yeo

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Author name code: yeo
ADS astronomy entries on 2022-09-14
author:Yeo, Kok Leng

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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: 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: 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 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.

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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.

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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.

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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.

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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: 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.

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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.

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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.

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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).

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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.

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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.

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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.

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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>

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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.

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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.

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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.

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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

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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.

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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: Analysis and modeling of solar irradiance variations
Authors: Yeo, K. L.
2014arXiv1412.3935Y Altcode:
A prominent manifestation of the solar dynamo is the 11-year activity
cycle, evident in indicators of solar activity, including solar
irradiance. Although a relationship between solar activity and the
brightness of the Sun had long been suspected, it was only directly
observed after regular satellite measurements became available with the
launch of Nimbus-7 in 1978. The measurement of solar irradiance from
space is accompanied by the development of models aimed at describing
the apparent variability by the intensity excess/deficit effected by
magnetic structures in the photosphere. The more sophisticated models,
termed semi-empirical, rely on the intensity spectra of photospheric
magnetic structures generated with radiative transfer codes from
semi-empirical model atmospheres. An established example of such
models is SATIRE-S (Spectral And Total Irradiance REconstruction
for the Satellite era). One key limitation of current semi-empirical
models is the fact that the radiant properties of network and faculae
are not adequately represented due to the use of plane-parallel model
atmospheres (as opposed to three-dimensional model atmospheres). This
thesis is the compilation of four publications, detailing the results
of investigations aimed at setting the groundwork necessary for the
eventual introduction of three-dimensional atmospheres into SATIRE-S
and a review of the current state of the measurement and modelling of
solar irradiance. Also presented is an update of the SATIRE-S model. We
generated a daily reconstruction of total and spectral solar irradiance,
covering 1974 to the present, that is more reliable and, in most cases,
extended than similar reconstructions from contemporary models.

---------------------------------------------------------
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: Analysis and modeling of solar irradiance variations
Authors: Yeo, Kok Leng
2014PhDT........92Y Altcode:
No abstract at ADS

---------------------------------------------------------
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.

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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.

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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.

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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: 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.