspot number for the last 4 centuries. We also discuss limits and
uncertainties of the model.

---------------------------------------------------------
Title: Is the current solar activity minimum special?
Authors: Solanki, Sami K.; Krivova, Natalie; Antunes Vieira, Luis
Eduardo
2010cosp...38.1693S Altcode: 2010cosp.meet.1693S
Is the current solar activity minimum special? Sami K. Solanki(1,2),
Natalie Krivova(1), Luis E. Vieira(1,3) (1) Max Planck Institute
for Solar System Research, Katlenburg-Lindau, Germany (2) Kyung-Hee
University, Yongin, Gyeonggi, Korea (3) Laboratory for Physics and
Chemistry of the Terrestrial Environment/CNRS, Or-leans, France In many
respects the current minimum of solar activity is rather different
from other minima during the last half century, which has seen the
Sun in a state of unusually high activity. This uncommon minimum
has fascinated solar physicists. Of particular importance for our
understanding is the behaviour of the magnetic field, which is the
source of all activity observed in the solar atmosphere, including the
heliosphere. Furthermore, the evolution of dark and bright magnetic
features on the solar surface modulates the solar irradiance, which
could directly influence the Earth's climate. In this presentation
observations and models of the evolution of the Sun's magnetic field,
sunspots and irradiance are reviewed. After discussing the variation
of these quantities over the solar cycle and from cycle to cycle,
the current minimum is considered in greater detail. It is compared
to earlier minima going back to the Maunder minimum and, in a more
averaged sense, to solar activity and irradiance during the whole
Holocene. Questions that will be onsidered are: Are we leaving the
period of high solar activity (a grand maximum) to enter into a phase
of more moderate activity, or even into a grand minimum? How could
such a change in the level of the magnetic field and irradiance aid
us in gaining further insight into the nature of the Sun's influence
on the Earth's climate?

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Title: Analysis of Ca II K spectroheliogram time-series for solar
irradiance studies
Authors: Ermolli, Ilaria; Krivova, Natalie; Solanki, Sami K.
2010cosp...38..133E Altcode: 2010cosp.meet..133E
Historic Ca II K observations constitute an extremely valuable
resource for many research topics, including investigations of solar
activity and irradiance variations on century timescales. In fact,
the intensities measured on these observations can serve as proxies for
magnetic activity, due to the strong correlation found between the Ca
II K emission and the line-of-sight magnetic flux density. We analyzed
three time-series of historic Ca II K observations, specifically those
obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson
spectroheliohram archives. We examined the quality of these data,
in order to estimate their value for irradiance studies focusing
on time-scales longer than the solar cycle. We employ different
processing techniques in order to investigate whether uniform results
on the evolution of the solar magnetic field, both in quiet and active
regions, can be obtained for the three analyzed series. ——-Session
A1.1 Atmosphere Chemistry and Physics (i) Solar Spectral Irradiance
Variations and Their Influence on the Earth's Atmosphere

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Title: Total and spectral irradiance comparisons between SIM and
the SATIRE model in the declining phase of cycle 23
Authors: Ball, Will; Unruh, Yvonne; Krivova, Natalie; Solanki, Sami
K.; Harder, Jerald
2010cosp...38..136B Altcode: 2010cosp.meet..136B
Climate models rely on accurate total and spectral solar irradiance
inputs, but until 2003 con-tinuous spectral irradiance information
across a large portion of the solar spectrum was lacking. Since the
launch of the Solar Radiation and Climate Experiment (SORCE), with the
Spectral Irradiance Monitor (SIM) observing the UV, visible and IR,
data have been accumulating and now cover a significant portion of
a cycle. For the first time this allows spectral models to be tested
over periods greater than a solar rotation. We present six years of
total and spectral irradiance reconstructions using the SATIRE model
that incorporates SOHO/MDI continuum and magnetogram images for the
period April 2004 to November 2009 in the declining phase of cycle 23
and through the recent unusual minimum. We compare these results with
the SIM instrument and so cover the spectral region 200 -1600 nm. While
detrended, short-term, variation is recreated well by the model, there
are discrepancies in longer-term trends between observations and the
model. This may become important when considering the radiative forcing
from the Sun used in climate research and so understanding why there is
such a significant disagreement is an important area of investigation.

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Title: Solar magnetic field and irradiance: how unusual is the
current minimum? (Invited)
Authors: Solanki, S. K.; Krivova, N.; Vieira, L. A.
2009AGUFM.U34A..01S Altcode:
In many respects the current minimum of solar activity is rather
different from other minima during the last half century, which has
seen the Sun in a state of unusually high activity. This uncommon
minimum has fascinated solar physicists. Of particular importance for
our understanding is the behaviour of the magnetic field, which is the
source of all activity observed in the solar atmosphere, including the
heliosphere. Furthermore, the evolution of dark and bright magnetic
features on the solar surface modulates the solar irradiance, which
could directly influence the Earth's climate. In this presentation
observations and models of the evolution of the Sun's magnetic field
and irradiance are reviewed. After discussing the variation of these
quantities over the solar cycle and from cycle to cycle, the current
minimum is considered in greater detail. It is compared to earlier
minima going back to the Maunder minimum and, in a more averaged sense,
to solar activity and irradiance during the whole Holocene. Questions
that will be considered are: Are we leaving the period of high solar
activity (a grand maximum) to enter into a phase of more moderate
activity, or even into a grand minimum? How could such a change in the
level of the magnetic field and irradiance aid us in gaining further
insight into the nature of the Sun's influence on the Earth's climate?

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Title: ACRIM-gap and total solar irradiance revisited: Is there a
secular trend between 1986 and 1996?
Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T.
2009GeoRL..3620101K Altcode: 2009arXiv0911.3817K
A gap in the total solar irradiance (TSI) measurements between
ACRIM-1 and ACRIM-2 led to the ongoing debate on the presence or not
of a secular trend between the minima preceding cycles 22 (in 1986)
and 23 (1996). It was recently proposed to use the SATIRE model of
solar irradiance variations to bridge this gap. When doing this, it is
important to use the appropriate SATIRE-based reconstruction, which we
do here, employing a reconstruction based on magnetograms. The accuracy
of this model on months to years timescales is significantly higher
than that of a model developed for long-term reconstructions used by
the ACRIM team for such an analysis. The constructed ‘mixed’ ACRIM
— SATIRE composite shows no increase in the TSI from 1986 to 1996,
in contrast to the ACRIM TSI composite.

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Title: Reconstruction of solar UV irradiance since 1974
Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T.; Podlipnik, B.
2009JGRD..114.0I04K Altcode: 2009arXiv0907.1500K; 2009JGRD..11400I04K
Variations of the solar UV irradiance are an important driver of
chemical and physical processes in the Earth's upper atmosphere and may
also influence global climate. Here we reconstruct solar UV irradiance
in the range 115-400 nm over the period 1974-2007 by making use of
the recently developed empirical extension of the Spectral And Total
Irradiance Reconstruction (SATIRE) models employing Solar Ultraviolet
Spectral Irradiance Monitor (SUSIM) data. The evolution of the solar
photospheric magnetic flux, which is a central input to the model,
is described by the magnetograms and continuum images recorded at
the Kitt Peak National Solar Observatory between 1974 and 2003 and
by the Michelson Doppler Imager instrument on SOHO since 1996. The
reconstruction extends the available observational record by 1.5
solar cycles. The reconstructed Ly-α irradiance agrees well with
the composite time series by Woods et al. (2000). The amplitude of
the irradiance variations grows with decreasing wavelength and in
the wavelength regions of special interest for studies of the Earth's
climate (Ly-α and oxygen absorption continuum and bands between 130
and 350 nm) is 1-2 orders of magnitude stronger than in the visible
or if integrated over all wavelengths (total solar irradiance).

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Title: Solar Surface Magnetism and Irradiance on Time Scales from
Days to the 11-Year Cycle
Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter,
M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.;
Unruh, Y.; Vögler, A.
2009SSRv..145..337D Altcode:
The uninterrupted measurement of the total solar irradiance during the
last three solar cycles and an increasing amount of solar spectral
irradiance measurements as well as solar imaging observations
(magnetograms and photometric data) have stimulated the development
of models attributing irradiance variations to solar surface
magnetism. Here we review the current status of solar irradiance
measurements and modelling efforts based on solar photospheric
magnetic fields. Thereby we restrict ourselves to the study of solar
variations from days to the solar cycle. Phenomenological models
of the solar atmosphere in combination with imaging observations of
solar electromagnetic radiation and measurements of the photospheric
magnetic field have reached high enough quality to show that a large
fraction (at least, about 80%) of the solar irradiance variability
can be explained by the radiative effects of the magnetic activity
present in the photosphere. Also, significant progress has been made
with magnetohydrodynamic simulations of convection that allow us to
relate the radiance of the photospheric magnetic structures to the
observations.

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Title: A homogeneous database of sunspot areas covering more than
130 years
Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S.
2009JGRA..114.7104B Altcode: 2009JGRA..11407104B; 2009arXiv0906.0942B
The historical record of sunspot areas is a valuable and widely used
proxy of solar activity and variability. The Royal Greenwich Observatory
regularly measured this and other parameters between 1874 and
1976. After that time records from a number of different observatories
are available. These, however, show systematic differences and often
have significant gaps. Our goal is to obtain a uniform and complete
sunspot area time series by combining different data sets. A homogeneous
composite of sunspot areas is essential for different applications
in solar physics, among others for irradiance reconstructions. Data
recorded simultaneously at different observatories are statistically
compared in order to determine the intercalibration factors. Using
these data we compile a complete and cross-calibrated time series. The
Greenwich data set is used as a basis until 1976, the Russian data
(a compilation of observations made at stations in the former USSR)
are used between 1977 and 1985, and data compiled by the USAF network
are used since 1986. Other data sets (Rome, Yunnan, and Catania)
are used to fill up the remaining gaps. Using the final sunspot areas
record the Photometric Sunspot Index is calculated. We also show that
the use of uncalibrated sunspot areas data sets can seriously affect
the estimate of irradiance variations. Our analysis implies that there
is no basis for the claim that UV irradiance variations have a much
smaller influence on climate than total solar irradiance variations.

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Title: Reconstructed and measured total solar irradiance: Is there
a secular trend between 1978 and 2003?
Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.
2009GeoRL..3611102W Altcode: 2009arXiv0909.2975W
Total solar irradiance reconstructed between 1978 and 2003 using solar
surface magnetic field distributions is compared with three composites
of total solar irradiance measurements. A good correspondence is found
with the total solar irradiance composite from PMOD/WRC, with no bias
between the three cycles. The agreement with the other composites
(the ACRIM composite, mainly based on the Active Cavity Radiometer
Irradiance Monitors I, II, and III, and the IRMB composite from the
Institut Royal Meteorologique Belgique) is significantly poorer. In
particular, a secular increase in the irradiance exhibited by these
composites is not present in the reconstructions. Hence any secular
trend in total solar irradiance between 1978 and 2003 is not due to
magnetic fields at the solar surface.

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Title: Comparison Among Ca II K Spectroheliogram Time Series with
an Application to Solar Activity Studies
Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.;
Ulrich, R. K.; Singh, J.
2009ApJ...698.1000E Altcode:
Various observatories around the globe started regular full-disk imaging
of the solar atmosphere in the Ca II K line in the early decades of
the 20th century. The archives made by these observations have the
potential of providing far more detailed information on solar magnetism
than just the sunspot number and area records to which most studies
of solar activity and irradiance changes are restricted. We evaluate
the image quality and contents of three Ca II K spectroheliogram time
series, specifically those obtained by the digitization of the Arcetri,
Kodaikanal, and Mt Wilson photographic archives, in order to estimate
their value for studies focusing on timescales longer than the solar
cycle. We analyze the quality of these data and compare the results
obtained with those achieved for similar present-day observations
taken with the Meudon spectroheliograph and with the Rome-PSPT. We
also investigate whether image-segmentation techniques, such as
those developed for identification of plage regions on present-day Ca
II K observations, can be used to process historic series. We show
that historic data suffer from stronger geometrical distortions and
photometric uncertainties than similar present-day observations. The
latter uncertainties mostly originate from the photographic calibration
of the original data and from stray-light effects. We also show that
the image contents of the three analyzed series vary in time. These
variations are probably due to instrument changes and aging of the
spectrographs used, as well as changes of the observing programs. The
segmentation technique tested in this study gives reasonably consistent
results for the three analyzed series after application of a simple
photographic calibration. Although the plage areas measured from the
three analyzed series differ somewhat, the difference to previously
published results is larger.

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Title: Faculae and Plague
Authors: Solanki, Sami; Krivova, Natalie
2009LanB...4B..153S Altcode: 2009LanB...4B.4124S
No abstract at ADS

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Title: Sunspots
Authors: Solanki, Sami; Krivova, Natalie
2009LanB...4B..141S Altcode: 2009LanB...4B.4123S
No abstract at ADS

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Title: Solar Activity Cycle
Authors: Solanki, Sami; Krivova, Natalie
2009LanB...4B..124S Altcode: 2009LanB...4B.4122S
No abstract at ADS

---------------------------------------------------------
Title: Spectral irradiance variations: comparison between observations
and the SATIRE model on solar rotation time scales
Authors: Unruh, Y. C.; Krivova, N. A.; Solanki, S. K.; Harder, J. W.;
Kopp, G.
2008A&A...486..311U Altcode: 2008arXiv0802.4178U
Aims: We test the reliability of the observed and calculated spectral
irradiance variations between 200 and 1600 nm over a time span of
three solar rotations in 2004. Methods: We compare our model
calculations to spectral irradiance observations taken with SORCE/SIM,
SoHO/VIRGO, and UARS/SUSIM. The calculations assume LTE and are based
on the SATIRE (Spectral And Total Irradiance REconstruction) model. We
analyse the variability as a function of wavelength and present time
series in a number of selected wavelength regions covering the UV to
the NIR. We also show the facular and spot contributions to the total
calculated variability. Results: In most wavelength regions,
the variability agrees well between all sets of observations and the
model calculations. The model does particularly well between 400 and
1300 nm, but fails below 220 nm, as well as for some of the strong NUV
lines. Our calculations clearly show the shift from faculae-dominated
variability in the NUV to spot-dominated variability above approximately
400 nm. We also discuss some of the remaining problems, such as the low
sensitivity of SUSIM and SORCE for wavelengths between approximately
310 and 350 nm, where currently the model calculations still provide
the best estimates of solar variability.

---------------------------------------------------------
Title: Models of solar irradiance variations: Current status
Authors: Krivova, Natalie A.; Solanki, Sami K.
2008JApA...29..151K Altcode:
Regular monitoring of solar irradiance has been carried out since 1978
to show that solar total and spectral irradiance varies at different
time scales. Whereas variations on time scales of minutes to hours
are due to solar oscillations and granulation, variations on longer
time scales are driven by the evolution of the solar surface magnetic
field. Here the most recent advances in modelling of solar irradiance
variations on time scales longer than a day are briefly reviewed.

---------------------------------------------------------
Title: On the potential value of Ca II K spectroheliogram time-series
for solar activity and irradiance studies
Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.;
Ulrich, R. K.; Singh, J.
2008arXiv0802.3806E Altcode:
Various observatories around the globe started regular full-disk imaging
of the solar atmosphere in the Ca II K line since the early decades
of the 20th century. The archives made by these observations have the
potential of providing far more detailed information on solar magnetism
than just the sunspot number and area records to which most studies
of solar activity and irradiance changes are restricted. We evaluate
the image contents of three Ca II K spectroheliogram time-series,
specifically those obtained by the digitization of the Arcetri,
Kodaikanal, and Mt Wilson photographic archives. We describe the
main problems afflicting these data and analyze their quality by
expressing the image contents through several quantities. We compare
the results obtained with those for similar present-day observations
taken with the Meudon spectroheliograph and with the Rome-PSPT. We show
that historic data suffer from stronger geometrical distortions and
photometric uncertainties than similar present-day observations. The
latter uncertainties mostly originate from the photographic calibration
of the original data and from stray-light effects. We also show that
the image contents of the three analyzed series vary in time. These
variations are probably due to instrument changes and aging of the
spectrographs used, as well as changes of the observing programs. Our
results imply that the main challenge for the analysis of historic data
is their accurate photometric calibration. This problem must be solved
before they can provide reliable information about solar magnetism and
activity over the last century. Moreover, inter-calibration of results
obtained from independent time-series is required to reliably trace
changes of solar properties with time from the analysis of such data.

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Title: Variability of solar irradiance from the UV to the NIR from
GOME and SCIAMACHY for use in atmospheric models
Authors: Ambrose Pagaran, Joseph; Weber, Mark; Ambrose Pagaran, Joseph;
Burrows, John P.; Krivova, Natalie; Solanki, Sami; Floyd, Linton
2008cosp...37.2329A Altcode: 2008cosp.meet.2329A
The Sun is the primary energy source that drives the Earth's
climate system. Its radiative output is known to vary in time,
for instance, with the 11-year solar cycle and 27-day solar
rotation period. Variations in the solar spectral irradiance (SSI)
affect the thermal structure and chemical composition of the Earth's
atmosphere. Although the largest solar variations are observed in the UV
spectral region, a large fraction of the total solar irradiance (TSI,
solar constant) variation over a solar cycle comes from the visible
and near IR spectral range. In order to understand how SSI variations
cause a detectable change in climate, we need to quantify UV, visible,
and near IR variation in the solar spectral irradiance to a high level
of certainty both over the short term solar rotation 27-day period and
11-year solar cycle. Using daily solar irradiance observations from
SUSIM (1992-2005), GOME (1995-present), and SCIAMACHY (2002-present),
we study solar variations over 27-day solar rotations from 120 nm to
1600 nm. The variability is modelled by parameterizing SSI in terms
of faculae brightening (using the Mg II core-to-wing ratio proxy)
and sunspot darkening (using the photospheric sunspot index). Since
the variations in the visible and NIR are well below 1

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Title: Investigation of influence of hypomagnetic conditions closely
similar to interplanetary magnetic filed on behavioral and vegetative
reactions of higher mammals
Authors: Krivova, Natalie; Trukhanov, Kiril; Zamotshina, Tatyana;
Zaeva, Olga; Khodanovich, Marina; Misina, Tatyana; Tukhvatulin, Ravil;
Suhko, Valery
2008cosp...37.1624K Altcode: 2008cosp.meet.1624K
To study the influence of long being under reduced magnetic field
on behavioral and vegetative reactions of higher mammals the white
rat males were put into the 700-1000 times reduced geomagnetic field
(50-70 nT) for 25 days. Such field was obtained by using automatic
compensation of the horizontal and vertical components of the GMF at
a frequencies up to 10 Hz by means of solenoids of the experimental
magnetic system. Control animals were located in the same room under
usual laboratory GMF conditions (52 uT). Two days before the experiment
the behavioral reactions were studied in the "open field" by means of
a set of tests, characterizing the level of emotionality, moving and
orientational-investigative activities of the animals under conditions
of unimpeded behavior. 60 white underbred rat males with the initial
body mass of 200 g were divided into three clusters. Animals with
average indices were selected for the experiment. We have judged
behavioral reaction disturbances of the rats under hypomagnetic
conditions using videotape recordings carried out in the entire course
of the chronic experiment. According to the obtained results during
the period of maximum activity (from 230 to 330 a.m.) the number
of interrelations between the individuals increased appreciably
for experimental rats including interrelations with aggressive
character. This was real during all 25 days of observation. We
observed a certain dynamics of this index differed from that of the
control group. We have also analyzed the final period of observation
from the 21th to the 25th days. In this period we studied the 24
hours' dynamics of interrelations which were noted during 5 minutes
in every hour around the clock. In the control group the number of
interrelation was at a constantly low level. For experimental animals
the number of interrelations was higher in the night hours than in
the day ones. Moreover it exceeded the similar indexes observed from
the 1st to the 20th day. For example from 300 to 305 a.m. on the
23th day we recorded 27 contacts of aggressive character between the
individuals. So, in hypomagnetic field conditions the irritability of
the animals' central nervous system grows, that expresses itself in
the increase of contacts of aggressive and non-aggressive character
between the individuals. Also we have carried out the Spirman
correlation analysis between studied indices of moving activity and
chemiluminescence of blood plasma and urine, electrolytic composition
of urine and muscles. For control animals the quantity of correlation
connections between electrolyte concentrations in studied substrata
was higher than for experimental animals. The physiological sense of
these correlation connections is discussed.

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Title: Solar Activity and Irradiance Studies with Ca II
Spectroheliograms: Potential and Problems
Authors: Ermolli, I.; Tlatov, A.; Solanki, S. K.; Krivova, N. A.;
Singh, J.
2007ASPC..368..533E Altcode:
Various observatories around the globe carried out synoptic full-disk
observations of the Sun since the beginning of the 20th century. The
archives created by these observations, especially those including
Ca II spectroheliograms, have the potential of providing far more
detailed information on solar activity than the indices usually used to
study activity variations, solar cycle and irradiance changes. However,
these data suffer significantly from various problems including numerous
defects in the photographic plates, missing or inaccurate calibration of
the blackening curve, changes in the positioning of the exit slit with
respect to the spectral line and variable seeing. Here we discuss the
quality of images obtained by the digitization of three historic Ca II
K time series, specifically those stored by the Arcetri, Kodaikanal
and Mt Wilson Observatories. The aim of this work is to evaluate
the potential value of these data for studies of solar activity and
variability. It also shows the importance of the detailed and accurate
image processing technique, in order to obtain uniform and trustable
results from images coming from different historic archives.

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Title: Reconstruction of the Long-Term Irradiance Variations
Authors: Balmaceda, L.; Krivova, N.; Solanki, S.
2007AGUSMGP54A..05B Altcode:
Solar irradiance variations have been recorded only since 1978. Clearly,
there is a need to extend these records into the past in order to
evaluate their possible influence on the Earth's climate. Here, a
reconstruction of solar irradiance back to the Maunder minimum from
the surface magnetic flux is presented. The reconstruction is based on
a simple physical model that builds on the sunspot number records and
sunspot areas where available. Since the sunspot area records generally
consist of a compilation of data from multiple observatories, a proper
cross-calibration is essential. The use of data of different sources
directly combined can lead to errors in estimating the increase of
solar irradiance during the past centuries. Thus, a brief description
of the cross-calibration of sunspot areas is also presented.

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Title: Reconstruction of solar total irradiance since 1700 from the
surface magnetic flux
Authors: Krivova, N. A.; Balmaceda, L.; Solanki, S. K.
2007A&A...467..335K Altcode:
Context: Total solar irradiance changes by about 0.1% between solar
activity maximum and minimum. Accurate measurements of this quantity are
only available since 1978 and do not provide information on longer-term
secular trends. Aims: In order to reliably evaluate the Sun's
role in recent global climate change, longer time series are, however,
needed. They can only be assessed with the help of suitable models. Methods: The total solar irradiance is reconstructed from the end of
the Maunder minimum to the present based on variations of the surface
distribution of the solar magnetic field. The latter is calculated
from the historical record of the sunspot number using a simple but
consistent physical model. Results: Our model successfully
reproduces three independent data sets: total solar irradiance
measurements available since 1978, total photospheric magnetic
flux since 1974 and the open magnetic flux since 1868 empirically
reconstructed using the geomagnetic aa-index. The model predicts an
increase in the solar total irradiance since the Maunder minimum of
1.3^+0.2_-0.4 Wm-2.

---------------------------------------------------------
Title: Properties of sunspots in cycle 23. I. Dependence of brightness
on sunspot size and cycle phase
Authors: Mathew, S. K.; Martínez Pillet, V.; Solanki, S. K.; Krivova,
N. A.
2007A&A...465..291M Altcode: 2007astro.ph..1401M
Aims:In this paper we investigate the dependence of umbral core
brightness, as well as the mean umbral and penumbral brightness on the
phase of the solar cycle and on the size of the sunspot. Methods:
Albregtsen &amp; Maltby (1978, Nature, 274, 41) reported an increase
in umbral core brightness from the early to the late phase of solar
cycle from the analysis of 13 sunspots which cover solar cycles 20
and 21. Here we revisit this topic by analysing continuum images
of more than 160 sunspots observed by the MDI instrument on board
the SOHO spacecraft for the period between 1998 March to 2004 March,
i.e. a sizable part of solar cycle 23. The advantage of this data set
is its homogeneity, with no seeing fluctuations. A careful stray light
correction, which is validated using the Mercury transit of 7th May,
2003, is carried out before the umbral and penumbral intensities are
determined. The influence of the Zeeman splitting of the nearby Ni I
spectral line on the measured "continuum" intensity is also taken into
account. Results: We did not observe any significant variation
in umbral core, mean umbral and mean penumbral intensities with solar
cycle, which is in contrast to earlier findings for the umbral core
intensity. We do find a strong and clear dependence of the umbral
brightness on sunspot size, however. The penumbral brightness also
displays a weak dependence. The brightness-radius relationship has
numerous implications, some of which, such as those for the energy
transport in umbrae, are pointed out.

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Title: Magnetic source of the solar cycle variation of the Mn I
539.4 nm line
Authors: Danilović, S.; Solanki, S. K.; Livingston, W.; Krivova,
N.; Vince, I.
2007msfa.conf..189D Altcode:
As a part of the long term program at KPNO, the Mn I 539.4 nm line has
been observed for nearly three solar cycles using the McMath telescope
and the 13.5 m spectrograph in double pass mode. These full-disk
spectrophotometric observations revealed an unusually large amplitude
change of its parameters over the solar cycle and its correlation with
Ca II K intensity. One of the proposed explanations for this phenomenon
is the optical pumping by the Mg II k line. With this work we would like
to show that this may not be the main mechanism behind the change. We
reconstructed the changes of the line parameters using a model that
takes into account only changes of the daily surface distributions of
magnetic field. This model has already been used to successfully model
total solar irradiance. We now apply it for modelling the Mn I line,
as well as its neighboring Fe I line using exactly the same value
of the free parameter as used for the reconstruction of total solar
irradiance. We reproduce well the Mn I and Fe I line changes over the
cycle purely with LTE modelling. This indicates that optical pumping
of the Mn I line by Mg II k is not the main cause of its solar cycle
change and sets an independent constraint on solar irradiance models.

---------------------------------------------------------
Title: Reconstruction of solar irradiance using the Group sunspot
number
Authors: Balmaceda, L.; Krivova, N. A.; Solanki, S. K.
2007AdSpR..40..986B Altcode: 2007astro.ph..3147B
We present a reconstruction of total solar irradiance since 1610 to
the present based on variations of the surface distribution of the
solar magnetic field. The latter is calculated from the historical
record of the Group sunspot number using a simple but consistent
physical model. Our model successfully reproduces three independent
data sets: total solar irradiance measurements available since 1978,
total photospheric magnetic flux from 1974 and the open magnetic
flux since 1868 (as empirically reconstructed from the geomagnetic
aa-index). The model predicts an increase in the total solar irradiance
since the Maunder Minimum of about 1.3 Wm -2.

---------------------------------------------------------
Title: Reconstruction of solar irradiance variations in cycles 21-23
based on surface magnetic fields
Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fröhlich, C.
2006A&A...460..583W Altcode:
Aims.We present a reconstruction of total solar irradiance (TSI) back
to 1974, i.e. from the minimum of cycle 21 to the declining phase of
cycle 23. We also present a cross-calibration between the magnetograms
obtained by the 512 channel magnetograph and the spectromagnetograph
at Kitt Peak. Methods: .The TSI reconstruction is carried out
using data from the 512-channel Diode Array Magnetograph and the newer
spectromagnetograph on Kitt Peak. The model is based on the assumption
that all irradiance changes on time-scales of a day and longer are
entirely due to the variations of the surface distribution of the
solar magnetic field. The reconstructed irradiance is compared with
the composite of total solar irradiance measurements from PMOD/WRC
(version 41). Results: .A good correspondence is found with the
PMOD TSI composite, with no bias between the three cycles on time-scales
longer than the solar rotation period, although the accuracy of the
TSI reconstruction is somewhat lower when 512 channel magnetograph
data are used. This suggests that the same driver of the irradiance
variations, namely the evolution of the magnetic flux at the solar
surface, is acting in cycles 21-23. Different methods of comparing
the magnetograms obtained by the two Kitt Peak magnetographs give
somewhat different results, with factors by which 512 channel data
must be divided in the range 1.38-1.63 being found. This is due to the
non-linearity of the relationship between the magnetic field measured
by the two instruments.

---------------------------------------------------------
Title: Solar Variability of Possible Relevance for Planetary Climates
Authors: Solanki, S. K.; Krivova, N. A.
2006SSRv..125...25S Altcode: 2006SSRv..tmp...97S
The global variability of the Sun of relevance for planetary climates
has been directly measured for the past few decades. For longer
stretches of time models are required. Semi-empirical models can now
accurately reproduce the measured records of solar total and spectral
irradiance, as well as of the magnetic flux. They can also provide
reconstructions of these quantities on longer time scales. Here a
summary is given of some of the modelling efforts and of the results
achieved so far.

---------------------------------------------------------
Title: Reconstruction of solar UV irradiance in cycle 23
Authors: Krivova, N. A.; Solanki, S. K.; Floyd, L.
2006A&A...452..631K Altcode:
Solar irradiance variations show a strong wavelength dependence. Whereas
the total solar irradiance varies by about 0.1% during the course
of the solar cycle, variations at the wavelengths around the Ly-α
emission line near 121.6 nm range up to 50-100%. These variations may
have a significant impact on the Earth's climate system. Being almost
completely absorbed in the upper atmosphere, solar UV radiation below
300 nm affects stratospheric chemistry and controls production and
destruction of ozone. Models of the solar UV irradiance remain far from
perfect, even though considerable progress has been made in modelling
the irradiance variations longwards of about 200-300 nm. We show that
after correcting for the exposure dependent degradation of the SUSIM
channels sampling irradiance at λ &gt;240 nm (making use of the Mg
II core-to-wing ratio) the agreement between model and measurement is
significantly improved. At shorter wavelengths the LTE approximation
usually made in such models fails, which makes a reconstruction of the
solar UV irradiance a rather intricate problem. We choose an alternative
approach and use the observed SUSIM UV spectra to extrapolate available
models to shorter wavelengths. The model reproduces observed solar
cycle variations of the irradiance at wavelengths down to 115 nm and
indicates an important role of UV irradiance variability: up to 60%
of the total irradiance variations over the solar cycle might be
produced at wavelengths below 400 nm.

---------------------------------------------------------
Title: Can surface magnetic fields reproduce solar irradiance
variations in cycles 22 and 23?
Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.
2005A&A...432.1057W Altcode:
Total solar irradiance is reconstructed using data from the
spectromagnetograph on Kitt Peak for 1992 2003 (covering parts of cycles
22 and 23) and compared with observational data. The model assumes
that solar irradiance variations are caused by surface magnetism and
its single free parameter is kept at the same value for both cycles. A
good correspondence is found for the whole period of time, with no bias
between the two cycles on time-scales longer than the solar rotation
period. This is in agreement with the recent results of [CITE], but it
is in contrast to the earlier analysis of [CITE], and suggests that the
source of the irradiance variations is the same for cycles 22 and 23,
namely the evolution of the magnetic flux at the solar surface.

---------------------------------------------------------
Title: Reconstruction of the solar UV irradiance back to 1974
Authors: Haberreiter, M.; Krivova, N. A.; Schmutz, W.; Wenzler, T.
2005AdSpR..35..365H Altcode:
The variability of the solar UV irradiance has strong effects on
the terrestrial atmosphere. In order to study the solar influence
for times when no UV observations are available, it is necessary to
reconstruct the variation of the UV irradiance with time on the basis
of proxies. We present reconstructions of the solar UV irradiance based
on the analysis of space-based and ground-based magnetograms of the
solar disk going back to 1974. With COde for Solar Irradiance (COSI)
we calculate solar intensity spectra for the quiet Sun and different
active regions and combine them according to their fractional area on
the solar disk, whereby their time-dependent contributions over the
solar cycle lead to a variability in radiation. COSI calculates the
continuum and line formation under conditions which are out of local
thermodynamic equilibrium (non-LTE). The applied temperature and
density structures include the chromosphere and transition region,
which is particularly important for the UV. The reconstructions are
compared with observations.

---------------------------------------------------------
Title: Modelling of irradiance variations through atmosphere models .
Authors: Krivova, N. A.; Solanki, S. K.
2005MmSAI..76..834K Altcode:
Regular space-based measurements of solar total and spectral irradiance
reveal its variations on time scales from minutes to decades. About
90% of these variations are reproduced by recent models assuming
that the evolution of the solar surface magnetic fields is their
main cause. Circumstantial evidence suggests that variations on yet
longer time scales, which are of special interest for climate studies,
are also possible. Once good understanding of the directly observed
variations has been gained, we can then attempt to extend the models
back in time. Here a brief overview of our recent efforts to reconstruct
solar total and spectral irradiance on time scales of days to centuries
is given.

---------------------------------------------------------
Title: A cross-calibrated sunspot areas time series since 1874
Authors: Balmaceda, L.; Solanki, S. K.; Krivova, N.
2005MmSAI..76..929B Altcode:
A complete and homogeneous historical record of sunspot areas is
a valuable proxy of solar variability, and is widely used, e.g.,
to understand the behaviour of total and spectral solar irradiance
at earlier times. Since 1874, the Royal Greenwich Observatory (RGO)
regularly carried out these and other measurements until December
1976. After that time the records from a number of different
observatories are available. These, however, show some systematic
differences and often have a lot of gaps. In order to compile a
complete and cross-calibrated time series we compare the data from
different observatories when they overlap and find the corresponding
correction factors. The Greenwich data set is used as a basis until
1976, the Russian data (stations from the former USSR) between 1977
and 1985 and the Mt. Wilson data since 1986. Other data sets (Rome,
Yunnan, Catania) are used to fill in the remaining gaps.

---------------------------------------------------------
Title: Reconstruction of solar UV irradiance
Authors: Krivova, N. A.; Solanki, S. K.
2005AdSpR..35..361K Altcode:
Understanding solar influence on the Earth’s climate requires
a reconstruction of solar irradiance for the pre-satellite
period. Considerable advances have been made in modelling the irradiance
variations at wavelengths longer than 200 nm. At shorter wavelengths,
however, the LTE approximation usually taken in such models fails, which
makes a reconstruction of the solar UV irradiance a rather intricate
problem. We choose an alternative approach and use the observed SUSIM
UV spectra to extrapolate available models to shorter wavelengths.

---------------------------------------------------------
Title: Irradiance models
Authors: Solanki, S. K.; Krivova, N. A.; Wenzler, T.
2005AdSpR..35..376S Altcode:
Measurements of solar irradiance have revealed variations at all the
sampled time scales (ranging from minutes to the length of the solar
cycle). One important task of models is to identify the causes of the
observed (total and spectral) irradiance variations. Another major
aim is to reconstruct irradiance over time scales longer than sampled
by direct measurements in order to consider if and to what extent
solar irradiance variations may be responsible for global climate
change. Here, we describe recent efforts to model solar irradiance
over the current and the previous two solar cycles. These irradiance
models are remarkably successful in reproducing the observed total and
spectral irradiance, although further improvements are still possible.

---------------------------------------------------------
Title: Comparison between KPVT/SPM and SoHO/MDI magnetograms with
an application to solar irradiance reconstructions
Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fluri, D. M.
2004A&A...427.1031W Altcode:
To be able to use both space- and ground-based solar magnetograms and
construct long time series of derived parameters it is important to
cross-calibrate them so that we can estimate their reliability and
combine them. Using two different techniques, we compare magnetograms
as well as continuum images recorded by the Spectropolarimeter (SPM)
on Kitt Peak and the Michelson Doppler Interferometer (MDI) on board
SoHO. We find that the result obtained depends on the method used. The
method we favour gives almost identical umbral and penumbral areas and
very similar total magnetic fluxes in faculae. The magnetic fluxes in
umbrae and penumbrae returned by the two instruments, however, differ
considerably. We also demonstrate that SPM data can be employed to
reconstruct total solar irradiance variations with almost the same
accuracy as recently shown for MDI data.

---------------------------------------------------------
Title: Solar Irradiance Variations: From Current Measurements to
Long-Term Estimates
Authors: Solanki, Sami K.; Krivova, Natalie A.
2004SoPh..224..197S Altcode: 2005SoPh..224..197S
Variations of solar total and spectral irradiance are prime
solar quantities purported to have an influence on the Earth's
climate. Quantitative estimates of irradiance over as long a time
as possible are needed to judge their effectiveness in forcing the
climate. In order to do this reliably, first the measured record must
be reproduced and a feeling for the physics underlying the irradiance
variations must be developed. With the help of this knowledge combined
with the available proxy data, reconstructions of irradiance in the
past, generally since the Maunder minimum, are attempted. Here a brief
introduction to some of the irradiance reconstruction work aiming at
irradiance on time scales of days to the solar cycle is given, followed
by a brief and incomplete overview of the longer-term reconstructions.

---------------------------------------------------------
Title: Effect of spatial resolution on estimating the Sun's magnetic
flux
Authors: Krivova, N. A.; Solanki, S. K.
2004A&A...417.1125K Altcode:
A critical question related to a possible secular trend in the Sun's
total magnetic flux and consequently in solar irradiance is the total
amount of magnetic flux present on the Sun and how it is distributed
between active regions and the quiet Sun. NSO/Kitt Peak synoptic
charts have in the past been used to estimate the total flux and the
fraction of the flux in active regions and in the quiet Sun. Since a
single pixel of these synoptic charts is much bigger than individual
small-scale magnetic elements and opposite polarities may be present
within the same pixel, some magnetic flux escaped notice. Here we
estimate the fraction of the magnetic flux escaping detection in Kitt
Peak synoptic charts. By artificially reducing the spatial resolution of
MDI full-disc and high-resolution magnetograms we study the influence
of the resolution on the measured total magnetic flux. Noise in the
data poses the main difficulty to this approach and is carefully
studied. It is concluded that at least half of the magnetic flux in
the quiet Sun remains undetected in Kitt Peak synoptic charts and that
the total flux present on the solar surface at maxima of activity is
around twice the flux present at activity minima.

---------------------------------------------------------
Title: Towards understanding the β Pictoris dust stream
Authors: Krivov, A. V.; Krivova, N. A.; Solanki, S. K.; Titov, V. B.
2004A&A...417..341K Altcode:
The recent radar detection by \citet{baggaley-2000} of a collimated
stream of interstellar meteoroids postulated to be sourced at β
Pictoris, a nearby star with a prominent dust disk, presents a challenge
to theoreticians. Two mechanisms of possible dust ejection from β
Pic have been proposed: ejection of dust by radiation pressure from
comets in eccentric orbits and by gravity of a hypothetical planet
in the disk. Here we re-examine observational data and reconsider
theoretical scenarios, substantiating them with detailed modeling
to test whether they can explain quantitatively and simultaneously
the masses, speeds, and fluxes. Our analysis of the stream geometry
and kinematics confirms that β Pic is the most likely source of the
stream and suggests that an intensive dust ejection phase took place
∼0.7 Myr ago. Our dynamical simulations show that high ejection
speeds retrieved from the observations can be explained by both
planetary ejection and radiation pressure mechanisms, providing,
however, several important constraints. In the planetary ejection
scenario, only a “hot Jupiter”-type planet with a semimajor axis
of less than 1 AU can be responsible for the stream, and only if the
disk was dynamically “heated” by a more distant massive planet. The
radiation pressure scenario also requires the presence of a relatively
massive planet at several AU or more, that had heated the cometesimal
disk before the ejection occurred. Finally, the dust flux measured at
Earth can be brought into reasonable agreement with both scenarios,
provided that β Pic's protoplanetary disk recently passed through an
intensive short-lasting (∼0.1 Myr) clearance stage by nascent giant
planets, similar to what took place in the early solar system.

---------------------------------------------------------
Title: Reconstruction of solar UV irradiance
Authors: Krivova, N. A.; Solanki, S. K.
2004cosp...35..504K Altcode: 2004cosp.meet..504K
Understanding solar influence on the Earth's climate requires
a reconstruction of solar irradiance for the pre-satellite
period. Considerable advances have been made in modelling the irradiance
variations at wavelengths longer than 300 nm. At shorter wavelengths,
however, the LTE approximation usually taken in such models fails, which
makes a reconstruction of the solar UV irradiance a rather intricate
problem. We choose an alternative approach and use the observed SUSIM
UV spectra to extrapolate available models to shorter wavelengths.

---------------------------------------------------------
Title: Understanding Solar Variability as a Groundwork for Planet
Transit Detection
Authors: Seleznyov, A. D.; Krivova, N. A.; Solanki, S. K.
2004IAUS..219..815S Altcode: 2003IAUS..219E...7K
Detection of planetary transits holds the greatest promise for
the search of terrestrial planets. However intrinsic stellar
variability can mask real transits or lead to 'false' planet transit
detections. Understanding the origin of stellar variability can
help to estimate the minimum sizes of planets detectable with this
technique around different types of stars and to identify the best
wavelength range for such measurements. The only star for which data
with sufficient photometric accuracy and temporal sampling exist is
the Sun. We analyze and model solar variability on timescales relevant
for planetary transits (hours to several days) using a variety of
components such as granulation network (supergranulation) faculae
and sunspots. This study extends our successful work to model solar
irradiance variations on days to years timescales (Fligge et al. 2000;
Krivova et al. 2003).

---------------------------------------------------------
Title: Reconstruction of solar UV irradiance back to 1974
Authors: Haberreiter, M.; Krivova, N. A.; Schmutz, W.; Wenzler, T.
2004cosp...35.2707H Altcode: 2004cosp.meet.2707H
The variability of the solar UV irradiance has strong effects on
the terrestrial atmosphere. In order to study the solar influence
for times when no UV observations are available, it is necessary to
reconstruct the variation of the UV irradiance with time on the basis
of proxies. We present reconstructions of the solar UV irradiance based
on the analysis of space-based and ground-based magnetograms of the
solar disk going back to 1974. With COSI (COde for Solar Irradiance)
we calculate solar intensity spectra for the quiet Sun and different
active regions and combine them according to their fractional area on
the solar disk, whereby their time-dependent contributions over the
solar cycle lead to a variability in radiation. COSI calculates the
continuum and line formation in non-local thermodynamic equilibrium
(non-LTE). The applied temperature and density structures include the
chromosphere and transition region, which is particularly important
for the UV. The reconstructions are compared with observations.

---------------------------------------------------------
Title: Solar variability and global warming: a statistical comparison
since 1850
Authors: Krivova, N. A.; Solanki, S. K.
2004AdSpR..34..361K Altcode:
The magnitude of the Sun's influence on climate has been a subject
of intense debate. Estimates of this magnitude are generally based on
assumptions regarding the forcing due to solar irradiance variations
entering climate modelling. Given the complexity of the climate system,
however, such modelling is perforce based on simplifying assumptions,
which leaves it open to criticism. We take a complementary approach. We
assume that the Sun has been responsible for climate change prior to
1970 and that their interrelation remained unchanged afterwards. Then,
employing reconstructions and measured records of relevant solar
quantities as well as of the cosmic-ray flux, we estimate statistically
which fraction of the dramatic temperature rise after that date could
be due to the influence of the Sun. We show that at least in the most
recent past (since about 1970) the solar influence on climate cannot
have been significant.

---------------------------------------------------------
Title: On the origin of solar variability, with an application to
the search for extrasolar planets
Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A.
2003ESASP.539..589S Altcode: 2003toed.conf..589S
Detection of planetary transits holds great promise for the search of
terrestrial planets. However, most stars are variable at the level of
the signal produced by the transit of an Earth-like planet. Hence,
intrinsic stellar variability can lead to "false" planet transit
detections. An understanding of the origin of the stellar variability
is needed to ensure reliable transit detections. We consider the
Sun as the closest and best studied star and analyze its variability
on timescales relevant to the transit effect, namely from an hour to
several days. Total and spectral solar irradiance measurements obtained
by the VIRGO instrument on board the SOHO spacecraft have been analyzed
by applying Fourier and wavelet techniques. Preliminary results suggest
that at the time scales of interest solar variability is driven partly
by solar magnetic activity, which dominates at longer time scales,
and convection, in particular solar granulation, which dominates at
shorter time scales. As part of a more quantitative analysis a simple
numerical model of the irradiance variations due to granulation has
been constructed. Irradiance variability of stars with different surface
gravity was calculated in the frequency band of relevance to transits.

---------------------------------------------------------
Title: Solar total and spectral irradiance: modelling and a possible
impact on climate
Authors: Krivova, N. A.; Solanki, S. K.
2003ESASP.535..275K Altcode: 2003iscs.symp..275K
There is growing evidence that solar variability influences the
Earth's climate, although the underlying mechanism is not yet
understood. Variations in the solar total and spectral irradiance
often play a central role within various processes that have been
suggested. Whereas changes in the total irradiance can affect the
overall energy balance of the Earth's atmosphere, variations in its
spectral distribution, in particular in the UV, have a pronounced effect
on the chemistry of the Earth's upper atmosphere. Measurements of the
solar total irradiance are only available since 1978 and the spectral
irradiance record is even shorter. This calls for a reconstruction of
irradiance variations at earlier times with the help of models. We first
outline our current understanding of the main mechanism responsible
for irradiance variations and describe the efforts to reconstruct
them. The reconstructed total and UV irradiance is then employed to
estimate the solar contribution to global warming, with particular
emphasis to the period since 1970.

---------------------------------------------------------
Title: Solar irradiance fluctuations on short timescales
Authors: Solanki, S. K.; Seleznyov, A. D.; Krivova, N. A.
2003ESASP.535..285S Altcode: 2003iscs.symp..285S
Although solar irradiance variability at time-scales of days to the
solar cycle has been well studied, comparatively little is known about
the causes of such variations on shorter time-scales. We present an
analysis that aims to distinguish between magnetic and convective
causes. It suggests that on time-scales longer than 1-2 days magnetic
structures are the dominant source while for time-scales shorter than
a few hours convection appears to dominate. We also present a simple
granulation model that includes the various paths of granule birth
and death and compare its output with VIRGO data.

---------------------------------------------------------
Title: Can solar variability explain global warming since 1970?
Authors: Solanki, S. K.; Krivova, N. A.
2003JGRA..108.1200S Altcode:
The magnitude of the Sun's influence on climate has been a subject
of intense debate. Estimates of this magnitude are generally based on
assumptions regarding the forcing due to solar irradiance variations
and climate modeling. This approach suffers from uncertainties that
are difficult to estimate. Such uncertainties are introduced because
the employed models may not include important but complex processes
or mechanisms or may treat these in too simplified a manner. Here
we take a more empirical approach. We employ time series of the most
relevant solar quantities, the total and UV irradiance between 1856
and 1999 and the cosmic rays flux between 1868 and 1999. The time
series are constructed using direct measurements wherever possible and
reconstructions based on models and proxies at earlier times. These
time series are compared with the climate record for the period 1856 to
1970. The solar records are scaled such that statistically the solar
contribution to climate is as large as possible in this period. Under
this assumption we repeat the comparison but now including the period
1970-1999. This comparison shows without requiring any recourse to
modeling that since roughly 1970 the solar influence on climate (through
the channels considered here) cannot have been dominant. In particular,
the Sun cannot have contributed more than 30% to the steep temperature
increase that has taken place since then, irrespective of which of the
three considered channels is the dominant one determining Sun-climate
interactions: tropospheric heating caused by changes in total solar
irradiance, stratospheric chemistry influenced by changes in the solar
UV spectrum, or cloud coverage affected by the cosmic ray flux.

---------------------------------------------------------
Title: A stream of particles from the begin {equation}ta Pictoris
disc: A possible ejection mechanism
Authors: Krivova, N. A.; Solanki, S. K.
2003A&A...402L...5K Altcode:
Recently, a stream of particles originating from the direction of begin
{equation}ta Pictoris, a young main sequence star surrounded by a dust
disc, has been reported (Baggaley \cite{baggaley-2000}). Standard
mechanisms of particle ejection from a disc fail to reproduce
the properties of this stream. We find that scattering by a giant
proto-planet with properties taken from the literature is consistent
with the observations. The fact that a straightforward ejection
mechanism reproduces the data supports the identification of the
particle stream's source with beta Pic. Our work also indicates that
protoplanetary dust discs form a potentially rich source of large
interstellar grains, as widely detected in the Solar System.

---------------------------------------------------------
Title: Ejection of Dust from Planetary Systems into Interstellar Space
Authors: Krivov, A.; Krivova, N.; Solanki, S.; Titov, V.
2003EAEJA.....4264K Altcode:
Extensive observational and theoretical research of the last two decades
have made it clear that young exoplanetary systems comprise not only
planets themselves, but also disks of small bodies and of dust-sized
material these bodies replenish. In much the same way as in the young
Solar System, interaction of the solids in a circumstellar disk with
planets at late stages of planet formation should hang the material on
the outskirts of the system, creating exosolar Kuiper belts and Oort
clouds. A portion of the material is ejected by the exoplanets from the
disks into interstellar space, which may represent an important source
of large interstellar dust grains in the Galaxy. Here we study how
the efficiency of the ejection mechanism and the involved timescales
depend on the mass and orbital parameters of the planet, as well
as on the `unperturbed' spatial distribution of disk particles. We
also derive the distribution of the final velocities of the ejected
material. The study is done semianalytically with the aid of the
Oepik-Weidenschilling statistical theory of planetary encounters and
numerically with direct Monte-Carlo orbital simulations. The results
may have particular implications to the beta Pictoris system: they
lend further support to the idea (Krivova and Solanki 2002) that a
collimated stream of interstellar radiometeors detected by Baggaley
(2000) originates at beta Pic and consists of grains ejected by a
presumed jovian planet orbiting this star.

---------------------------------------------------------
Title: Cycles and cyclicities of the Sun
Authors: Solanki, S. K.; Krivova, N. A.
2003ASPC..292..423S Altcode: 2003ipc..conf..423S
The solar activity cycle is discussed in the broader context of solar
variability. It is pointed out that the Sun exhibits periodic, cyclic,
chaotic and stochastic phenomena. The origins of solar variability
are mentioned and the importance of secular variations of the solar
cycle for a putative influence of the Sun on the Earth's climate is
briefly discussed.

---------------------------------------------------------
Title: Reconstruction of solar irradiance variations in cycle 23:
Is solar surface magnetism the cause?
Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M.; Unruh, Y. C.
2003A&A...399L...1K Altcode:
A model of solar irradiance variations is presented which is based
on the assumption that solar surface magnetism is responsible for all
total irradiance changes on time scales of days to years. A time series
of daily magnetograms and empirical models of the thermal structure
of magnetic features (sunspots, faculae) are combined to reconstruct
total (and spectral) irradiance from 1996 to 2002. Comparisons with
observational data reveal an excellent correspondence, although the
model only contains a single free parameter. This provides strong
support for the hypothesis that solar irradiance variations are caused
by changes in the amount and distribution of magnetic flux at the
solar surface.

---------------------------------------------------------
Title: Was one sunspot cycle in the 18th century really lost?
Authors: Krivova, N. A.; Solanki, S. K.; Beer, J.
2002A&A...396..235K Altcode:
The unusually long 4th solar cycle has recently been proposed by
Usoskin et al. (2001) to be composed of two cycles. They argue that
a weak and short cycle might have been lost in sparse sunspot data at
the end of the 18th century. Here we check this hypothesis in different
ways. First, we consider the sunspot number record in greater detail
and compare in a statistical sense the sunspot observations of the
period in question with those at other times. In a statistical sense the
sunspot numbers recorded at the time of the proposed new cycle minimum
are extremely untypical for other minima in the solar cycle record,
but quite usual for the declining phase of the solar cycle. We also
analyse other available proxies of solar activity, such as variations
of the cosmogenic nuclides 10Be and 14C as well
as auroral activity. These historical records are sufficiently long
and provide an independent testimony of the cyclic behaviour of solar
activity at the end of the 18th century. We found no evidence for a lost
cycle in any of these data sets. Finally, we compare the proposed new
cycle with the other cycles in the sunspot record. This reveals that
the proposed “missing” cycle has very unusual properties, much more
so than the original, standard cycle 4. Taken together, the evidence
from these various tests strongly suggests that no cycle was missed and
that the official sunspot cycle numbering and parameters are correct.

---------------------------------------------------------
Title: Search for a relationship between solar cycle amplitude
and length
Authors: Solanki, S. K.; Krivova, N. A.; Schüssler, M.; Fligge, M.
2002A&A...396.1029S Altcode:
The cross-correlation between time series of solar cycle length
and amplitude suggests that the length precedes the amplitude. The
relationship between the two is found to be more complex than a simple
lag or phase shift, however. A simple empirical model is constructed
which allows the amplitude of a given cycle to be predicted with
relatively high accuracy from the lengths of earlier cycles. This
result not only adds to the means at our disposal for predicting the
amplitudes of future cycles, but also implies that the solar dynamo
carries a memory of the length of one cycle over into the next. It may
also have a bearing on why solar cycle length correlates better with
the Earth's temperature record than cycle amplitude (Friis-Christensen
&amp; Lassen \cite{Friis-Christensen:Lassen:1991}). Thoughts on possible
physical causes are presented.

---------------------------------------------------------
Title: The 1.3-year and 156-day periodicities in sunspot data:
Wavelet analysis suggests a common origin
Authors: Krivova, N. A.; Solanki, S. K.
2002A&A...394..701K Altcode:
Helioseismic data have revealed a 1.3-year periodicity in the solar
rotation rate near the bottom of the solar convection zone. In order
to test whether these rotation rate variations have a significant
impact on the solar dynamo, we search for such a periodicity in
tracers of relatively freshly emerged flux at the solar surface,
namely sunspots. Sunspot areas and sunspot number time series are
studied with the help of the wavelet transform. Significant power
at this period (1.28 years) is indeed found and is observed to vary
strongly with time. This provides independent support for the presence
of a 1.3 year periodicity in solar data. The power at the 154-158-day
Rieger period of solar flares is seen to vary approximately in phase
with the 1.28-year period. Based on this we propose that the Rieger
period is the third harmonic (3x 156 days= 1.28 years) of the 1.3-year
period. If the rotation rate of the Sun does vary with 1.3 years then
the enhanced flaring with the Rieger period may finally be driven by
the 1.3 year periodicity. However, the power in both periods is also
found to approximately follow the total number of sunspots. Therefore
we cannot rule out that the 1.3-year and 156-day periods are harmonics
of the solar activity cycle. Finally, our analysis of a calibrated
sunspot area record reveals that the 156-day period continues into
the most recent cycles, in contrast to earlier results.

---------------------------------------------------------
Title: How large is the Sun's total magnetic flux?
Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M.
2002ESASP.505..461K Altcode: 2002solm.conf..461K; 2002IAUCo.188..461K
Variations in the radiative output of the Sun are directly allied
to changes in the amount and distribution of solar surface magnetic
field. The variability of irradiance on time-scales significantly
shorter than a solar cycle can be produced by the spatial and temporal
evolution of active regions, whereas its secular variations ensue
from changes of the quiet-Sun magnetic flux on longer time-scales. The
relative amounts of the magnetic flux in active regions and in the quiet
Sun as well as their cyclic evolution have been studied by Harvey (1994)
on the basis of the NSO/Kitt Peak synoptic maps. Since a single pixel of
such a map is much bigger than individual small-scale magnetic elements
and opposite polarities may be present within the same pixel, some
magnetic flux went uncounted, so that the flux obtained by Harvey (1994)
represents a lower limit to the true amount of magnetic flux present
on the Sun. We use MDI full-disc and high-resolution magnetograms to
estimate the total magnetic flux of the Sun.

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Title: Total solar magnetic flux: dependence on spatial resolution
of magnetograms
Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M.
2002ESASP.508..155K Altcode: 2002soho...11..155K
The variability of the solar radiative output is tied to the evolution
of the surface magnetic field. Irradiance changes on time-scales of the
solar rotation are governed by the varying distribution of the magnetic
features and the evolution of sunspots and individual active regions,
whereas the total amount of magnetic flux in particular small-scale
magnetic elements seems to dominate the irradiance variations on the
time scale of the solar cycle. Possibly of even greater relevance for
climate on Earth are secular variations of solar irradiance, which
are at least partly caused by changes in the quiet-Sun magnetic flux
on long time scales. Using NSO/Kitt Peak synoptic charts Harvey (1994)
found that the total magnetic flux in active regions at activity maximum
is about 3 times higher than the flux at activity minimum. The size of
small-scale magnetic elements is, however, far below the resolution of
currently obtainable magnetograms. At the same time, their distribution
on the solar surface is highly non-uniform, with magnetic elements of
opposite polarities often being grouped close together. This leads to
an apparent cancellation of the flux within a relatively large pixel
of a Kitt Peak synoptic chart and underestimates the total magnetic
flux, mainly in the quiet Sun. Using MDI full-disc and high-resolution
magnetograms and artificially reducing their spatial resolution by
binning several pixels together we study the influence of the resolution
on the measured total magnetic flux.

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Title: Solar variability and global warming
Authors: Krivova, N.; Solanki, S.
2002cosp...34E.274K Altcode: 2002cosp.meetE.274K
The magnitude of the Sun's influence on climate has been a subject
of intense debate. Estimates of this magnitude are generally based on
assumptions regarding the forcing due to solar irradiance variations
entering climate modelling. Given the complexity of the climate system,
however, such modelling is perforce based on simplifying assumptions,
which leaves it open to criticism. We take a complementary approach. We
assume that the Sun has been responsible for climate change prior to
1970. Then, using reconstructions and measured records of relevant
solar quantities as well as of the cosmic-ray flux, we estimate which
fraction of the dramatic temperature rise after that date could be
due to the influence of the Sun. We show that at least in the most
recent past (since 1970) the solar influence on climate cannot have
been significant.

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Title: Dust in young solar systems (Invited)
Authors: Krivova, N. A.
2002dsso.conf..201K Altcode: 2002IAUCo.181..201K
No abstract at ADS

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Title: Irradiance Medels based on Magnetic Activity
Authors: Solanki, S.; Krivova, N.; Unruh, Y.
2002cosp...34E1357S Altcode: 2002cosp.meetE1357S
An introduction is given to the modelling of irradiance variations
based on solar surface magnetism. We describe how a time series of daily
magnetograms and empirical models of the thermal structure of magnetic
features (sunspots, magnetic elements) are combined to reconstruct total
and spectral irradiance on a time scale of weeks to years. Comparisons
with observational data reveal an excellent correspondence. On a
longer time scale of decades to centuries we first need to know how
the Sun's magnetic field evolved. Hence reconstructions of the Sun's
total magnetic flux are briefly introduced, before we present longer
term reconstructions of the total and spectral irradiance since the
Maunder minimum.

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Title: Size distributions of dust in circumstellar debris discs
Authors: Krivov, A. V.; Mann, I.; Krivova, N. A.
2000A&A...362.1127K Altcode:
The size distribution of particles in a dust disc is determined by,
and holds the key to, sources, sinks and dynamics of grains. Here we
derive the size distribution in circumstellar debris discs, exemplified
by the disc of begin {equation}ta Pictoris, by modelling the dynamical
evolution of the circumstellar dust, dominated by collisions. The whole
disc is considered as consisting of two dust populations: larger grains
moving in bound orbits (alpha -meteoroids) and smaller ones blown away
by radiation pressure (begin {equation}ta-meteoroids). Although begin
{equation}ta-meteoroids leave the disc in hyperbolic trajectories,
they are continuously replenished by collisions, so that at any time
the disc contains a substantial population of small particles. As a
consequence, the fragmentation of alpha -meteoroids not only by mutual
collisions, but also by impacts of begin {equation}ta-meteoroids becomes
significant. This flattens the distribution of alpha -meteoroids
in the size regime adjacent to the blow-out limit and shifts the
cross section-dominating sizes from a few micrometres to ga 10 mum
. The overall distribution shows essentially three different slopes:
steeper ones for both begin {equation}ta-meteoroids and large alpha
-meteoroids and a gentler one for alpha -meteoroids with sizes just
above the blow-out limit. This resembles the size distribution of
interplanetary dust particles in the Solar system which, however,
is shaped by different mechanisms. The basic features of the modelled
size distribution (the presence of a substantial population of small
hyperbolic particles in the disc, the dominance of grains ~ 10 mum
in size) well agree with the observational data available. Although
particular calculations were made for the begin {equation}ta Pic disc,
our basic qualitative conclusions directly apply to the debris discs
around other <ASTROBJ>Vega</ASTROBJ>-type stars with low gas contents
and similar or somewhat lower optical depths.

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Title: Porous Dust Grains in the Shells of Herbig Ae/Be Stars
Authors: Il'In, V. B.; Krivova, N. A.
2000AstL...26..379I Altcode:
The transfer of polarized radiation in inhomogeneous circumstellar
shells with a spheroidal spatial distribution of porous dust particles
is computed. The grains are modeled by an MRN mixture of silicate
and graphite particles. The optical properties of porous particles
(considered separately in the Appendix) are computed by using
effective medium theory and Mie theory. The following observational
characteristics have been computed for WW Vul, a typical Herbig Ae
star with Algol-like minima: the spectral energy distribution from
the ultraviolet to the far infrared, the color-magnitude diagrams,
the wavelength dependence of linear polarization, and the shell
brightness distribution. The effect of grain porosity on the results
is considered. It has been found that only moderate particle porosity
(the volume fraction of matter is f ~ 0.5) can explain available
observational data in terms of the approach used. Since radiation
pressure must rapidly sweep submicron-sized grains out of the vicinity
of Herbig Ae/Be stars, we briefly discuss how particle porosity can
affect this process.

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Title: Size Distribution of Dust in the Disk of β Pictoris
Authors: Krivova, N. A.; Krivov, A. V.; Mann, I.
2000ASPC..219..387K Altcode: 2000dpp..conf..387K
No abstract at ADS

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Title: WWW database of optical constants for astronomy
Authors: Henning, Th.; Il'In, V. B.; Krivova, N. A.; Michel, B.;
Voshchinnikov, N. V.
1999A&AS..136..405H Altcode:
The database we announce contains references to the papers,
data files and links to the Internet resources related to
measurements and calculations of the optical constants of the
materials of astronomical interest: different silicates, ices,
oxides, sulfides, carbides, carbonaceous species from amorphous
carbon to graphite and diamonds, etc. We describe the general
structure and content of the database which has now free access via
Internet: http://www.astro.spbu.ru/JPDOC/entry.html\ or \ http://
www. astro.uni-jena.de/Users/database/entry.html

---------------------------------------------------------
Title: Polarization of the Zodiacal cloud and other Solar system -
type debris disks
Authors: Krivova, N.; Mann, I.; Krivov, A.
1999md98.conf..291K Altcode:
No abstract at ADS

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Title: On the Behaviour of Interstellar CO and CN Molecules in
Individual Clouds
Authors: Sikorski, J.; Krelowski, J.; Gnacinski, P.; Kaczmarczyk,
G.; Snow, T. P.; Krivova, N.
1998ESASP.413..505S Altcode: 1998uabi.conf..505S
No abstract at ADS

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Title: Dust shells around Herbig Ae/Be stars with algol-like minima:
Modeling of photometric observations
Authors: Krivova, N. A.; Il'In, V. B.
1997AstL...23..791K Altcode: 1997PAZh...23..908K
No abstract at ADS

---------------------------------------------------------
Title: Herbig Ae stars with Algol-like minima: Modeling of the
spectral energy distribution and the behavior of colors at minima
Authors: Krivova, N. A.
1997AstL...23..327K Altcode: 1997PAZh...23..371K
No abstract at ADS

---------------------------------------------------------
Title: Dust shells around Herbig Ae/Be stars with Algol-like minima:
results of a combined modelling of different observational data
Authors: Krivova, N. A.; Il'In, V. B.; Kimura, H.
1997IAUJD..13E...8K Altcode:
Herbig Ae/Be stars (HAeBe), young stars surrounded by dust shells, are
believed to be precursors of beta Pic-like stars, and the dust around
them is thought to be a possible source material for the formation of
planets. A group of HAeBe stars (UX Ori-like) shows large brightness
variations. The dust surrounding these stars is responsible for their
excess emission in the infrared, anomalous extinction at the ultraviolet
and visible wavelengths, "blueing" effect in colour-magnitude diagrams,
and intrinsic polarization increasing in deep minima. An investigation
of all the effects mentioned can provide a good insight into the
properties of the circumstellar dust. As distinct from previous studies
of the dust shells, we take an advantage of a simultaneous modelling
of different kinds of the observational data that give us additional
constraints on the model parameters. We use spherical and spheroidal
models of the shells and different dust grain models. We also consider
some effects related with the possible porosity of the grains. The
data available for several UX Ori-like stars are interpreted, and the
obtained parameters of the shells are compared with the results of
earlier studies.

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Title: Dust around Herbig Ae stars: additional constraints from
their photometric and polarimetric variability.
Authors: Krivova, N. A.; Il'in, V. B.; Fischer, O.
1996NASCP3343...37K Altcode: 1996NASCP3339...37K; 1996ASPC..122...37K
Using a Monte-Carlo code for polarized radiation transfer the
authors compared the results obtained for different models with the
observational data available. They found that the photometric and
polarimeteric behaviour of the stars provided essential additional
constraints on the circumstellar dust models.

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Title: Expected Features of Martian Dust Belts
Authors: Krivov, A. V.; Krivova, N. A.
1996pacm.conf..235K Altcode:
No abstract at ADS

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Title: Modelling of the IR Intensity Maps for HAEBE Stars with
Algol-Like Minima
Authors: Il'in, V.; Krivova, N.; Men'shchikov, A.
1996rdfs.conf..183I Altcode:
No abstract at ADS

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Title: Multiwavelength Study of HAEBE Stars with Algol-Like Minima
Authors: Krivova, N.; Il'in, V.
1996rdfs.conf..187K Altcode:
No abstract at ADS