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Author name code: faurobert
ADS astronomy entries on 2022-09-14
author:"Faurobert, Marianne"
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Title: Solar-cycle variations of internetwork magnetic fields
Authors: Faurobert, M.
2022fysr.confE..40F Altcode:
Small-scale magnetic fields in the quiet Sun contain in total more
flux than active regions and represent an important reservoir of
magnetic energy. But the origin and evolution of these fields still
remain largely unknown. We present a study of the solar-cycle and
center-to-limb variations of the magnetic-flux structures at small
scales in the solar internetwork. We used Hinode SOT/Spectropolarimetric
data from the irradiance program from 2008 to 2016 and applied
a deconvolution to the Stokes profiles to correct them from the
smearing due the Point Spread Function of the telescope. Then we
performed a Fourier spectral analysis of the spatial fluctuations of
the magnetic-flux density in 10"x10" internetwork regions spanning
a wide range of latitudes. At low and mid latitudes and away from
the active latitudes present at solar maximum, the power spectra
do not vary significantly with the solar cycle. At high latitudes
variations in opposition of phase with the solar cycle are observed
at granular scales. Whatever the latitude the power of the magnetic
fluctuations at scales smaller than 0.5" remain constant throughout
the solar cycle. These results are in favor of a small-scale dynamo
that operates in the internetwork.
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Title: HiRISE - High-Resolution Imaging and Spectroscopy Explorer
- Ultrahigh resolution, interferometric and external occulting
coronagraphic science
Authors: Erdélyi, Robertus; Damé, Luc; Fludra, Andrzej; Mathioudakis,
Mihalis; Amari, T.; Belucz, B.; Berrilli, F.; Bogachev, S.; Bolsée,
D.; Bothmer, V.; Brun, S.; Dewitte, S.; de Wit, T. Dudok; Faurobert,
M.; Gizon, L.; Gyenge, N.; Korsós, M. B.; Labrosse, N.; Matthews,
S.; Meftah, M.; Morgan, H.; Pallé, P.; Rochus, P.; Rozanov, E.;
Schmieder, B.; Tsinganos, K.; Verwichte, E.; Zharkov, S.; Zuccarello,
F.; Wimmer-Schweingruber, R.
2022ExA...tmp...21E Altcode:
Recent solar physics missions have shown the definite role of waves and
magnetic fields deep in the inner corona, at the chromosphere-corona
interface, where dramatic and physically dominant changes occur. HiRISE
(High Resolution Imaging and Spectroscopy Explorer), the ambitious new
generation ultra-high resolution, interferometric, and coronagraphic,
solar physics mission, proposed in response to the ESA Voyage 2050
Call, would address these issues and provide the best-ever and most
complete solar observatory, capable of ultra-high spatial, spectral,
and temporal resolution observations of the solar atmosphere, from the
photosphere to the corona, and of new insights of the solar interior
from the core to the photosphere. HiRISE, at the L1 Lagrangian
point, would provide meter class FUV imaging and spectro-imaging,
EUV and XUV imaging and spectroscopy, magnetic fields measurements,
and ambitious and comprehensive coronagraphy by a remote external
occulter (two satellites formation flying 375 m apart, with a
coronagraph on a chaser satellite). This major and state-of-the-art
payload would allow us to characterize temperatures, densities, and
velocities in the solar upper chromosphere, transition zone, and inner
corona with, in particular, 2D very high resolution multi-spectral
imaging-spectroscopy, and, direct coronal magnetic field measurement,
thus providing a unique set of tools to understand the structure and
onset of coronal heating. HiRISE's objectives are natural complements
to the Parker Solar Probe and Solar Orbiter-type missions. We present
the science case for HiRISE which will address: i) the fine structure
of the chromosphere-corona interface by 2D spectroscopy in FUV at
very high resolution; ii) coronal heating roots in the inner corona by
ambitious externally-occulted coronagraphy; iii) resolved and global
helioseismology thanks to continuity and stability of observing at the
L1 Lagrange point; and iv) solar variability and space climate with,
in addition, a global comprehensive view of UV variability.
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Title: Solar-cycle variations of internetwork magnetic fields
Authors: Faurobert, M.; Ricort, G.
2021sf2a.conf..218F Altcode:
Small-scale magnetic fields in the quiet Sun contain in total more
flux than active regions and represent an important reservoir of
magnetic energy. But the origin and evolution of these fields still
remain largely unknown. We present a study of the solar-cycle and
center-to-limb variations of the magnetic-flux structures at small
scales in the solar internetwork. We used Hinode SOT/Spectropolarimetric
data from the irradiance program from 2008 to 2016 and applied a
deconvolution to the intensity and polarization profiles to correct them
from the smearing due the Point Spread Function of the telescope. Then
we performed a Fourier spectral analysis of the spatial fluctuations of
the magnetic-flux density in 10”x10” internetwork regions spanning
a wide range of latitudes. At low and mid latitudes and away from
the active latitudes present at solar maximum, the power spectra
do not vary significantly with the solar cycle. At high latitudes
variations in opposition of phase with the solar cycle are observed
at granular scales. Whatever the latitude the power of the magnetic
fluctuations at scales smaller than 0.5” remain constant throughout
the solar cycle. These results are in favor of a small-scale dynamo
that operates in the internetwork.
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Title: New boundary conditions for the approximate flux-limited
diffusion radiative transfer in circumstellar environments. Test
case study for spherically symmetric envelopes
Authors: Perdigon, J.; Niccolini, G.; Faurobert, M.
2021A&A...653A.139P Altcode: 2021arXiv210713993P
Context. In order to constrain the models describing circumstellar
environments, it is necessary to solve the radiative transfer equation
in the presence of absorption and scattering, coupled with the equation
for radiative equilibrium. However, solving this problem requires much
CPU time, which makes the use of automatic minimisation procedures
to characterise these environments challenging. <BR /> Aims: In this
context, the use of approximate methods is of primary interest. One
promising candidate method is the flux-limited diffusion (FLD), which
recasts the radiative transfer problem into a non-linear diffusion
equation. One important aspect for the accuracy of the method lies in
the implementation of appropriate boundary conditions (BCs). We present
new BCs for the FLD approximation in circumstellar environments that
we apply here to spherically symmetric envelopes. <BR /> Methods: At
the inner boundary, the entering flux (coming from the star and from
the envelope itself) may be written in the FLD formalism and provides
us with an adequate BC. At the free outer boundary, we used the FLD
formalism to constrain the ratio of the mean radiation intensity over
the emerging flux. In both cases we derived non-linear mixed BCs
relating the surface values of the mean specific intensity and its
gradient. We implemented these conditions and compared the results
with previous benchmarks and the results of a Monte Carlo radiative
transfer code. A comparison with results derived from BCs that were
previously proposed in other contexts is presented as well. <BR />
Results: For all the tested cases, the average relative difference
with the benchmark results is below 2% for the temperature profile
and below 6% for the corresponding spectral energy distribution or the
emerging flux. We point out that the FLD method together with the new
outer BC also allows us to derive an approximation for the emerging
flux. This feature avoids additional formal solutions for the radiative
transfer equation in a set of rays (ray-tracing computations). <BR />
Conclusions: The FLD approximation together with the proposed new
BCs performs well and captures the main physical properties of the
radiative equilibrium in spherical circumstellar envelopes.
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Title: Magnetic flux structuring of the quiet Sun
internetwork. Center-to-limb analysis of solar-cycle variations
Authors: Faurobert, M.; Ricort, G.
2021A&A...651A..21F Altcode: 2021arXiv210508657F
Context. The small-scale magnetism of the quiet Sun has been
investigated by various means in recent decades. It is now well
established that the quiet Sun contains in total more magnetic flux
than active regions and represents an important reservoir of magnetic
energy. But the nature and evolution of these fields remain largely
unknown. <BR /> Aims: We investigate the solar-cycle and center-to-limb
variations of magnetic-flux structures at small scales in internetwork
regions of the quiet Sun. <BR /> Methods: We used Hinode SOT/SP
data from the irradiance program between 2008 and 2016. Maps of the
magnetic-flux density are derived from the center-of gravity method
applied to the circular polarization profiles in the FeI 630.15 nm
and FeI 630.25 nm lines. To correct the maps from the instrumental
smearing of the telescope, we applied a deconvolution method based
on a principal component analysis of the line profiles and on a
Richardson-Lucy deconvolution of their coefficients. We took defocus
effects and the diffraction of the SOT telescope into account. We
then performed a spectral analysis of the spatial fluctuations of the
magnetic-flux density in 10″ × 10″ internetwork regions spanning
a wide range of latitudes from ±70° to the equator. <BR /> Results:
At low and mid latitudes the power spectra normalized by the mean
value of the unsigned flux in the regions do not vary significantly
with the solar cycle. However at solar maximum for one scan in the
activity belt showing an enhanced network, a marginal increase in
the power of the magnetic fluctuations is observed at granular and
larger scales in the internetwork. At high latitudes, we observe
variations at granular and larger scales where the power decreases
at solar maximum. At all the latitudes the power of the magnetic
fluctuations at scales smaller than 0.5″ remains constant throughout
the solar cycle. <BR /> Conclusions: At the equator the unsigned flux
density is related to the vertical component of the magnetic field,
whereas at high latitudes this flux density is mainly related to the
horizontal component and probe higher altitudes. Our results favor a
small-scale dynamo that operates in the internetwork, but they show
that the global dynamo also contributes to the internetwork fields. At
solar maximum the high-latitude horizontal internetwork fields seem to
be depleted from the structures at granular and larger scales that are
seen at solar minimum, whereas the internetwork within enhanced network
regions show more structures at those scales than at solar minimum.
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Title: A new spectroscopic method for measuring the temperature
gradient in the solar photosphere. Generalized application in
magnetized regions
Authors: Faurobert, M.; Criscuoli, S.; Carbillet, M.; Contursi, G.
2020A&A...642A.186F Altcode:
Context. The contribution of quiet-Sun regions to the solar irradiance
variability is currently unclear. Certain solar-cycle variations of
the quiet-Sun's physical structure, such as the temperature gradient,
might affect the irradiance. Accurate measurements of this quantity
over the course of the activity cycle would improve our understanding
of long-term irradiance variations. <BR /> Aims: In a previous work,
we introduced and successfully tested a new spectroscopic method
for measuring the photospheric temperature gradient directly on a
geometric scale in the case of non-magnetic regions. In this paper,
we generalize this method for moderately magnetized regions that
may be encountered in the quiet solar photosphere. <BR /> Methods:
To simulate spectroscopic observations, we used synthetic Stokes
profiles I and V of the magnetic FeI 630.15 nm line and intensity
profiles of the non-magnetic FeI 709 nm line computed from realistic
three-dimensional magneto-hydrodynamical simulations of the photospheric
granulation and line radiative transfer under local thermodynamical
equilibrium conditions. We then obtained maps at different levels
in the line-wings by convolution with the instrumental point spread
function (PSF) under various conditions of atmospheric turbulence -
with and without correction by an adaptive optics (AO) system. The
PSF were obtained with the PAOLA software and the AO performance is
inspired by the system that will be operating on the Daniel K. Inouye
Solar Telescope. <BR /> Results: We considered different conditions
of atmospheric turbulence and photospheric regions with different
mean magnetic strengths of 100 G and 200 G. As in non-magnetic cases
studied in our previous work, the image correction by the AO system
is mandatory for obtaining accurate measurements of the temperature
gradient. We show that the non-magnetic line at 709 nm may be safely
used in all the cases we have investigated. However, the intensity
profile of the magnetic-sensitive line is broadened by the Zeeman
effect, which would bias our temperature-gradient measurement. We thus
implemented a correction procedure of the line profile for this magnetic
broadening in the case of weakly magnetized regions. In doing so, we
remarked that in the weak-field regime, the right- and left-hand (I +
V and I - V) components have similar shapes, however, they are shifted
in opposite directions due to the Zeeman effect. We thus reconstructed
the intensity profile by shifting back the I + V and I - V profiles
and by adding the re-centered profiles. The measurement then proceeds
as in the non-magnetic case. We find that this correction procedure
is efficient in regions where the mean magnetic strength is smaller or
on the order of 100 G. <BR /> Conclusions: The new method we implement
here may be used to measure the temperature gradient in the quiet Sun
from ground-based telescopes equipped with an efficient AO system. We
stress that we derive the gradient on a geometrical scale and not on an
optical-depth scale as we would do with other standard methods. This
allows us to avoid any confusion due to the effect of temperature
variations on the continuum opacity in the solar photosphere.
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Title: Chapter 8 - Solar and Stellar Variability
Authors: Faurobert, Marianne
2019sgsp.book..267F Altcode:
The Sun, like many cool stars on the main sequence, is
variable. Changing dark and bright features, sunspots, and faculae
appear and disappear on its surface. These variations are cyclic and
related to a dynamo mechanism operating in the solar interior. The basic
principles of the dynamo theories were presented in the previous chapter
within the formalism of the mean field theory and its two branches,
the flux-transport dynamos and the tachocline dynamos. However, many
important problems remain to be solved to understand the magnetic
activity of the Sun and stars. <P />In this chapter, we focus on
observational studies of solar and stellar variability. This subject has
been evolving quickly within the past decades thanks to the development
of new space-borne and ground-based instruments, which have provided
us with a wealth of data of unprecedented quality for both the Sun and
stars. We will emphasize how knowledge and methods developed in the
context of solar physics can serve as guides for stellar investigations
and how, in turn, observation of stellar activity offers us a unique
possibility of testing the dynamo theories through varying stellar
parameters. <P />Because magnetic activity affects the radiative output
of stars, we will examine variations in solar and stellar irradiance
and discuss their modeling. This problem is related to the hotly
debated subject of the impact of solar activity on the terrestrial
climate and also to stars hosting exoplanets regarding the issue of
accessing their habitability conditions.
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Title: Temperature gradient in the solar photosphere. Test of a new
spectroscopic method and study of its feasibility for ground-based
telescopes
Authors: Faurobert, M.; Carbillet, M.; Marquis, L.; Chiavassa, A.;
Ricort, G.
2018A&A...616A.133F Altcode: 2018arXiv180900893F
Context. The contribution of quiet-Sun regions to the solar irradiance
variability is currently unclear. Some solar-cycle variations of
the quiet-Sun physical structure, such as the temperature gradient,
might affect the irradiance. The synoptic measurement of this
quantity along the activity cycle would improve our understanding
of long-term irradiance variations. <BR /> Aims: We intend to
test a method previously introduced for measuring the photospheric
temperature gradient from high-resolution spectroscopic observation
and to study its feasibility with ground-based instruments with and
without adaptative optics. <BR /> Methods: We used synthetic profiles
of the FeI 630.15 nm obtained from realistic three-dimensional
hydrodynamical simulations of the photospheric granulation and line
radiative transfer computations under local thermodynamical equilibrium
conditions. Synthetic granulation images at different levels in the
line are obtained by convolution with the instrumental point spread
function (PSF) under various conditions of atmospheric turbulence,
with and without correction by an adaptative optics (AO) system. The
PSF are obtained with the PAOLA software, and the AO performances are
inspired by the system that will be operating on the Daniel K. Inouye
Solar Telescope. <BR /> Results: We consider two different conditions
of atmospheric turbulence, with Fried parameters of 7 cm and 5 cm,
respectively. We show that the degraded images lead to both a bias and
a loss of precision in the temperature-gradient measurement, and that
the correction with the AO system allows us to drastically improve the
measurement quality. <BR /> Conclusions: Long-term synoptic observations
of the temperature gradient in the solar photosphere can be undertaken
by implementing this method on ground-based solar telescopes that are
equipped with an AO correction system.
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Title: Quiet Sun magnetic fields
Authors: Faurobert, M.
2017POBeo..96...39F Altcode:
No abstract at ADS
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Title: Inference of magnetic fields in inhomogeneous prominences
Authors: Milić, I.; Faurobert, M.; Atanacković, O.
2017A&A...597A..31M Altcode: 2016A&A...597A..31M; 2016arXiv160904954M
Context. Most of the quantitative information about the magnetic
field vector in solar prominences comes from the analysis of the
Hanle effect acting on lines formed by scattering. As these lines can
be of non-negligible optical thickness, it is of interest to study
the line formation process further. <BR /> Aims: We investigate the
multidimensional effects on the interpretation of spectropolarimetric
observations, particularly on the inference of the magnetic field
vector. We do this by analyzing the differences between multidimensional
models, which involve fully self-consistent radiative transfer
computations in the presence of spatial inhomogeneities and velocity
fields, and those which rely on simple one-dimensional geometry. <BR
/> Methods: We study the formation of a prototype line in ad hoc
inhomogeneous, isothermal 2D prominence models. We solve the NLTE
polarized line formation problem in the presence of a large-scale
oriented magnetic field. The resulting polarized line profiles are
then interpreted (I.e. inverted) assuming a simple 1D slab model. <BR
/> Results: We find that differences between input and the inferred
magnetic field vector are non-negligible. Namely, we almost universally
find that the inferred field is weaker and more horizontal than the
input field. <BR /> Conclusions: Spatial inhomogeneities and radiative
transfer have a strong effect on scattering line polarization in the
optically thick lines. In real-life situations, ignoring these effects
could lead to a serious misinterpretation of spectropolarimetric
observations of chromospheric objects such as prominences.
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Title: Variation of the photospheric temperature gradient with
magnetic activity
Authors: Faurobert, Marianne
2017psio.confE..16F Altcode:
No abstract at ADS
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Title: Variation of the temperature gradient in the solar photosphere
with magnetic activity
Authors: Faurobert, M.; Balasubramanian, R.; Ricort, G.
2016A&A...595A..71F Altcode:
Context. The contribution of quiet-Sun regions to the solar irradiance
variability is currently unclear. Some solar-cycle variations of the
quiet-Sun physical structure, such as the temperature gradient or
the photospheric radius, might affect the irradiance. <BR /> Aims:
We intend to investigate possible variations of the photospheric
temperature gradient with magnetic activity. <BR /> Methods: We used
high-resolution center-to-limb observations of the FeI 630.15 nm line
profile in the quiet Sun performed onboard the Hinode satellite on 2007,
December 19, and on 2013, December 7, that is, close to a minimum and
a maximum of magnetic activity, respectively. We analyzed samples of
10″ × 10″ internetwork regions. The wings of the FeI 630.15 nm line
were used in a non-standard way to recover images at roughly constant
continuum optical depths above the continuum formation level. The image
formation height is derived from measuring its perspective shift with
respect to the continuum image, both observed away from disk center. The
measurement relies on a cross-spectral method that is not limited by
the spatial resolution of the SOT telescope and does not rely on any
radiative transfer computation. The radiation temperature measured
in the images is related to the photospheric temperature at their
respective formation height. <BR /> Results: The method allows us to
investigate the temperature gradient in the low photosphere at altitudes
of between 0 and 60 km above the 500 nm continuum formation height. In
this layer the internetwork temperature gradient appears steeper in
our 2013 sample than in the sample of 2007 in the northern hemisphere,
whereas we detect no significant change in the southern hemisphere. We
argue that this might be related to some strong hemispheric asymmetry
of the magnetic activity at the solar maximum of cycle 24. <BR />
Conclusions: Structural changes have been observed in numerical
simulations of the magneto-convection at the surface of the Sun where
the increase of the ambient sub-surface magnetic fields leads to some
steepening of the temperature gradient in the internetwork. Hemispheric
asymmetry of the activity has been reported for the last solar cycles
with successive dominant north and south hemisphere during the activity
maximum. Our results seem consistent with this global physical picture,
but need to be confirmed by additional studies.
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Title: Fast inversion of Zeeman line profiles using central
moments. II. Stokes V moments and determination of vector magnetic
fields
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2016A&A...591A..64M Altcode:
Context. In the case of unresolved solar structures or stray light
contamination, inversion techniques using four Stokes parameters
of Zeeman profiles cannot disentangle the combined contributions of
magnetic and nonmagnetic areas to the observed Stokes I. <BR /> Aims:
In the framework of a two-component model atmosphere with filling factor
f, we propose an inversion method restricting input data to Q , U, and
V profiles, thus overcoming ambiguities from stray light and spatial
mixing. <BR /> Methods: The V-moments inversion (VMI) method uses
shifts S<SUB>V</SUB> derived from moments of V-profiles and integrals
of Q<SUP>2</SUP>, U<SUP>2</SUP>, and V<SUP>2</SUP> to determine the
strength B and inclination ψ of a magnetic field vector through
least-squares polynomial fits and with very few iterations. Moment
calculations are optimized to reduce data noise effects. To specify the
model atmosphere of the magnetic component, an additional parameter
δ, deduced from the shape of V-profiles, is used to interpolate
between expansions corresponding to two basic models. <BR /> Results:
We perform inversions of HINODE SOT/SP data for inclination ranges 0
<ψ< 60° and 120 <ψ< 180° for the 630.2 nm Fe I line. A
damping coefficient is fitted to take instrumental line broadening into
account. We estimate errors from data noise. Magnetic field strengths
and inclinations deduced from VMI inversion are compared with results
from the inversion codes UNNOFIT and MERLIN. <BR /> Conclusions:
The VMI inversion method is insensitive to the dependence of Stokes I
profiles on the thermodynamic structure in nonmagnetic areas. In the
range of Bf products larger than 200 G, mean field strengths exceed
1000 G and there is not a very significant departure from the UNNOFIT
results because of differences between magnetic and nonmagnetic model
atmospheres. Further improvements might include additional parameters
deduced from the shape of Stokes V profiles and from large sets of
3D-MHD simulations, especially for unresolved magnetic flux tubes.
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Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical,
and near-infrared
Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini,
Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis
Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini,
Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio,
Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne;
Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio;
Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska,
Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza,
Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca;
Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele;
Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini,
Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre,
Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio;
Zuccarello, Francesca
2015JATIS...1d4006B Altcode:
Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept
for a small solar and space weather mission with a budget compatible
with an European Space Agency (ESA) S-class mission, including launch,
and a fast development cycle. ADAHELI was submitted to the European
Space Agency by a European-wide consortium of solar physics research
institutes in response to the "Call for a small mission opportunity
for a launch in 2017," of March 9, 2012. The ADAHELI project builds
on the heritage of the former ADAHELI mission, which had successfully
completed its phase-A study under the Italian Space Agency 2007 Small
Mission Programme, thus proving the soundness and feasibility of
its innovative low-budget design. ADAHELI is a solar space mission
with two main instruments: ISODY: an imager, based on Fabry-Pérot
interferometers, whose design is optimized to the acquisition of
highest cadence, long-duration, multiline spectropolarimetric images
in the visible/near-infrared region of the solar spectrum. XSPO: an
x-ray polarimeter for solar flares in x-rays with energies in the 15
to 35 keV range. ADAHELI is capable of performing observations that
cannot be addressed by other currently planned solar space missions,
due to their limited telemetry, or by ground-based facilities, due to
the problematic effect of the terrestrial atmosphere.
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Title: Do the quiet sun magnetic fields vary with the solar cycle?
Authors: Faurobert, Marianne; Ricort, Gilbert; Lites, Bruce
2015IAUS..305...22F Altcode:
The quiet Sun observed in polarized light exhibits a rich and
complex magnetic structuring which is still not fully resolved nor
understood. The present work is intended to contribute to the debate
about the origin of the quiet sun magnetic fields, in relation or
not to the global solar dynamo. We present analysis of center-to-limb
polarization measurements obtained with the SOT/SP spectropolarimeter
onboard the Hinode satellite outside active regions, in 2007 and 2013,
i.e. at a minimum and a maximum of the solar cycle, respectively. We
compare the spatial fluctuation Fourier spectra of unsigned circular and
linear polarization images after corrections for polarization bias and
focus variations between the two data sets. The decay of active regions
is clearly a source of magnetic fields in the quiet Sun. It leads to
a global increase of the polarization fluctuation power spectrum in
2013 in the network. In the internetwork, we observe no variation of
the polarization fluctuation power at mesogranular and granular scales,
whereas it increases at sub-granular scales. We interpret these results
in the following way. At the mesogranular and granular scales very
efficient mechanisms of magnetic field removal are operating in the
internetwork, that leads to a dissipation or a concentration of magnetic
fields on smaller scales. So the cycle-invariant magnetic signal that
we detect at mesogranular and granular scales must be continuously
created by a dynamo mechanism which is independent of the solar cycle.
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Title: Solar-cycle variations of the internetwork magnetic field
Authors: Faurobert, M.; Ricort, G.
2015A&A...582A..95F Altcode:
Context. The quiet Sun exhibits a rich and complex magnetic structuring
that is still not fully resolved or understood. <BR /> Aims: We intend
to contribute to the debate about the origin of the internetwork
magnetic fields and whether or not they are related to the global
solar dynamo. <BR /> Methods: We analyzed center-to-limb polarization
measurements obtained with the SOT/SP spectropolarimeter onboard the
Hinode satellite outside active regions in 2007 and 2013, that is, at a
minimum and a maximum of the solar cycle, respectively. We examined 10”
× 10” maps of the unsigned circular and linear polarization in the
FeI 630.25 nm line in regions located away from network elements. The
maps were corrected for bias and focus variations between the two
data sets. Then we applied a Fourier spectral analysis to examine
wether the spatial structuring of the internetwork magnetic fields
shows significant differences between the minimum and maximum of
the cycle. <BR /> Results: Neither the mean values of the unsigned
circular and linear polarizations in the selected 10” × 10”
maps nor their spatial fluctuation power spectra show significant
center-to-limb variations. For the unsigned circular polarization the
power of the spatial fluctuations is lower in 2013 than in 2007, but
the spectral slope is unchanged. The linear polarization spectra show
no significant differences in 2013 and 2007, but the spectrum of 2013
is more strongly affected by noise. <BR /> Conclusions: The small-scale
magnetic structuring in the internetwork is different in our 2013 and
2007 data. Surprisingly, we find a lower spatial fluctuation power
at the solar maximum in the internetwork magnetic structuring. This
indicates some complex interactions between the small-scale magnetic
structures in the quiet Sun and the global dynamo, as predicted by
recent numerical simulations. This result has to be confirmed by
further statistical studies with larger data sets.
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Title: Inhomogeneity and velocity fields effects on scattering
polarization in solar prominences
Authors: Milić, I.; Faurobert, M.
2015IAUS..305..238M Altcode:
One of the methods for diagnosing vector magnetic fields in solar
prominences is the so called "inversion" of observed polarized spectral
lines. This inversion usually assumes a fairly simple generative model
and in this contribution we aim to study the possible systematic errors
that are introduced by this assumption. On two-dimensional toy model
of a prominence, we first demonstrate importance of multidimensional
radiative transfer and horizontal inhomogeneities. These are able to
induce a significant level of polarization in Stokes U, without the
need for the magnetic field. We then compute emergent Stokes spectrum
from a prominence which is pervaded by the vector magnetic field and
use a simple, one-dimensional model to interpret these synthetic
observations. We find that inferred values for the magnetic field
vector generally differ from the original ones. Most importantly,
the magnetic field might seem more inclined than it really is.
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Title: New view on exoplanet transits. Transit of Venus described
using three-dimensional solar atmosphere STAGGER-grid simulations
Authors: Chiavassa, A.; Pere, C.; Faurobert, M.; Ricort, G.; Tanga,
P.; Magic, Z.; Collet, R.; Asplund, M.
2015A&A...576A..13C Altcode: 2015arXiv150106207C
Context. An important benchmark for current observational techniques and
theoretical modeling of exoplanet atmospheres is the transit of Venus
(ToV). Stellar activity and, in particular, convection-related surface
structures, potentially cause fluctuations that can affect the transit
light curves. Surface convection simulations can help interpreting the
ToV as well as other transits outside our solar system. <BR /> Aims:
We used the realistic three-dimensional (3D) radiative hydrodynamical
(RHD) simulation of the Sun from the Stagger-grid and synthetic images
computed with the radiative transfer code Optim3D to predict the transit
of Venus (ToV) in 2004 that was observed by the satellite ACRIMSAT. <BR
/> Methods: We computed intensity maps from the RHD simulation of the
Sun and produced a synthetic stellar disk image as an observer would
see, accounting for the center-to-limb variations. The contribution of
the solar granulation was considered during the ToV. We computed the
light curve and compared it to the ACRIMSAT observations as well as
to light curves obtained with solar surface representations carried
out using radial profiles with different limb-darkening laws. We
also applied the same spherical tile imaging method as used for RHD
simulation to the observations of center-to-limb solar granulation with
Hinode. <BR /> Results: We explain ACRIMSAT observations of 2004 ToV and
show that the granulation pattern causes fluctuations in the transit
light curve. We compared different limb-darkening models to the RHD
simulation and evaluated the contribution of the granulation to the
ToV. We showed that the granulation pattern can partially explain the
observed discrepancies between models and data. Moreover, we found
that the overall agreement between real and RHD solar granulation
is good, either in terms of depth or ingress/egress slopes of the
transit curve. This confirms that the limb-darkening and granulation
pattern simulated in 3D RHD of the Sun represent well what is imaged
by Hinode. In the end, we found that the contribution of the Venusean
aureole during ToV is ~10<SUP>-6</SUP> times less intense than the
solar photosphere, and thus, accurate measurements of this phenomena
are extremely challenging. <BR /> Conclusions: The prospects for
planet detection and characterization with transiting methods are
excellent with access to large a amount of data for stars. Being able
to consistently explain the data of 2004 ToV is a new step forward
for 3D RHD simulations, which are becoming essential for detecting
and characterizing exoplanets. They show that granulation has to
be considered as an intrinsic uncertainty (as a result of stellar
variability) on precise measurements of exoplanet transits of, most
likely, planets with small diameters. In this context, it is mandatory
to obtain a comprehensive knowledge of the host star, including a
detailed study of the stellar surface convection.
---------------------------------------------------------
Title: Scattering line polarization in rotating, optically thick disks
Authors: Milić, I.; Faurobert, M.
2014A&A...571A..79M Altcode: 2014arXiv1409.2654M
Context. To interpret observations of astrophysical disks, it is
essential to understand the formation process of the emitted light. If
the disk is optically thick, scattering dominated and permeated by a
Keplerian velocity field, non-local thermodynamic equilibrium (NLTE)
radiative transfer modeling must be done to compute the emergent
spectrum from a given disk model. <BR /> Aims: We investigate NLTE
polarized line formation in different simple disk models and aim to
demonstrate the importance of both radiative transfer effects and
scattering, as well as the effects of velocity fields. <BR /> Methods:
We self-consistently solve the coupled equations of radiative transfer
and statistical equilibrium for a two-level atom model by means of
Jacobi iteration. We use the short characteristics method of formal
solution in two-dimensional axisymmetric media and compute scattering
polarization, that is Q/I and U/I line profiles, using the reduced
intensity formalism. We account for the presence of Keplerian velocity
fields by casting the radiative transfer equation in the observer's
frame. <BR /> Results: Relatively simple (homogeneous and isothermal)
disk models show complex intensity profiles that owe their shape
to the interplay of multidimensional NLTE radiative transfer and
the presence of rotation. The degree of scattering polarization is
significantly influenced not only by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and
the presence of rotation. Stokes U/I shows double-lobed profiles with
amplitude that increases with the disk rotation. <BR /> Conclusions: Our
results suggest that the line profiles, especially the polarized ones,
emerging from gaseous disks differ significantly from the profiles
predicted by simple approximations. Even in the case of the simple
two-level atom model, we obtain line profiles that are diverse in shape,
but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which
might prove to be a useful diagnostic tool. We also demonstrate that,
for moderate rotational velocities, an approximate treatment can be
used, where NLTE radiative transfer is done in the velocity field-free
approximation, and Doppler shift is applied in the process of spatial
integration over the whole emitting surface.
---------------------------------------------------------
Title: Possible measurements of the magnetic field in eruptive
prominences using the PROBA-3 coronagraph
Authors: Serge, Koutchmy; Zhukov, Andrei; Dolla, Laurent; Heinzel,
Petr; Lamy, Philippe; Bazin, Cyrille; Bommier, Veronique; Faurobert,
Marianne
2014cosp...40E2971S Altcode:
The PROBA-3 mission will fly a spacecraft put in the shadow of a
precisely occulting sister satellite orbiting “in formation” at a
distance of 150 m in front of it to make artificial total eclipses. The
region right above the solar limb will be studied for the first time
over a coronal background not polluted by any spurious light. Although
the priority will be the high-resolution fast imaging of the dynamic
white-light corona, the use of a narrow filter centered on a low
excitation D3 line of He I, is planned for imaging prominences. Adding
the linear polarization analysis would permit the measurements of the
magnetic field using the Hanle effect. We evaluate the possibility
offered during the eruptive phase of a CME with prominence material
inserted inside, for studying the associated magnetic field changes
related to both the heating process and the ejection of material. The
background highly polarized K-corona is taken into account. Sequences of
quasi- simultaneous white-light processed images at high resolution are
an additional feature of great interest for interpreting the overall
magnetic structure.
---------------------------------------------------------
Title: Multidimensional and inhomogeneity effects on scattering
polarization in solar prominences
Authors: Milić, Ivan; Faurobert, Marianne
2014IAUS..300..453M Altcode:
Measurements of magnetic fields in solar prominences via Hanle effect
usually assume either single scattering approximation or simple,
one-dimensional, slab model in order to perform an inversion and find
the unknown magnitude and the orientation of the magnetic field from
spectropolarimetric observations. Here we perform self-consistent
NLTE modeling of scattering polarization in inhomogeneous 2D slab,
illuminated from its sides by the solar continuum radiation. We show
that even in the absence of a magnetic field, in the non-optically thin
regime, significant non-zero Stokes U is to be expected. Neglecting
these effects, in principle, could cause systematic errors in
spectropolarimetric inversions, in the case when the prominence is
optically thick.
---------------------------------------------------------
Title: Distribution of magnetic fields in the quiet Sun
Authors: Faurobert, M.; Ricort, G.
2013A&A...560A..62F Altcode:
Context. The distribution of magnetic fields in the atmosphere of the
quiet Sun has been extensively studied with various methods, but it is
still a matter of debate. Previous analyses mostly rely on the inversion
of spectro-polarimetric data and different methods lead to somewhat
different results. <BR /> Aims: Here we do not intend to determine the
magnetic field vector, but we use total polarization images as tracers
of the magnetic field distribution and we study their cross-correlations
with granulation and reversed-granulation images. <BR /> Methods: We
used high-resolution spectro-polarimetric scans obtained in the 630
nm FeI line pair at varying heliocentric angles along the north-south
polar axis of the Sun, with the Solar Optical Telescope (SOT) onboard
Hinode. We obtain polarization images by summing the total polarization
(linear and circular) in each line. We compute the 2D correlation of
polarization images and their cross-correlations with images of the
granulation observed simultaneously in the line wings and with the
reversed granulation observed in the line cores, and we examine their
center-to-limb variations. <BR /> Results: The correlation-widths of
polarization images have on the average, the same value (1.3”) as the
correlation-width of the granulation, showing that the granular scale is
a characteristic scale of the quiet Sun magnetic field distribution. At
disk center the cross-correlation between total polarization and the
granulation shows a negative peak. Out of disk-center both a negative
and a positive peak are detected. The cross-correlation of polarization
images with the reversed granulation always shows a positive peak. We
assign these cross-correlation signals to the presence of two kinds
of magnetized regions spatially separated, one of them is located in
the intergranular lanes (anti-correlated with the granulation), the
other one lies within the bright granules. For images obtained out of
disk center, the correlation and anti-correlation peaks are shifted
along the north-south direction and the shifts measured at the same
limb-distance in the southern and northern hemispheres have opposite
values. A consistent interpretation of these shifts is proposed
in terms of a perspective effect arising when two images formed at
different heights are observed at an angle. We were able to measure
the perspective effect for the magnetic component correlated with the
reversed granulation. Its polarization signals observed in the FeI
630.25 nm line and in the 630.15 nm line, are formed respectively 100
km and 150 km below the bright features seen at line centers.
---------------------------------------------------------
Title: Boundary conditions for polarized radiative transfer with
incident radiation
Authors: Faurobert, M.; Milić, I.; Atanacković, O.
2013A&A...559A..68F Altcode: 2013arXiv1309.4160F
Context. Polarized radiative transfer in the presence of scattering
in spectral lines and/or in continua may be cast in a reduced form for
six reduced components of the radiation field. In this formalism, the
six components of the reduced source function are angle-independent
quantities. It thus drastically reduces the storage requirement
of numerical codes and it is very well suited to solving polarized
non-local thermodynamic equilibrium radiative transfer problems in 3D
media. <BR /> Aims: This approach encounters a fundamental problem
when the medium is illuminated by a polarized incident radiation,
because there is a priori no way of relating the known (and measurable)
Stokes parameters of the incident radiation to boundary conditions
for the reduced equations. The origin of this problem is that there
is no unique way of deriving the radiation-reduced components from its
Stokes parameters (only the inverse operation is clearly defined). The
method proposed here aims at enabling to work with arbitrary
incident radiation field (polarized or unpolarized). <BR /> Methods:
In previous studies, an ad-hoc treatment of the boundary conditions,
applied to cases where the incident radiation is unpolarized, has been
used. In this paper, we show that it is possible to account for the
incident radiation in a rigorous way without any assumption on its
properties by expressing the radiation field as the sum of a directly
transmitted radiation and of a diffuse radiation. This approach was
first used by Chandrasekhar to solve the problem of diffuse reflection
by planetary atmospheres illuminated by their host star. <BR /> Results:
The diffuse radiation field obeys a transfer equation with no incident
radiation that may be solved in the reduced form. The first scattering
of the incident radiation introduces primary creation terms in the six
components of the reduced source function. Once the reduced polarized
transfer problem is solved for the diffuse radiation field, its Stokes
parameters can be computed. The full radiation field is then obtained
by adding the directly transmitted radiation field computed in the
Stokes formalism. <BR /> Conclusions: In the case of an unpolarized
incident radiation, the diffuse field approach allows us to validate
the previously introduced ad-hoc expressions. The diffuse field
approach however leads to more accurate computation of the source
terms in the case where the incident radiation is anisotropic. It
is the only possible approach when the incident radiation field is
polarized. We perform numerical computations of test cases, showing
that the emergent line-polarization may be significantly affected by
the polarization of the incident radiation.
---------------------------------------------------------
Title: Empirical determination of the temperature stratification in
the photosphere of the quiet Sun
Authors: Faurobert, M.; Ricort, G.; Aime, C.
2013A&A...554A.116F Altcode:
Context. Detailed realistic 3D simulations of the photosphere of the Sun
are now available, but 1D models of the average quiet-Sun photosphere
are still widely used, in particular for spectro-polarimetric
inversions. <BR /> Aims: Here we present an empirical determination
of the average radiation temperature variations as a function of the
geometrical height above the continuum formation level in the solar
photosphere. <BR /> Methods: We used high resolution spectroscopic
scans in the 630 nm Fe i line pair at varying heliocentric angles
along the north-south polar axis of the Sun, made with SOT onboard
Hinode. Implementing a new method for image reconstruction, we obtained
images of the photospheric granulation at constant continuum opacity
levels, from the upper photosphere seen at line centers to the low
photosphere. The Fourier cross-spectra of images at different opacity
levels were computed, and we derived the formation depths of images
without referring to any atmospheric model, by measuring the slope of
the cross-spectrum phase. <BR /> Results: A modified Milne-Eddington
model for the line formation was tested by comparing it with the
average line-intensity profiles observed at solar disk center. It
yields consistent results for the FeI 630.2 nm line, whereas the FeI
line at 630.1 nm is not well reproduced by the model. We ascribe this
discrepancy to non-LTE effects in the line formation processes. The
average image intensities at the different FeI 630.2 nm levels were used
to determine the depth-variation of the temperature for an average 1D
model of the quiet photosphere. We compared our empirical temperature
model with the widely used FALC model. Both models agree well for
the temperature variations with the continuum optical depth. But in
the low photosphere, the temperature gradient we measure with respect
to the geometrical height is significantly softer than in Model C. We
argue that some of the assumptions used to solve the pseudohydrostatic
equilibrium in semi-empirical models are probably at fault. We also
derived empirical values for the 630 nm continuum absorption coefficient
as a function of the geometrical height in the low photosphere. Finally,
we were able to measure the altitude of the base of the granulation
contrast inversion layer, which is found at about 130 km above the
base of the photosphere, in agreement with 3D MHD simulations.
---------------------------------------------------------
Title: A cross-correlation method for measuring line formation
heights in the solar photosphere
Authors: Faurobert, M.; Ricort, G.; Aime, C.
2012A&A...548A..80F Altcode:
Context. Detailed 3D-simulations of magneto-convection in the solar
photosphere are now available. They intend to capture the main
physical mechanisms at play in this boundary layer, where complex
physical phenomena, such as convective overshooting and small scale
magnetic dynamo take place. But numerical limitations in spatial
resolution and in box-size are likely to affect the description of
some relevant physical scales, so simulations need to be compared to
independent observations allowing us to explore the full height range
of the photosphere. <BR /> Aims: Here we focus on a model-independent
method for measuring line formation depths. We construct images of
the photosphere at constant continuum opacity levels from the low
to the upper photosphere and we show how they can be used to measure
systematic displacements of granular structures with height. The method
is applied to determine the formation height of the 630 nm Fe i line
pair. We compare our measurements to the results of 3D simulations. <BR
/> Methods: We analyze high resolution spectroscopic scans obtained
in the 630 nm Fe i line pair at varying heliocentric angles along the
north-south polar axis of the Sun, with SOT onboard Hinode. We implement
a new strategy for correcting the images observed at different line
cords from spurious Doppler effects. The cross-correlations between
continuum images and line core images show a clear anti-correlation
peak due to the contrast inversion of the granulation in the upper
photosphere, as predicted by magnetohydrodynamic (MHD) simulations. <BR
/> Results: The anti-correlation peak is shifted by the perspective
effect and by horizontal velocity effects. Both effects may be
distinguished because they have different center-to-limb variations. The
measurement of the perspective shift allows us to determine the line
core formation heights and their center-to-limb variations. The results
are in good agreement with 3D- MHD simulations for images close to
disk center, but close to the solar limb we measure larger formation
heights than what is predicted by the simulations, which seem to fail
in modeling properly the upper layers of the photosphere. As the
granulation contrast inversion is observed at line centers, we can
safely conclude that the height of the contrast inversion layer is
smaller than 200 km.
---------------------------------------------------------
Title: Searching For Weak Solar Magnetic Fields. What Can We Learn
From the Hanle Effect?
Authors: Faurobert, M.
2012POBeo..91...35F Altcode:
Many dynamical phenomena taking place in the solar atmosphere, such as
flares and coronal mass ejections, are driven by the interplay between
the magnetic field and the turbulent solar plasma. Long standing
questions on the physical origin of coronal heating and solar wind
acceleration are also probably related to magnetic mechanisms. So
the issue of how to measure magnetic fields in the solar atmosphere,
from the photosphere up to the corona, is of crucial importance in
solar physics. Because of the high value of the Reynolds magnetic
number in the solar atmosphere, magnetic structures may develop from
large to very small spatial scales, typically ranging from the scale
of the solar diameter down to few tens of kilometers. The magnetic
field strength also shows a broad dispersion from several thousands
of Gauss in active regions like sunspots, down to a few Gauss in the
quiet Sun. The amount of magnetic energy stored in magnetic structures
is still unkown, and, according to recent numerical simulations, a
significant fraction may be stored in weak, small scale fields, which
cover a large fraction of the solar surface. However these field are
hardly detectable in magnetograms based on the Zeeman effect because of
their limited performances both in terms of polarimetric sensitivity and
of spatial resolution. The Hanle effect is a valuable alternative for
the diagnostics of weak fields with mixed polarity at small scales. It
affects the linear polarization of spectral lines formed by scattering
of photons in the solar atmosphere. I will present how weak magnetic
fields investigations have changed our vision of the solar magnetism and
discuss future research directions, in the context of solar polarimetry
projects with ground based or space based intruments.
---------------------------------------------------------
Title: The quiet Sun magnetism: What can we learn from the Hanle
effect?
Authors: Faurobert, M.
2012IAUSS...6E.301F Altcode:
The physics of the outer layers of the Sun is mostly driven by magnetic
phenomena. This is the reason why high resolution investigations of the
magnetic fields in the hot and dilute outer atmosphere of the Sun, from
the photosphere to the chromosphere and corona, are the major objectives
of future large solar telescopes, such as the Advanced Technology Solar
Telescope (ATST), or the European Solar Telescope (EST). The so-called
"quiet Sun" is filled in with magnetic fields distributed in strengths
and over a wide range of spatial scales. The magnetic energy content
of this distribution of fields is a crucial issue, related to the long
standing questions of the coronal and chromospheric heatings. Zeeman
diagnostics of the magnetic fields depend crucially on the spatial
resolution of the observations, whereas diagnostics based on the Hanle
effect do provide valuable information on the average field strength
even if the magnetic structures are not resolved. However, they rely on
the precise radiative transfer modeling of polarized lines formed under
non-LTE conditions. The use of the differential Hanle effect on lines
with different magnetic sensitivities is a method of choice to obtain
model-independent diagnostics. Another promissing way explored nowdays
is to make use of the complementary diagnostics provided by both the
Zeeman and Hanle effects when they can be observed in the same lines.
---------------------------------------------------------
Title: Resonance Line Polarization in Moving Optically Thick
Structures
Authors: Milic, I.; Faurobert, M.
2012POBeo..91...45M Altcode:
We compute the scattering polarization of lines formed in moving
slabs of moderate optical thickness (τ=1 and τ=10) illuminated
by a linearly polarized radiation field showing a broad absorption
feature. Slabs are one-dimensional and horizontal, placed at a finite
height H above a semi-infinite atmosphere. This model is an academic
case which represents the formation of emergent radiation in solar
filaments. The slabs have a macroscopic velocity in the radial direction
with respect to the atmosphere, and are observed at different angles
(i.e. at different locations over the solar disk). We investigate
the sensitivity of the outgoing polarization to the slab velocity and
observing angle. We show that outgoing polarization profiles are at
least as sensitive to macroscopic velocity as are intensity profiles.
---------------------------------------------------------
Title: Hanle diagnostics of weak solar magnetic fields: . Inversion
of scattering polarization in C<SUB>2</SUB> and MgH molecular lines
Authors: Milić, I.; Faurobert, M.
2012A&A...547A..38M Altcode:
Context. The quiet Sun magnetism has been intensively investigated
in recent years by various observational techniques. But several
issues, such as the question of the isotropy and of the energy density
spectrum of the mixed polarity turbulent magnetic fields, are still
under debate. <BR /> Aims: Here we present an inversion method that
allows us to constrain the depth-dependence of the magnetic field
strength. We use the center-to-limb variations of linear scattering
polarization measured in molecular lines of C<SUB>2</SUB> and MgH
molecules with different sensitivities to the Hanle effect. We consider
six C<SUB>2</SUB>-triplets and one MgH line in the spectral range
between 515.7 nm and 516.1 nm observed with the THEMIS Telescope. <BR
/> Methods: One of the delicate problems with Hanle diagnostics is
to disentangle the effects of elastic depolarizing collisions from
the depolarization due to the Hanle effect of the magnetic field. By
making use of the different sensitivities of the molecular lines in
our spectral range to microturbulent magnetic fields and, by using
a non-LTE radiative transfer modeling of the line formation, we are
able to determine both the depolarizing collision cross-section and the
magnetic strength. We use a standard 1D quiet Sun atmospheric model and
we invert the full set of center-to-limb polarization rates measured
at line centers, with a depth-dependent magnetic field described by
three free parameters. The depolarizing collision cross-section is
also treated as a free parameter. A downhill simplex method is used to
find the best-fitting values for the collisional and magnetic strength
parameters. <BR /> Results: For the elastic depolarizing collisions
cross-section for the C<SUB>2</SUB> lines we obtain α<SUP>(2)</SUP>
= 1.6 ± 0.4 × 10<SUP>-9</SUP> cm<SUP>3</SUP> s<SUP>-1</SUP>, which
is within an order of magnitude of the value previously obtained for
MgH lines from a differential Hanle effect analysis. The observational
constraints provided by the MgH and C<SUB>2</SUB> line polarization
give access to the altitude range between z = 200 km and z = 400 km
above the base of the photosphere. We find that the turbulent magnetic
field strength decreases from 95 Gauss at the altitude z = 200 km to 5
Gauss at z = 400 km. <BR /> Conclusions: The turbulent magnetic field
strength that we derive from the Hanle effect shows a strong vertical
gradient in the upper photosphere. We point out that this behavior
may explain why very different turbulent magnetic field strengths
have been inferred from the interpretation of Hanle depolarization
when using different lines formed at different altitudes. We notice
that the presence of a strong depth gradient is not compatible with
the assumption of isotropy of the turbulent field.
---------------------------------------------------------
Title: Numerical modeling of the linear polarization in molecular
lines of the solar flash spectrum
Authors: Milić, I.; Faurobert, M.
2012EAS....55...65M Altcode:
Molecular lines formed in the upper photosphere of the Sun show
significant degree of linear scattering polarization, when one observes
close to the solar limb. Those lines prove to be important tools for
turbulent magnetic field diagnostic via the Hanle effect. In order to
correctly model the line formation in regions close to the Solar limb,
one has to take into account the sphericity of the atmosphere and to
model depolarizing collisions and NLTE line formation in details. We
present computations of scattering polarization in C2 lines in a 1D
spherically symmetric medium described by FALC and FALX models, and
compare those computations to observations of the Solar flash spectrum.
---------------------------------------------------------
Title: Extinction and Sky Brightness at Dome C
Authors: Faurobert, M.; Arnaud, J.; Vernisse, Y.
2012EAS....55..365F Altcode:
We have installed a small telescope to monitor the sky brightness
around the sun at the French-Italian station Concordia at Dome
C in Antarctica. Previous campaigns have been performed with the
same instrument at Haleakala in Hawai and Sunspot in New Mexico. We
compare here the results of the first year of the campaign at Dome C
(2008) to the purest sky observed at Haleakala. We show that Dome C
is an outstanding site for coronal observations. Compared to Haleaka,
it appears to be more transparent, and to contain less aerosols. Its
water vapour content is also significantly smaller. These results
still have to be confirmed by the analysis of the 2009 and 2010 data.
---------------------------------------------------------
Title: Height of the granulation contrast inversion layer derived
from a cross-correlation method
Authors: Ricort, G.; Faurobert, M.; Aime, C.; Roudon, S.
2012EAS....55...79R Altcode:
Using spectroscopic data from Hinode/SOT we have obtained monochromatic
images of the solar granulation at different levels in the Fe i 603
nm lines. The granulation contrast inversion is clearly detected as
a negative peak in the cross-correlation of images at the continuum
level and at line center levels. Taking advantage of the perspective
effect which appears in images taken out of the solar disk center,
we were able to measure the formation heights of the lines and the
height of the contrast inversion layer.
---------------------------------------------------------
Title: Inversion of Zeeman Line Profiles Using Central Moments
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2012EAS....55...83M Altcode:
A new inversion method derived from central moments of Zeeman line
profiles (ICM), is used to determine magnetic field vectors (Mein et
al. 2011). Two quantities A<SUB>1</SUB> and A<SUB>2</SUB> combining
moments of profiles I ± S (S = Q,U,V) are nearly linear functions
of the longitudinal and transverse components and lead to the field
components through very fast iterations. Optimized exponents reduce
noise effects. The ICM inversion does not require Milne Eddington
approximation and can be used in a wide range of solar models.
---------------------------------------------------------
Title: Foreword
Authors: Faurobert, M.; Fang, C.; Corbard, T.
2012EAS....55....1F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: ESCAPE : a first step to high resolution solar coronagraphy
in Antarctica
Authors: Damé, L.; Abe, L.; Faurobert, M.; Fineschi, S.; Kuzin, S.;
Lamy, P.; Meftah, M.; Vives, S.
2012EAS....55..359D Altcode:
The Dome C high plateau is unique for coronagraphic observations: sky
brightness is reduced, water vapour is low, seeing is excellent and
continuity of observations on several weeks is possible. ESCAPE (the
Extreme Solar Coronagraphy Antarctic Program Experiment) will perform
2-dimensional spectroscopy of the forbidden line of FeXIV at 530.285 nm:
precise line profile analysis will allow the diagnostic of the nature
of waves by simultaneous measurements of velocities and intensities in
the corona. ESCAPE is proposed to Institut Paul-Emile Victor (IPEV)
for a campaign in 2012-2013 at Dome C/Concordia since all subsystems
are available in particular thanks to an ESA STARTIGER 2010 R&D
"Toward a New Generation of Formation Flying Coronagraph". Using
state-of-the-art technologies developed for Space missions (a
Three Mirrors Anastigmat telescope, the TMA, a 4 stages Liquid
Crystal Tunable-filter Polarimeter, the LCTP) allows us to propose
an automated Coronal Green Line full-field Polarimeter for unique
observations (waves nature and intensity to address coronal heating)
with the best possible performances on Earth and for preparing and
testing the technologies for the next steps in Space. No other site
would allow such coronagraphic performances (the sky brightness is a
factor 2 to 4 better than in Hawaï) and with high spatial resolution
(better than an arcsec is possible).
---------------------------------------------------------
Title: Understanding Solar Activity: Advances and Challenges
Authors: Faurobert, M.; Fang, C.; Corbard, T.
2012EAS....55.....F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modeling scattering polarization in molecular solar lines in
spherical geometry
Authors: Milić, I.; Faurobert, M.
2012A&A...539A..10M Altcode:
Context. The atmosphere of the Sun is permeated by a vast amount
of magnetic flux that remains invisible in magnetograms based on the
Zeeman effect. A model-independent way of measuring weak hidden magnetic
fields makes use of the differential Hanle effect on the scattering
polarization of molecular lines with different sensitivities to magnetic
fields. <BR /> Aims: The observed line scattering polarization steeply
increases at the solar limb. Here we are interested in interpreting
observations performed at the solar limb, where plane-parallel
semi-infinite geometry is not valid. The main reason is that the
sphericity of the atmosphere means that the line-of-sight optical path
intersects only a finite part of the solar atmosphere. In this paper
we revisit the modeling of scattering polarization in two molecular
lines of C<SUB>2</SUB> and MgH in the spectral range from 515.60 nm to
516.20 nm, where observations performed both inside and above the solar
limb are available. <BR /> Methods: The solar atmosphere is described
by a one-dimensional, spherically symmetric medium following either
the FALC or the FALX quiet Sun model. Both the line and background
continuum scattering polarizations are computed by means of the
"along-the-ray" approach. We assume a two-level atom formalism for
the line source function, and we compute the molecule number densities
and line opacities assuming LTE. We estimate the elastic and inelastic
collision rates by fitting the line intensity and linear polarization
in several couples of lines of the Second Solar Spectrum Atlas. <BR
/> Results: The limb variations of scattering polarization, both
in the lines and in the continuum, are strongly modified when the
sphericity of the solar atmosphere is accounted for. We show that the
line polarization goes through a maximum at 0.4” above the limb, for
both MgH and C<SUB>2</SUB> lines. The contribution of the line rapidly
goes to zero at a larger limb distance, but continuum polarization
keeps increasing. The maximum polarization rates have an amplitude of
2% to 2.5% when the FALC model is used, which agrees with previous
observations, whereas the FALX model leads to much higher rates. We
then investigate the Hanle effect of microturbulent magnetic fields on
the C<SUB>2</SUB> line linear polarization. We show that polarization
observed close to the limb would provide valuable diagnostics of weak
magnetic fields in the region of the temperature minimum.
---------------------------------------------------------
Title: Scattering polarization of molecular lines at the solar limb
Authors: Milić, I.; Faurobert, M.
2011sf2a.conf..401M Altcode:
Molecular lines formed in the upper photosphere of the Sun show linear
scattering polarization, when one observes close to the solar limb. This
provides us with a diagnostic tool for measuring weak magnetic fields
in the solar photosphere through the differential Hanle effect
in these lines. However, in order to interpret polarization ratio
measured in different lines of different optical thickness, one has
to model accurately enough the line formation processes. Observations
performed close to and above the solar limb give access to the still
poorly known region of the temperature minimum between the photosphere
and the chromosphere. The modeling of such observations requires to
account for the spherical geometry of the solar atmosphere. Here we
revisit the modeling of molecular solar line scattering polarization
in spherical geometry and we investigate its diagnostics potential.
---------------------------------------------------------
Title: Fast inversion of Zeeman line profiles using central moments
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2011A&A...535A..45M Altcode:
Context. Many inversion techniques derive vector magnetic fields and
other parameters of the solar atmosphere from Stokes profiles with an
iterative process. <BR /> Aims: We propose a new inversion method, using
functions derived from central moments (ICM), to determine magnetic
field vectors with very few iterations. <BR /> Methods: Two quantities
A<SUB>1</SUB> and A<SUB>2</SUB> that combine moments of profiles I ±
S (S = Q,U,V) are proposed. They are nearly linear functions of the
longitudinal and transverse components of the magnetic field, and lead
to estimates of the field components through a least-squares polynomial
fit. A third quantity A<SUB>D</SUB> can be used to interpolate between
expansions that correspond to two basic models. Exponents β<SUB>1</SUB>
and β<SUB>2</SUB> in the moment expressions are adjusted to minimize
the sensitivity to data noise. <BR /> Results: Inversion coefficients
are computed for magnetic fields up to 3000 G in the case of the 630.2
Fe i line by forward modeling in two selected 1D model atmospheres
(FALC and MALTM). After inversion of synthetic profiles computed with
four models at disk center (FALA, FALC, FALF, MALTM), the mean standard
deviations with respect to the input fields do not exceed 5 G for both
components over the full range 0-3000 G. A comparison of ICM results
with inversion by the UNNOFIT code of profiles observed with THEMIS/MTR
shows good agreement. The typical computing time for a solar map of 100
000 points is less than 30 s. <BR /> Conclusions: The ICM inversions
are almost insensitive to thermodynamic properties and solve for vector
magnetic fields in a wide range of solar conditions, ranging from plage
to spot, with very little computational effort. They are, therefore,
extremely suitable for large data sets. Further improvements should
take into account instrumental profiles and effects of limited spatial
resolution by using filling factors. Extensions using more parameters
and models with large departures from the Milne Eddington approximation
could also be considered.
---------------------------------------------------------
Title: Measurement of Line Formation Depths from a Super Resolving
Analysis of Photospheric Layers
Authors: Faurobert, M.; Aime, C.; Ricort, G.; Uitenbroek, H.; Grec, C.
2011ASPC..437...51F Altcode:
We present the results of an experiment aimed at measuring the formation
depths of the Fe I line pair at 630 nm in the solar photosphere. We use
images of the granulation obtained at different levels in the lines,
from line center up to the continuum level. When we observe out of
disc center their difference in formation depths is projected into a
radial shift of the images by the perspective effect. We measure this
shift by implementing a cross-spectral method similar to a technique
previously developed for stellar applications (Aime et al. 1984). The
signal-to-noise ratio is increased by averaging the cross spectra
over a large number of images. This technique allows us to measure
very small displacements, below the telescope resolution. <P />We
show results we obtained on HINODE/SP observations and compare them to
numerical simulations. The difference of formation depths between the
two line cores is determined with a very high accuracy and compares
quite well to LTE model calculation using either 1D solar model
or full 3D calculations in snapshots of the granulation. However
it shows significant variations in magnetic regions. The difference
between line core and continuum formation depths is more difficult to
measure precisely because line core and continuum images are not well
correlated, due to contrast inversion and depth-dependence of granular
structures. We solve this problem by measuring the perspective shifts
between similar enough images taken at successive steps along the
line profile, and by integrating the shifts from the continuum level
to the line center forming layer.
---------------------------------------------------------
Title: Measuring line formation depths by cross-spectral
analysis. Numerical simulations for the 630 nm Fe I line pair
Authors: Grec, C.; Uitenbroek, H.; Faurobert, M.; Aime, C.
2010A&A...514A..91G Altcode:
Context. Numerical three-dimensional simulations of the solar
photosphere have progressed greatly in the last 15 years. Observational
tests are needed to independently verify the realism of these
simulations. <BR /> Aims: We aim to measure the perspective shift
between monochromatic images at different wavelengths taken away from
disk center. We investigate the feasibility of our method by applying
it to simulated spectra of the Fe i line pair at 630.15 and 630.25 nm
calculated from several snapshots of a three-dimensional simulation
of solar magneto-convection. <BR /> Methods: We present a method
to determine line formation depths from spectroscopic observations
without relying on assumptions about an atmospheric model. Our method is
based on the measurement of a perspective shift, which is detected as a
linear phase term in the cross-spectrum of the images. In principle this
detection is independent of the spatial resolution of the observations,
and provides a valuable test for numerical simulations of the solar
photosphere. <BR /> Results: To obtain accurate formation heights we
need to correct spectra for convective Doppler shifts, and we need to
accumulate successive phase shifts between images in nearby wavelengths,
rather than compare images from the continuum and core directly. The
comparison of images provides large dissimilarities, which result from
the temperature contrast inversion in the granulation with height. We
verify that the cross-spectrum phase of the simulated images shows
the expected linear behavior with spatial frequency when considering
two close enough wavelengths in a spectral line profile. This linear
behavior is however only obtained at small spatial frequencies,
i.e. for large granular structures. Derived line formation heights of
the two lines range from 239 and 287 km above the continuum formation
height for the 630.15 nm line, and from 138 to 201 km for the 630.25
nm line, with significant variation between snapshots. Formation
height estimates from optical depth unity give on average 319 km and
244 km respectively. <BR /> Conclusions: Our numerical tests validate
measurements of line formation depths from cross-spectra between images
at different wavelengths and stress the value of measuring the phase
of the cross-spectra as an important test for numerical simulations.
---------------------------------------------------------
Title: Solar chromospheric and coronal magnetic structure: science
cases for Dome C
Authors: Faurobert, M.
2010EAS....40..467F Altcode:
Magnetic fields play a crucial role for the physics of the dilute and
hot upper parts of the solar atmosphere, from the low chromosphere up
to the outer corona. Numerical modeling of these regions by MHD codes
and 3D radiative transfer in spectral lines has started, we urgently
need direct measurement of the magnetic and velocity fields in order to
confirm and improve such modeling. Direct measurements of the magnetic
field vector rely on spectro-polarimetric observations of the Hanle
or Zeeman effects in spectral lines. In the upper solar atmosphere
the magnetic field strength tends to decrease with height and lines
are broaedened both thermally and by turbulent and wave motions. The
sensitivity of spectral lines to Zeeman effect is thus low under such
conditions, but it increases linearly with wavelength. The infrared
is the domain of choice for Zeeman observations in chromospheric
and coronal lines. Moreover, as chromospheric and coronal structures
develop on a broad range of spatial scales, from very fine to large
scales, high spatial resolution observations are required. These
observations are very demanding in photometric sensitivity, together
with spatial and temporal resolution. Dome C conditions of both very
good day-time seeing and high coronal quality are unique and make it
a site of choice for a possible breakthrough in our understanding of
the physical mechanisms at play in the outer solar atmosphere.
---------------------------------------------------------
Title: Direct measurement of the formation height difference of the
630 nm Fe I solar lines
Authors: Faurobert, M.; Aime, C.; Périni, C.; Uitenbroek, H.; Grec,
C.; Arnaud, J.; Ricort, G.
2009A&A...507L..29F Altcode:
Context: Spectral lines formed over a limited height range in either
a stellar or planetary atmosphere provide us with information about
the physical conditions within this height range. In this context,
an important quantity is the so-called line formation depth. It is
usually determined from numerical calculation of the atmospheric
opacity in the line of interest and then converted into geometrical
depth by using atmospheric models. <BR />Aims: We develop a radically
different approach, which allows us to measure directly line formation
depths from spectroscopic observations without relying on assumptions
about an atmospheric model. This method requires spatially resolved
observations, which up to now have been available only for solar
or planetary studies. We apply this method to images of the solar
granulation. <BR />Methods: The method was presented and tested
numerically in previous papers. It is based on the measurement
of the perspective shift between images at different wavelengths,
formed at different heights, when they are observed away from disk
center. Because of the Fourier transform properties, this shift gives
rise to a deterministic linear phase term in the cross spectrum of the
images. <BR />Results: The method is applied to observations of solar
quiet regions performed with the SOT spectropolarimeter on HINODE in
the Fe i line pair at 630.15 and 630.25 nm. We derive the difference
in formation heights between the two lines and its center-to-limb
variations. We show that the high sensitivity of the measurements allows
us to detect variations in the line formation heights between magnetized
and non-magnetized regions of the solar atmosphere. <BR />Conclusions:
Our results are the first direct measurements of line formation depths
in the solar photosphere. Cross spectral analysis provides us with a
new observable quantity, which may be measured with an accuracy well
bellow the spatial resolution of the observations. We recall that
the Fe i line pair at 630.15 and 630.25 nm is often used to determine
solar magnetic fields by spectropolarimetric observations and inversion
methods. The difference in the line formation heights that we measure
should be taken into account in the inversion procedures.
---------------------------------------------------------
Title: Non-LTE Modeling of the Ba II D2 Line Resonance Polarization
Authors: Faurobert, M.; Bommier, V.; Derouich, M.
2009ASPC..405...35F Altcode:
The Ba II resonance line at 455.4 nm is formed in the low solar
chromosphere. It shows significant linear polarization outside active
regions close to the solar limb. This so-called resonance polarization
is sensitive to the Hanle effect of weak magnetic fields. We report
on numerical simulations of the intensity and resonance polarization
center-to-limb variations in the line and in the adjacent continuum,
in a quiet solar atmosphere and we compare them to observations
performed at THEMIS in August 2007. In the simulations we take
into account non-LTE multilevel coupling, multiple scattering and
partial frequency redistribution, and we neglect the hyperfine
structure of the odd isotopes of Barium. As resonance polarization
and partial frequency redistribution effects are very sensitive to
elastic collisions with hydrogen atoms we use accurate depolarizing
collisional rates recently computed for this line by a semi-classical
method. Our radiative transfer calculations allow us to model the
central part of the line core and the wings quite well. We show that
the line polarization depends very much indeed on partial frequency
redistribution effects. Then we investigate its diagnostic potential
for weak unresolved magnetic fields in the low chromosphere. We
find that the observed polarization rates are in good agreement with
the simulations if we take into account the Hanle effect due to an
isotropic turbulent magnetic field of the order of 30 Gauss at the
altitude where the line core is formed, i.e. between 900 km and 1300
km above the basis of the photosphere.
---------------------------------------------------------
Title: Hanle effect in the solar Ba II D2 line: a diagnostic tool
for chromospheric weak magnetic fields
Authors: Faurobert, M.; Derouich, M.; Bommier, V.; Arnaud, J.
2009A&A...493..201F Altcode: 2008arXiv0811.1180F
Context: The physics of the solar chromosphere depends in a crucial way
on its magnetic structure. However there are presently very few direct
magnetic field diagnostics available for this region. <BR />Aims:
Here we investigate the diagnostic potential of the Hanle effect
on the Ba II D2 line resonance polarization for the determination
of weak chromospheric turbulent magnetic fields. <BR />Methods:
The line formation is described with a non-LTE polarized radiative
transfer model taking into account partial frequency redistribution
with an equivalent two-level atom approximation, in the presence of
depolarizing collisions and the Hanle effect. We investigate the line
sensitivity to temperature variations in the low chromosphere and
to elastic collision with hydrogen atoms. We compare center-to-limb
variations of the intensity and linear polarization profiles observed
at THEMIS in 2007 to our numerical results. <BR />Results: We show that
the line resonance polarization is very strongly affected by partial
frequency redistribution effects both in the line central peak and
in the wings. Complete frequency redistribution cannot reproduce the
polarization observed in the line wings. The line is weakly sensitive
to temperature differences between warm and cold components of the
chromosphere. The effects of elastic collisions with hydrogen atoms and
of alignment transfer due to multi-level coupling with the metastable
^2D{5/2} levels have been studied in a recent paper showing that they
depolarize the ^2P{3/2} level of the line. In the conditions where
the line is formed we estimate the amount of depolarization due to
this mechanism as a factor of 0.7 to 0.65. If we first neglect this
effect and determine the turbulent magnetic field strength required to
account for the observed line polarization, we obtain values between
20 G and 30 G. We show that this overestimates the magnetic strength
by a factor between 1.7 and 2. Applying these correction factors
to our previous estimates, we find that the turbulent magnetic field
strength is between 10 G and 18 G. <BR />Conclusions: Because of its low
sensitivity to temperature variations, the solar Ba II D2 line appears
as a very good candidate for the diagnosis of weak magnetic fields in
the low chromosphere (z ≥ 900 km) by means of the Hanle effect.
---------------------------------------------------------
Title: Apodized apertures for solar coronagraphy
Authors: Carlotti, Alexis; Aime, Claude; Arnaud, Jean; Faurobert,
Marianne; Ferrari, André; Grec, Catherine; Ricort, Gilbert
2008SPIE.7014E..15C Altcode: 2008SPIE.7014E..38C
An apodized aperture should make it possible to observe the solar corona
without the need of a Lyot coronagraph. We show in this communication
that Sonine functions are much better apodizers for the observation
of the solar corona than the generalized prolate spheroidal functions
previously proposed. For a perfect circular aperture of diameter
unity operated in space, a simple Sonine apodization of the form (1 -
4r<SUP>2</SUP>), with |r| <= 1/2 should sufficiently reduce the
diffraction halo produced by the solar disc to observe the corona
very close to the solar limb (a few arcsec). The throughput is just
one third of the clear aperture.
---------------------------------------------------------
Title: Apodized apertures for solar coronagraphy
Authors: Aime, C.; Arnaud, J.; Carlotti, A.; Faurobert, M.; Ferrari,
A.; Grec, C.; Ricort, G.
2007sf2a.conf..574A Altcode:
The solar corona cannot be studied without the help of a coronagraph. A
telescope with an apodized aperture is, as described by Aime (2007),
an alternative to the classical Lyot coronagraph. A spheroidal prolate
apodization will modify the PSF of the telescope and optimize the energy
concentration in the focal plane. A strong apodization (prolatness
parameter c ≈ 10) would reduce the diffraction halo by a factor
10^5 at a cost of intensity throughput reduced at 10 %. In a site with
outstanding daytime seeing, like Dome C, this method should allow to
observe the corona extremely close to the solar limb and also, much
better than otherwise, the rich chromospheric weak emission spectrum.
---------------------------------------------------------
Title: Resonance polarization of the solar 455.4 nm BaII line:
diagnostics of chromospheric magnetic fields
Authors: Michel, C.; Faurobert, M.; Arnaud, J.; Malherbe, J. M.
2007sf2a.conf..607M Altcode:
The BaII resonance line at 455.4 nm is formed in the low solar
chromopshere. It is significantly linearly polarized outside active
regions and close to the solar limb. This so-called resonance
polarization is sensitive to the Hanle effect of weak magnetic
fields. We report on numerical simulations of the intensity and
resonance polarization profiles in the line and in the adjacent
continuum, in the quiet solar atmosphere and we compare them to
observations performed at the Jean Rosch refractor at the Pic du Midi
Observatory. In the simulations we take into account non-LTE multilevel
coupling, multiple scattering and partial frequency redistribution, and
we neglect the hyperfine structure of the odd isotopes. This allows to
model the central part of the line core and the wings quite well. Then
we investigate the diagnostic potential of the line core polarization
for weak unresolved magnetic fields in the low chromosphere. We find
that the observed polarization rates are in good agreement with the
simulations if we take into account the Hanle effect of weak magnetic
fields on the order of 60 to 75 Gauss.
---------------------------------------------------------
Title: The Prolate Apodized Solar Coronagraph
Authors: Aimé, C.; Arnaud, J.; Carlotti, A.; Faurobert, M.; Ferrari,
A.; Grec, C.; Ricort, G.
2007lyot.confE..40A Altcode:
We present the project of a new solar coronagraph that makes it possible
to observe the solar corona very close to the solar limb, without
using Lyot's mask and stop technique. The high dynamic capability is
obtained using a strongly apodized aperture. A good choice for the
aperture transmission is the generalized prolate spheroidal function
of prolateness coefficient c on the order of 10. Such an instrument
operated in space, could reduce the diffraction halo produced by the
Sun by a factor 100 000, at the cost of an intensity throughput of 10
%. The classical resolution, in terms of equivalent width of the PSF,
is reduced by a factor of about 1.7 while the MTF of the telescope
becomes similar to a Gaussian function with unchanged cut-off frequency
D/lambda. The telescope design is an unobstructed circular aperture
and the variable transmission produced directly at the entrance
window. This concept, although demanding in terms of mechanical and
optical achievements, is preferred to the more classical re-imaging
of a clear aperture and subsequent apodisation, for image quality
concerns. It does not need a wide field telescope and may take advantage
of adaptive optics for ground based observations. It is expected that
such a system should give much better images than the classical Lyot
coronagraph very close to the solar limb. Such observations should
give access to coronal heating processes expected to occur close to
the solar surface, and provide information on Coronal Mass Ejections
mechanisms at work in the very low corona.
---------------------------------------------------------
Title: Differential speckle interferometry: in-depth analysis of
the solar photosphere
Authors: Grec, C.; Aime, C.; Faurobert, M.; Ricort, G.; Paletou, F.
2007A&A...463.1125G Altcode:
Aims:We present the results of an experiment performed at the solar
telescope THEMIS in 2002 to measure the depth over which the solar
granulation extends in the photosphere. <BR />Methods: Observations
made in the 523.3 nm and 557.6 nm photospheric non-magnetic iron lines
were correlated with images in the continuum using spectrograms. The
difference in depth between the different levels in the photosphere
is projected into a difference of position along the slit of the
spectrograph, using a perspective effect similar to the well-known
Wilson effect for sunspots. This requires measuring displacements,
ones much smaller than the telescope resolution. This is made
possible by using a differential speckle interferometric technique,
cross-correlating images taken in the continuum and the line. The
method is not adapted to following displacements of structures in
the core of strong lines, due to their difference in shapes with
the structures observed in the continuum. In this case, a sequential
cross-spectrum method is developed to cross-correlate images taken
at close wavelengths. <BR />Results: The raw results are surprising:
displacements measured in the blue and the red wings of a line have
opposite signs! North and South observations, however, clearly show
the expected behavior attributed to a perspective effect. After
a description of the observations, we give a first interpretation
of the results. The main part of the observed displacement comes
from the effect of unresolved Doppler shifts produced by horizontal
velocities in the solar photosphere. The perspective effect we seek
appears as a second-order term; we find that its amplitude is 2 or
3 times larger than predicted by theoretical 1D models. In the core
of strong lines we detect a contrast inversion that also shows up in
the cross-correlation function as an anti-correlation peak at line
center. <BR />Conclusions: .This first use of the differential speckle
interferometry technique on the Sun is quite promising for 3D studies
at high spatial resolution. Further observations with very good image
quality are needed to take advantage of this new technique. <P />THEMIS
is operated on the Island of Tenerife by CNRS-CNR in the Spanish
Observatorio del Teide of the Instituto de Astrofísica de Canarias.
---------------------------------------------------------
Title: Magnetic flux tubes observed with THEMIS/MSDP
Authors: Mein, P.; Mein, N.; Faurobert, M.; Aulanier, G.; Malherbe,
J. -M.
2007A&A...463..727M Altcode:
Aims:We use spectro-polarimetric THEMIS/MSDP data to investigate the 3D
structure of solar magnetic-flux tubes across the upper photosphere. <BR
/>Methods: Profiles of the sodium D1 line 589.6 nm are analysed by
the bisector method at different wavelengths from the core to the
wings, for several bright features. They are compared to synthetic
profiles derived from 2D magnetic models of flux tubes and from the
MULTI code for NLTE line profiles. Three different magnetic models of
flux tubes are investigated. Model (I) consists of a single flux tube
that compensates for the horizontal Lorentz forces exactly, while model
(II) uses a compromise between horizontal and vertical components. Model
(III), a conglomerate of thinner flux tubes, leads to the best agreement
with observations. <BR />Results: (1) The combination of seeing effects
(small filling factor) with slopes of line profiles, which are different
in the flux tubes and the neighbouring quiet sun, account for the
decrease in observed magnetic field from line core to line wings in
central parts of magnetic features, as well as the decrease in magnetic
fluxes integrated over the whole magnetic features. (2) The expansion
with height of single magnetic flux tubes (models I and II) accounts for
the increase in the size of magnetic features from line wings to line
core. (3) Pure thermodynamical criteria characterising Dopplershifts
and line-intensity fluctuations of magnetic and non-magnetic features
have been proven by observations. <BR />Conclusions: . We could account
for differential Zeeman effects along the D1 line profile by combining
expansion of flux tubes with height, low gas pressure inside flux tubes,
and small filling factor due to seeing effects. Better agreement with
observations, in particular with respect to magnetic field amplitudes,
will probably need 3D models that take velocity fields and horizontal
gradients of temperature into account.
---------------------------------------------------------
Title: Magnetic flux tubes observed with THEMIS/MSDP .
Authors: Mein, P.; Mein, N.; Faurobert, M.; Aulanier, G.; Malherbe,
J. -M.
2007MmSAI..78...92M Altcode:
We use 2D spectro-polarimetric data of the NaD1 line to investigate
magnetic flux tubes at several levels of the solar photosphere: <P />-
magnetic and non-magnetic bright features can be discriminated by simple
criteria of intensities and dopplershifts. <P />- 2D magnetic models
and NLTE line profiles are compared to observations : combination of
seeing effects and departures between slopes of line profiles in flux
tubes and neighbouring photosphere account for vertical gradients of
line-of-sight (LOS) magnetic field measurements. <P />- Best qualitative
agreements are obtained with clusters of magnetic flux tubes.
---------------------------------------------------------
Title: Dome C: An exceptional site for solar observations .
Authors: Arnaud, J.; Faurobert, M.; Fossat, E.
2007MmSAI..78..105A Altcode:
Dome C, on the Antarctica plateau, may be the best site on Earth
for astronomy, thanks to outstanding image quality and very pure and
cold atmosphere. This is of particular interest for solar physics,
namely for very high-resolution studies of the solar surface and
for magnetometry of the innermost solar corona. Here we review Dome
C unique atmospheric properties and present two projects aimed at
quantitatively qualify this site for solar observations.
---------------------------------------------------------
Title: Solar Coronagraphy at Dome C: Site Testing and Prospects
Authors: Arnaud, J.; Faurobert, M.; Grec, G.; Renaud, C.
2007EAS....25...81A Altcode:
Progress in our understanding of solar magnetism and activity rest to a
large part on our ability to improve spatial resolution for resolving
the solar surface magnetic fields fine structure. The solar corona
is permeated by magnetic fields emerging from the sun and, in turn,
ejects particules and magnetic field into the heliosphere. It is of
prime importance to measure the coronal magnetic field to understand
mechanisms at work in the corona. We explain why Dome C is expected to
be an outstanding site for such observations and describe the Solar
Brightness Monitor we prepare to probe sky background and aerosols
levels at this site. We also discuss coronal observations DomeC could
allow to obtain.
---------------------------------------------------------
Title: Contrast inversion in the 557.6 nm line detected with
differential speckle interferometry
Authors: Grec, C.; Aime, C.; Faurobert, M.; Ricort, G.; Paletou, F.
2007MmSAI..78...48G Altcode:
We report on some aspects of the use of a Differential speckle
interferometry technique on the Sun. The method consists in
cross-correlating images of the granulation taken in the line
absorption and in the continuum, outside the solar disk center. Due
to a perspective effect, the difference in depth between different
photospheric levels results in a difference in position along the
spectrograph slit. Observations were done in 2002, 2005 and 2006, at
the telescope THEMIS in the 557.6 nm iron line. As expected from the
perspective effect, we obtain opposite results at opposite latitudes
on the Sun disk. Surprisingly, the displacements measured in the blue
and the red wings of the line have opposite signs. This may be the
result of unresolved Doppler shifts produced by horizontal granular
velocity fields. We also detect an anti-correlation peak in the core
of strong lines, that is the signature of a contrast inversion.
---------------------------------------------------------
Title: Preliminary results from the March 29, 2006 total eclipse
observations in Egypt
Authors: Koutchmy, S.; Daniel, J. -Y.; Mouette., J.; Vilinga, J.;
Noëns, J. -C.; Damé, L.; Faurobert, M.; Dara, H.; Hady, A.; Semeida,
M.; Sabry, M.; Domenech, A.; Munier, J. -M.; Jimenez, R.; Legault,
Th.; Viladrich, Ch.; Kuzin, S.; Pertsov, A.; O. A. Team
2006sf2a.conf..547K Altcode:
A coordinated effort has been carried in the framework of
the French-Egyptian scientific cooperation to permit joined
simultaneous eclipse observations of the solar corona during the
total solar eclipse of March 29, 2006. Spaceborne EIT and Lasco (SoHO)
observations were also planned at the same time and were successfully
collected. Scientists from other countries collaborated on different
experiments. The synthetic image showing the magnetic coronal structure
of this quasi-minimum corona seen in W-L is given. Some preliminary
results are presented; a White Light (W.L.) movie has been also taken
during the totality.
---------------------------------------------------------
Title: Solar Magnetometry: recent advances
Authors: Faurobert, M.
2005sf2a.conf....7F Altcode:
Magnetic fields play a crucial role in the phenomena related to solar
and stellar activity. However the determination of magnetic fields in
astrophysical plasma is very often a difficult task, which relies on
spectro-polarimetric methods. The PNST (Programme National Soleil-Terre)
supports the collaboration of several groups investigating solar
magnetic fields with various methods. I present some results recently
obtained by PNST groups using dedicated instruments, such as THEMIS.
---------------------------------------------------------
Title: Coronal Magnetometry
Authors: Arnaud, J.; Faurobert, M.; Grec, G.; Vial, J. -C.
2005EAS....14...95A Altcode:
Magnetic fields emerging in the solar atmosphere control the structure,
dynamics and heating of the solar corona. Those fields remain
essentially unattainable with present low corona instrumentation. We
present the most direct way of magnetic field remote measurements
in the internal corona, namely visible and infrared coronal emission
lines magnetometry and we explain why Dome C should be investigated as
a likely outstanding site for such observations. We develop a strategy
towards a large aperture coronal magnetometer at Dome C in the context
of the other main instrumental projects underway for solar physics.
---------------------------------------------------------
Title: Center-to-limb variation of scattering polarization in
molecular solar lines: Observations and modeling
Authors: Faurobert, M.; Arnaud, J.
2003A&A...412..555F Altcode:
We present center-to-limb measurements of the scattering polarization
observed in molecular lines of C<SUB>2</SUB> and MgH in the spectral
range between 515.7 and 516.1 nm, together with a radiative transfer
model for the formation of these lines. The observations were performed
in July 2000 with THEMIS inside the south polar limb. We were able
to measure the polarization at distances between 1 arcsec and 50
arcsec from the solar limb. The lines appear as very weak absorption
features in the intensity spectrum but their linear polarization
clearly dominates in the polarization spectrum. We introduce here
a simple radiative transfer model which allows to interpret the
observed center-to-limb variations of both the intensity and linear
polarization. The basic assumption is that molecular lines are
formed higher in the photosphere than the continuous photospheric
radiation. Molecules are thus illuminated by the polarized continuum
photospheric radiation field. We account for a possible Hanle effect
due to weak unresolved magnetic fields but we neglect depolarizing
collisions. The model depends on four parameters which are determined
by fitting both the intensity and polarization in 9 molecular lines of
the observed spectral domain. Making use of the differential Hanle
effect in the different lines of C<SUB>2</SUB> we show that the
C<SUB>2</SUB> lines are affected by the Hanle effect due to a weak
unresolved magnetic field. Its mean strength is on the order of 15
Gauss in the upper photosphere and increases to values on the order of
50 Gauss at larger depths. These results are in good agreement with
those derived previously (Faurobert et al. \cite{Faurobert1}) from
the linear polarization of the SrI 460.7 nm line which was observed
simultaneously. Such a weak field has almost no effect on the MgH lines.
---------------------------------------------------------
Title: Molecular lines observations with THEMIS
Authors: Arnaud, J.; Faurobert, M.; Raouafi, N. -E.; Solanki, S. K.
2003sf2a.conf..111A Altcode: 2003sf2a.confE..39A
Lines of molecules like MgH, C2, TiO, ... represent powerful tools
for the study of the solar atmosphere. Their observations on the
Sun can also help to constrain the polarizability and Lande factors
calculations which are much more complex than for atomic lines. We will
present THEMIS polarimetric observations of these lines to illustrate
some aspects of their use for solar physics.
---------------------------------------------------------
Title: Diagnostics of Solar Magnetic fields based on the Hanle effect
Authors: Faurobert, M.
2003EAS.....9...77F Altcode:
The Hanle effect was first used in solar physics to determine weak
magnetic fields in prominences (see Leroy [CITE]). More recently the
development of a new generation of very accurate spectro-polarimeters
has open a new window for the diagnostics of weak magnetic fields in
the solar photosphere and chromosphere, based on the Hanle effect. I
present here such recent research programs performed with the solar
telescope THEMIS and with other polarimeters such as ZIMPOL and the
LOCARNO polarimeter.
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Title: Second Solar Spectrum Observations at THEMIS
Authors: Faurobert, M.; Arnaud, J.
2003ASPC..307..431F Altcode:
No abstract at ADS
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Title: Hanle effect with angle-dependent partial redistribution
Authors: Nagendra, K. N.; Frisch, H.; Faurobert, M.
2002A&A...395..305N Altcode:
The polarized line transfer equation for the Hanle effect is solved in
the framework of an exact partial frequency redistribution (PRD) theory
developed by Bommier (1997a,b). In that theory the effect of collisions
on the Hanle effect is considered self-consistently. We follow that
approach in the line transfer computations presented here. The theory
formulated by Bommier clearly recognizes two levels of approximations
for exact PRD, in order to facilitate the solution of the line transfer
equation. The second level employs angle-dependent redistribution
functions, and numerically represents a more difficult problem compared
to the third level, which involves only the use of angle-averaged
frequency redistribution functions. We present a method which can
solve the problem in both the levels of approximation. The method
is based on a perturbative approach to line polarization. Although
computationally expensive, it offers the only practical means of solving
the angle-dependent Hanle PRD problem. We discuss the numerical aspects
of assembling the so called “frequency domain dependent redistribution
matrices”, and also an efficient way of computing the scattering
integral. Some examples are presented to illustrate the interesting
aspects of the Hanle-PRD problem with angle-dependent frequency
redistribution. A comparison of the emergent profiles computed under
angle-averaged and angle-dependent redistribution is carried out, and
the effect of collisions is investigated. We show that it is necessary
to incorporate an angle-dependent redistribution mechanism especially
in the computation of the Stokes U parameter. We demonstrate that the
use of simple frequency domains is good enough in practical applications
of the Hanle PRD theory.
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Title: Scattering polarization of molecular emission lines in the
quiet solar chromosphere
Authors: Faurobert, M.; Arnaud, J.
2002A&A...382L..17F Altcode:
We present scattering polarization measurements performed with THEMIS
in July 2000 near the south polar limb. The low level of scattered
light at THEMIS, which is the only large solar telescope to include
a superpolished primary mirror, allows, in good seeing conditions,
to observe the emission spectrum of the low chromosphere above
the limb. These are, as far as we know, the first high spectral
resolution observations of the intensity and the first measurements
of the polarization of C<SUB>2</SUB> and MgH molecular lines in
emission above the limb. Molecules are present in a thin layer in
the region of the temperature minimum between the photosphere and
the chromosphere. We present a very simple model for the formation
of the polarized lines and we relate the observed polarization
rates to the so-called intrinsic line polarizability coefficients
W<SUB>2</SUB>. Those quantities may be derived from quantum mechanical
computations involving the solution of the Schrödinger equation for
the molecular potential. Solar observations provide a direct way of
checking these heavy computations and contribute to the improvement
of our knowledge in molecular physics. Nine C<SUB>2</SUB> molecular
transitions and two MgH transitions are present in our spectral window;
we find that for the C<SUB>2</SUB> transitions, the polarizability
is between 0.13 and 0.26 and that it takes higher values, namely 0.41
and 0.46, for the two MgH transitions.
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Title: Polarimetric Imaging of The Solar Corona During The 21 June
2001 Total Solar Eclipse
Authors: Habbal, S. R.; Arnaud, J.; Arndt, M.; Ene, A.; Esser, R.;
Faurobert, M.; Hale, J.; Hegwer, S.; Johnson, J.; Woo, R.
2002EGSGA..27.6098H Altcode:
We report on the first successful simultaneous eclipse measurements
of the inten- sity and polarization brightness of the K-Corona and the
near-infrared Fe XIII 1074.7 nm emission line. Those observations were
obtained during the total solar eclipse of 21 June 2001. The technique
used for those observations will be presented. Coronal emission lines
polarization measurements are the only tools to date that can yield
the direction of the coronal magnetic field.
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Title: Vertical structure of sunspots from THEMIS observations
Authors: Eibe, M. T.; Aulanier, G.; Faurobert, M.; Mein, P.; Malherbe,
J. M.
2002A&A...381..290E Altcode:
We have analysed two-dimensional spectro-polarimetric data taken
with the MSDP observing mode of THEMIS in the Na I D<SUB>1</SUB>
line to investigate the height variation of the magnetic field
in sunspot umbrae. From the Zeeman-induced circular polarization
measured at individual MSDP channels within the line profile, maps of
the longitudinal magnetic field have been computed. A method based
on Response Functions has been developed to estimate the depth in
the atmosphere at which the Zeeman measurements are originated,
thus providing the line-of-sight field at different altitudes
in the photosphere. The magnetogram corresponding to the deepest
level has served as a boundary condition to perform the potential
field extrapolation into the corona. We have found that the spatial
distribution of vertical field gradient contours predicted from
extrapolation is in qualitatively good agreement with that inferred from
observations. Quantitatively, however, the longitudinal field gradients
obtained with both methods differ about one order of magnitude, being
larger for observations. The origin of this discrepancy has been
discussed with respect to possible observation biases, as well as to
idealizations used for field extrapolation. This is a crucial problem
to be addressed in future work, and may have important implications
for the physics of how the magnetic field evolves through sunspots
and how the flux is distributed in the corona.
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Title: Polarized Intensity Measurements of the Corona during the 21
June 2001 Total Solar Eclipse
Authors: Habbal, S. R.; Arnaud, J.; Johnson, J.; Hegwer, S.; Ene, A.;
Hale, J.; Esser, R.; Arndt, M.; Kohl, J. L.; Daw, A.; Faurobert, M.;
Woo, R.; Habbal, F.; Havasy, R.; Alford, J. N.
2001AGUFMSH11C0716H Altcode:
We report on the first successful simultaneous polarimetric measurements
of the brightness of the Thompson-scattered white light and intensity
of the near-infrared Fe XIII 10747 Å line, the strongest of the
coronal iron forbidden lines. These observations which extended out
to 3 R<SUB>s</SUB> in the corona were obtained during the total solar
eclipse of 21 June 2001. The novel technique used to acquire these
measurements will be presented. Polarized intensity measurements of the
resonantly scattered component of coronal emission lines are the only
tools to date that can yield the direction of the coronal magnetic
field. Through these simultaneous measurements, we show how the
direction of the coronal magnetic field can be placed in the context
of coronal density structures. We also discuss the implications of
these simultaneous measurements for the source of the solar wind.
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Title: Investigation of weak solar magnetic fields. New observational
results for the SrI 460.7 nm linear polarization and radiative
transfer modeling
Authors: Faurobert, M.; Arnaud, J.; Vigneau, J.; Frisch, H.
2001A&A...378..627F Altcode:
Scattering polarization measurements were obtained with THEMIS in July
2000, close to the solar south Pole and to the east Equator and in
a period of maximum solar activity. Using the THEMIS multi-lines
spectro-polarimetric mode (MTR), we observed simultaneously
four spectral domains containing the 460.7 nm Sr i line, several
molecular lines around 515.9 nm and the Na i D<SUB>1</SUB> and Na i
D<SUB>2</SUB> lines. This allows us to scan different altitudes in the
solar atmosphere at the same time and provides us with a large set
of constraints to study the behaviour of the magnetic field. This
paper is devoted to the Sr i line which exhibits quite a strong
linear polarization peak outside active regions. A detailed radiative
transfer modeling is performed in order to interpret the observed
center-to-limb variations of the line intensity and polarization. It
was shown previously (Faurobert-Scholl \cite{Faurobert-Scholl1})
that this line, which is sensitive to the Hanle effect, can be used
as a diagnostic tool for the presence of weak turbulent magnetic
fields in the solar photosphere outside active regions. The line
polarization rates that we measured in July 2000 are 25% lower
than what has been reported previously, for observations near the
minimum, or in the increasing phase, of the activity cycle (Stenflo
et al. \cite{Stenflo1}). They are in agreement with other observations
performed with a different observational set-up in August 2000 (Bommier
& Molodij \cite{Bommier4}). We show that they are consistent with
the presence of a weak turbulent magnetic field with an average strength
between 20 G and 30 G in the upper solar photosphere. This is about
twice the value which was derived from previous observations. This
result raises the possiblity of a long-term variation of the turbulent
photospheric magnetic field with the activity cycle.
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Title: Investigation of temperature and velocity fluctuations through
the solar photosphere with the Na I D lines
Authors: Eibe, M. T.; Mein, P.; Roudier, Th.; Faurobert, M.
2001A&A...371.1128E Altcode:
In this work we explore the diagnostic properties of the Na I D
resonance lines by calculating the response functions of their line
profiles to temperature and velocity perturbations in the atmosphere. We
propose a method to transform spectral line intensity fluctuations
measured at several wavelengths into temperature and velocity
disturbances at different height levels. Results from tests done with
several theoretical models of perturbations are discussed. Perturbations
that vary as linear functions of depth are efficiently reproduced. The
method also provides good estimations for exponential models. As an
example we present an application to solar granulation data obtained
with Multichannel Subtractive Double Pass (MSDP) spectroscopy.
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Title: Temporal height properties of the exploding granules
Authors: Roudier, Th.; Eibe, M. T.; Malherbe, J. M.; Rieutord, M.;
Mein, P.; Mein, N.; Faurobert, M.
2001A&A...368..652R Altcode:
Based on time series of 2D MSDP spectrograms, taken at the Turret Dome
in Pic du Midi, we present the temporal evolution of exploding granules
in intensity and Doppler velocity through the solar photosphere. We
describe the penetration of exploding granules in the solar photosphere
during their lifes and the related phenomena like the “Bright Plumes”
located in the downflowing plasma just on the edge of the granule. We
suggest a possible scenario of the exploding granule evolution in the
solar photosphere.
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Title: Scattering Polarization and Hanle Effect: On the Importance
of Angle-Dependent Frequency Redistribution
Authors: Faurobert, M.; Frisch, H.; Nagendra, K. N.
2001ASPC..248..145F Altcode: 2001mfah.conf..145F
No abstract at ADS
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Title: Scattering Induced Polarization and Hanle Effect Observations
with THEMIS
Authors: Arnaud, J.; Faurobert, M.; Vigneau, J.; Paletou, F.
2001ASPC..248...93A Altcode: 2001mfah.conf...93A
No abstract at ADS
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Title: Using the Na I D resonance lines to probe the solar photosphere
Authors: Eibe, M. T.; Mein, P.; Faurobert, M.; Roudier, Th.
2001ESASP.464..199E Altcode: 2001soho...10..199E
Observational techniques in helioseismology are often based on
measurements made in the Na I D resonance lines. A good knowledge of
their spectral profiles is crucial for the purposes of calibration and
interpretation of data. Here we explore their diagnostic properties by
calculating response functions of the D<SUB>1</SUB> and D<SUB>2</SUB>
line profiles to temperature and velocity perturbations in the
atmosphere. We propose a method to transform spectral line intensity
fluctuations measured at several wavelenths into temperature and
velocity disturbances at different height levels. The possibilities
of such a procedure to trace the vertical structure of the photosphere
are discussed.
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Title: The Hanle Effect with Angle Dependent Redistribution Functions
Authors: Frisch, H.; Faurobert, M.; Nagendra, K. N.
2001ASPC..236..197F Altcode: 2001aspt.conf..197F
No abstract at ADS
---------------------------------------------------------
Title: Scattering Polarization Measurements with THÉMIS
Authors: Arnaud, J.; Vigneau, J.; Faurobert, M.; Paletou, F.
2001ASPC..236..151A Altcode: 2001aspt.conf..151A
No abstract at ADS
---------------------------------------------------------
Title: Hanle Effect of Weak Solar Magnetic Fields
Authors: Faurobert, M.
2000ASPC..205..156F Altcode: 2000ltse.conf..156F
The Hanle effect provides a diagnostic tool for weak magnetic fields,
which do not give rise to a measurable Zeeman effect, such as turbulent
fields or magnetic canopies in the chromosphere. The lines which are
sensitive to the Hanle effect are formed under non-LTE conditions
by scattering of photons. After a brief description of the physical
mechanism at hand, I present some approximate expressions for the
linear polarization of such lines in the presence of a weak magnetic
field and show how the Hanle effect may be used for the diagnostics
of magnetic canopies.
---------------------------------------------------------
Title: Investigating the Vertical Structure of the Solar Granulation
with the Sodium D<SUB>2</SUB> Line
Authors: Eibe, M. T.; Roudier, Th.; Mein, P.; Faurobert, M.
2000ESASP.463..309E Altcode: 2000sctc.proc..309E
No abstract at ADS
---------------------------------------------------------
Title: Fast Numerical Methods for Polarized Line Radiative Transfer
in the Presence of Hanle Effect
Authors: Faurobert, M.; Frisch, H.; Nagendra, K. N.
1999ASPC..184...28F Altcode:
The Hanle effect provides a diagnostic tool for weak magnetic fields
which do not give rise to a measurable Zeeman effect, such as turbulent
fields or magnetic canopies. The lines which are sensitive to the
Hanle effect are formed under non-LTE conditions, by scattering of
photons. Inversion methods for such diagnostics require to solve the
non-LTE polarized transfer equation for a large number of magnetic
configurations. Fast numerical methods are thus highly required. We
present an Approximate Lambda Iteration method to treat the Hanle effect
for lines formed with complete frequency redistribution. Referred to
as PALI-H, this method is an extension of ALI methods first developed
for non polarized line transfer. The starting point is to recast the
polarized transfer equation into a vectorial integral equation for a
6-component source function. We show that the convergence of the method
is independent of the strength and direction of the magnetic field. The
method is very fast and allows to handle any type of depth-dependent
magnetic field.
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Title: An operator perturbation method for polarized line transfer IV:
Applications to the Hanle effect with partial frequency redistribution
Authors: Nagendra, K. N.; Paletou, F.; Frisch, H.; Faurobert-Scholl, M.
1999ASSL..243..127N Altcode: 1999sopo.conf..127N
No abstract at ADS
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Title: Polarized radiation transfer in 2D geometry
Authors: Paletou, F.; Bommier, V.; Faurobert-Scholl, M.
1999ASSL..243..189P Altcode: 1999sopo.conf..189P
No abstract at ADS
---------------------------------------------------------
Title: Polarized redistribution matrix for Hanle effect: Numerical
tests
Authors: Faurobert-Scholl, M.; Paletou, F.; Bommier, V.
1999ASSL..243..115F Altcode: 1999sopo.conf..115F
No abstract at ADS
---------------------------------------------------------
Title: An operator perturbation method for polarized line
transfer. III. Applications to the Hanle effect in 1D media
Authors: Nagendra, K. N.; Frisch, H.; Faurobert-Scholl, M.
1998A&A...332..610N Altcode:
In this paper we present an Approximate Lambda Iteration method to
treat the Hanle effect (resonance scattering in the presence of
a weak magnetic field) for lines formed with complete frequency
redistribution. The Hanle effect is maximum in the line core and
goes to zero in the line wings. Referred to as PALI-H, this method is
an extension to non-axisymmetric radiative transfer problems of the
PALI method presented in Faurobert-Scholl et al. (1997), hereafter
referred to as Paper I. It makes use of a Fourier decomposition of the
radiation field with respect to the azimuthal angle which is somewhat
more general than the decomposition introduced in Faurobert-Scholl
(1991, hereafter referred to as FS91). The starting point of the
method is a vector integral equation for a six-component source vector
representing the non-axisymmetric polarized radiation field. As
in Paper I, the Approximate Lambda operator is a block diagonal
matrix. The convergence rate of the PALI-H method is independent
of the polarization rate and hence of the strength and direction
of the magnetic field. Also this method is more reliable than the
perturbation method used in FS91. The PALI-H method can handle any
type of depth-dependent magnetic field. Here it is used to examine
the dependence of the six-component source vector on the co-latitude,
azimuthal angle and strength of the magnetic field. The dependence of
the surface polarization on the direction of the line-of-sight and on
the magnetic field is illustrated with polarization diagrams showing
Q/I versus U/I at line center. The analysis of the results show that
the full six-dimension problem can be approximated by a two-component
modified resonance polarization problem, producing errors of at most
20% on the surface polarization at line center.
---------------------------------------------------------
Title: An operator perturbation method for polarized line
transfer. II. Resonance polarization with partial frequency
redistribution effects
Authors: Paletou, Frederic; Faurobert-Scholl, Marianne
1997A&A...328..343P Altcode:
The effects of partial frequency redistribution are implemented in the
Polarized Accelerated Lambda Iteration (PALI) method of Faurobert-Scholl
et al. (1997). The numerical scheme is an extension of the core-wing
technique of Paletou & Auer (1995) originally developed for
non-polarized line transfer problems. Using a new code, we validate
theoretical results against those given by a Feautrier type code.
---------------------------------------------------------
Title: An operator perturbation method for polarized line
transfer. I. Non-magnetic regime in 1D media.
Authors: Faurobert-Scholl, M.; Frisch, H.; Nagendra, K. N.
1997A&A...322..896F Altcode:
In this paper we generalize an Approximate Lambda Iteration (ALI)
technique developed for scalar transfer problems to a vectorial transfer
problem for polarized radiation. Scalar ALI techniques are based on a
suitable decomposition of the Lambda operator governing the integral
form of the transfer equation. Lambda operators for scalar transfer
equations are diagonally dominant, offering thus the possibility to
use iterative methods of the Jacobi type where the iteration process
is based on the diagonal of the Lambda operator (Olson et al., 1986,
JQSRT 35, 431). Here we consider resonance polarization, created by
the scattering of an anisotropic radiation field, for spectral lines
formed with complete frequency redistribution in a 1D axisymmetric
medium. The problem can be formulated as an integral equation for a
2-component vector (Rees, 1978PASJ...30..455R) or, as shown by Ivanov
(1995A&A...303..621I), as an integral equation for a (2x2) matrix
source function which involves the same generalized Lambda operator as
the vector integral equation. We find that this equation holds also in
the presence of a weak turbulent magnetic field. The generalized Lambda
operator is a (2x2) matrix operator. The element {11} describes the
propagation of the intensity and is identical to the Lambda operator of
non-polarized problems. The element {22} describes the propagation of
the polarization. The off-diagonal terms weakly couple the intensity and
the polarization. We propose a block Jacobi iterative method and show
that its convergence properties are controlled by the propagator for
the intensity. We also show that convergence can be accelerated by an
Ng acceleration method applied to each element of the source matrix. We
extend to polarized transfer a convergence criterion introduced by
Auer et al. (1994A&A...292..599A) based on the grid truncation
error of the converged solution.
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Title: On the use of CA I triplet lines as luminosity indicators.
Authors: Cayrel, R.; Faurobert-Scholl, M.; Feautrier, N.; Spielfieldel,
A.; Thevenin, F.
1996A&A...312..549C Altcode:
Many strong lines formed in stellar atmospheres are well known to
be dominated by pressure broadening, therefore their wings are a
precious help for determining the surface gravities of dwarf or
subgiant stars. In this paper, we explore the possibility for using
the wings of the Ca I triplet lines 6102, 6122, 6162A for which recent
theoretical computations of the damping are available. A NLTE code for
radiative transfer was used to check these new computations against
the integrated Solar Spectrum. We report here that (i) there is a
spectacular improvement in the fit of the computed profile with the
observed one when the new quantum mechanical computations are used
instead of the classical Unsoeld's expression of the Van der Waals
damping constant (ii) the departures from LTE in the computation of
the wings of these lines are negligible, and this justifies the use
of LTE computations for the determination of the surface gravities of
G type stars, by this method.
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Title: Transfert de rayonnement hors-ETL et applications en physique
solaire.
Authors: Faurobert-Scholl, M.; Paletou, F.; Frisch, H.
1996JAF....53...24F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Diagnostics with the Hanle Effect
Authors: Faurobert-Scholl, M.
1996SoPh..164...79F Altcode:
The Hanle effect has been extensively used for the determination of
the magnetic field strength and direction in solar prominences. Here
we address the problem of the diagnostics of weak magnetic fields in
the solar photosphere and chromosphere by means of their Hanle effect
in some selected absorption lines. As this is a relatively new area
we will focus on the diagnostic methods and summarize some results
that relate to the presence of a weak, turbulent magnetic field in the
photosphere and to the chromospheric magnetic canopy. Finally we will
outline some directions for future work.
---------------------------------------------------------
Title: Turbulent magnetic fields in the solar photosphere: diagnostics
and interpretation.
Authors: Faurobert-Scholl, M.; Feautrier, N.; Machefert, F.; Petrovay,
K.; Spielfiedel, A.
1995A&A...298..289F Altcode:
Turbulent magnetic fields in the solar photosphere may be determined
by their depolarizing Hanle effect on the linear polarization of
some solar absorption lines formed outside active regions (Stenflo
1982). In Faurobert-Scholl (1993) the center-to-limb variation of
the linear polarization in the SrI 4607 A line was analyzed. It was
shown that it is affected by the Hanle effect due to a weak magnetic
field with mixed polarity at small scales. The accurate determination
of the turbulent magnetic field strength requires radiative transfer
calculations taking into account the effect of collisions and magnetic
fields on the frequency and angular redistribution of the light. Here
we present the different steps in the theory of redistribution,
together with the first precise quantum calculations of the collisional
cross-sections between hydrogen and SrI and CaI atoms. These new results
are then used to perform a more accurate determination of the turbulent
magnetic field strength in the solar photosphere. It is shown that the
center-to-limb variation of the linear polarization in the SrI line
is mainly sensitive to the average value of the field strength between
the altitudes 200 and 400km. According to the observations this average
value is between 20 and 10G. A theoretical interpretation is given in
terms of a one-dimensional passive magnetic field transport equilibrium
model with first-order smoothing. The resulting <|B_x_|>(z)
equilibrium distribution reproduces the observations rather well, but
the uncertainty in the amplitude, height-dependence and interpretation
of microturbulence suggests that the present model could be brought to
an even better agreement with the observations if a microturbulence
different from the VAL3C model is used. It is finally pointed out
that in the upper photosphere the turbulent field becomes force-free
instead of being passively transported. It is found that this does
not seriously modify our conclusions.
---------------------------------------------------------
Title: Hanle effect of magnetic canopies in the solar chromosphere
Authors: Faurobert-Scholl, M.
1994A&A...285..655F Altcode:
The center-to-limb variations of the linear polarization observed by
Stenflo et al. (1980) outside active regions, in the resonance lines
of CaI at 4227 A and of SrII at 4078 A, are analyzed. Both lines
are formed in the low chromosphere by multiple scattering. Their
linear polarization is a consequence of the coherent scattering of
the anisotropic radiation field by the atoms. It is very sensitive to
partial frequency redistribution, depolarizing collisions, turbulent
velocity fields and to the Hanle effect due to weak magnetic
fields. The rates of depolarizing collisions may be determined
by fitting the polarization profiles observed in the line wings,
which are not affected by the Hanle effect. The quadratic mean of
the turbulent velocity field is adjusted in order to recover the
observed line core intensity. It is then possible to estimate the
resonance polarization in the line cores. These estimates are in good
agreement with the observations performed close to the solar limb (at
heliocentric angles larger than about 70deg). Closer to disk center
the observed polarization rates are significantly larger than what is
expected in the absence of a magnetic field. We investigate the Hanle
effect of an almost horizontal magnetic field of random azimuthal
orientation lying in the chromosphere, which may constitute the base
of a magnetic canopy. If the base of the canopy lies slightly below
the depth where the line core is formed, i.e. between 700km and 1000km
above τ_5000_=1, then the line core polarization close to disk center
is actually enhanced as compared to its non-magnetic value. This
surprising effect is a consequence of multiple scattering of line
photons in the chromosphere. The enhancement is however somewhat
smaller than what is required to fit the observations. In contrast,
the polarization is reduced when the canopy base lies in the region
where the line core is formed, i.e. between 1000km and 1300km above
τ_5000_=1; it becomes insensitive to any horizontal magnetic field
lying higher in the chromosphere.
---------------------------------------------------------
Title: Diagnostics of turbulent magnetic fields in the solar
photosphere
Authors: Faurobert-Scholl, M.
1994smf..conf..331F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Diagnostics of Magnetic Canopies in the Solar Chromosphere
Authors: Faurobert-Scholl, M.
1994emsp.conf...69F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Investigation of microturbulent magnetic fields in the solar
photosphere by their Hanle effect in the Sr I 4607 angstroms line.
Authors: Faurobert-Scholl, M.
1993A&A...268..765F Altcode:
A detailed analysis is made of the center-to-limb variations of the
Sr I 4607 A line, recorded by Stenflo et al. (1980). The effect of
multiple scattering, depolarizing collisions, and partial frequency
redistribution are fully taken into account. We find that the observed
polarization rates are hard to explain without depolarizing effects,
which following Stenflo (1982) we ascribe to a weak turbulent magnetic
field producing depolarization by Hanle effect. As the spatial scale of
a weak turbulent magnetic field is likely to be smaller than 300 km, the
effect of the turbulent magnetic field is treated in the microturbulent
limit. In the transfer equation for the Stokes parameters, the Hanle
phase matrix is thus replaced by its average over the magnetic field
distribution. We show that the center-to-limb observations of the Sr
I 4607 A line polarization indicate the presence in the photosphere
of a depth-dependent turbulent magnetic field. Its intensity decreases
from values in the range 30 G to 60 G at the altitude z = 150 km above
tau 5000 = 1, to values in the range l0 G to 30 G at the altitude z =
250 km. It then remains roughly constant at higher altitudes.
---------------------------------------------------------
Title: Hanle effect with partial frequency redistribution. II -
Linear polarization of the solar CA I 4227 A line
Authors: Faurobert-Scholl, M.
1992A&A...258..521F Altcode:
An interpretation is presented of observations of the linear
polarization in the resonance line of Ca I at 4227 A over solar
active and nonactive regions performed by Stenflo (1982) and Stenflo
et al. (1980). Theoretical polarization profiles of the Ca I line
are calculated by solving a non-LTE polarized transfer problem with
a two-level atom interative method which takes into account partial
frequency redistribution and the Hanle effect. For Ca I depolarization
is caused by collisions with neutral hydrogen atoms, and the collision
rate is determined by gamma(vw), the van der Waals coefficient of
neutral calcium. Comparisons of the theoretical line wing polarization
with observational data from nonmagnetic regions suggest an empirical
value for gamma(vw) which is in agreement with the results of previous
theoretical calculations.
---------------------------------------------------------
Title: Hanle effect with partial frequency
redistribution. I. Numerical methods and first applications.
Authors: Faurobert-Scholl, M.
1991A&A...246..469F Altcode:
The Hanle effect on optically thick resonance lines formed with partial
frequency redistribution is investigated. In order to take into account
partial frequency redistribution, two numerical methods were used for
solving directly the vectorial transfer equation with Hanle effect
for a two-level atom in a 1D medium: (1) a generalization of the
Feautrier method and (2) a method that relies on a Fourier expansion
of the radiation field with respect to the azimuth of the direction
of propagation. Complete and partial frequency redistribution results
are compared, and the effect of partial frequency redistribution on
the determination of magnetic fields by means of the Hanle effect
is evaluated.
---------------------------------------------------------
Title: Quelques problèmes de transfert en physique solaire.
Authors: Frisch, H.; Faurobert-Scholl, M.
1991sed..conf..151F Altcode:
(1) Introduction. (2) Formation ETL des raies: (2.1) Intensité
du rayonnement et équation de transfert. (2.2) Approximation
de diffusion. (2.3) Equation de transfert ETL pour les raies. (3)
Formation non-ETL des raies: (3.1) Fonction source d'un atome à deux
niveaux. (3.2) Fonction de redistribution. (3.3) Equation de transfert
non-ETL. (3.4) Analyse asymptotique. (3.5) Transfert non-ETL dans
une couche plane. (3.6) Probabilité d'échappement. (3.7) Méthodes
numériques en transfert non-ETL. (3.8) Construction de modèles
d'atmosphères. (4) Transfert de rayonnement polarisé. (4.1)
Les paramètres de Stokes. (4.2) Mécanismes de polarisation des
raies spectrales. (4.3) Equation de transfert d'un rayonnement
polarisé. (4.4) Effet Hanle. (4.5) Effet Zeeman.
---------------------------------------------------------
Title: Hanle Effect with Partial Frequency Redistribution
Authors: Faurobert-Scholl, M.
1990PDHO....7..264F Altcode: 1990dysu.conf..264F; 1990ESPM....6..264F
The author investigates the Hanle effect on optically thick lines formed
by multiple scattering of photons. In particular the importance of
the correlation between the frequencies of the absorbed and reemitted
photons at each scattering event (partial frequency redistribution)
is outlined.
---------------------------------------------------------
Title: Asymptotic analysis of resonance polarization and escape
probability approximations
Authors: Faurobert-Scholl, M.; Frisch, H.
1989A&A...219..338F Altcode:
Polarized transfer asymptotic and first order escape probability methods
developed for the nonpolarized case are generalized to include linear
polarization produced by the scattering of anisotropic radiation in
the absence of magnetic fields. The analyses are based on a coupled
integral equation for two-angle-dependent source functions. Some general
properties, such as the order of magnitude of the surface polarization
and approximations for the source functions, are deduced. The escape
probability approximations are compared with exact solutions and found
to be more accurate than first order perturbation techniques for lines
of moderate optical thickness in solar isothermal prominences.
---------------------------------------------------------
Title: A Model for the Penetration of Lyman Alpha in the Solar
Chromosphere
Authors: Faurobert, M.; Frisch, H.; Skumanich, A.
1988ApJ...328..856F Altcode:
A multilevel calculation of the hydrogen spectrum by Skumanich and
Lites, followed by a two-level representation of each transition, shows
that all the solar Lyα photons are created in an optically thick layer
in the high chromosphere above τ<SUB>0</SUB> ≈ 10<SUP>2</SUP>. The
authors give a simplified model that demonstrates the penetration of
Lyα into the lower chromosphere below the creation region, i.e., for
τ<SUB>0</SUB> > 10<SUP>2</SUP>. A second-order escape probability
approximation yields a simple analytical expression which accurately
reproduces the behavior of the Lyα source function for 10<SUP>2</SUP>
⪉ τ<SUB>0</SUB> ⪉ 5×10<SUP>6</SUP> provided the destruction
probability, ɛ is set close to 10<SUP>-6</SUP>.
---------------------------------------------------------
Title: Linear polarization of resonance lines in the absence of
magnetic fields. II - Semi-infinite atmospheres
Authors: Faurobert, M.
1988A&A...194..268F Altcode:
Polarization profiles of resonance lines formed in magnetic free
semiinfinite atmospheres having either a uniform temperature or a
chromospheric-type temperature rise are considered. In the isothermal
case, a comparison is made between profiles calculated with the angle-
and frequency-dependent R(II) partial redistribution and those given
by the angle-averaged form of R(II), which is a simplified version of
it introduced by Ayres (1985) and the Kneer (1975) approximation. It
is found that, in chromospheric models, a maximum of polarization
perpendicular to the surface appears at frequencies which are formed
in the region of the temperature rise.
---------------------------------------------------------
Title: Transfert de rayonnement polarisé et non-polarisé avec
redistribution en fréquence : méthodes asymptotiques et applications
---------------------------------------------------------
Title: Transfert de rayonnement polarisé et non-polarisé avec
redistribution en fréquence : méthodes asymptotiques et applications
---------------------------------------------------------
Title: Transfer of polarized and non-polarized radiation with
frequency redistribution: asymptotic methods and applications;
Authors: Faurobert, Marianne
1988PhDT........83F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Linear polarization of resonance lines in the absence of
magnetic fields. I - Slabs of finite optical thickness
Authors: Faurobert, M.
1987A&A...178..269F Altcode:
Effects of partial frequency redistribution on resonance polarization
of spectral lines in the absence of magnetic fields are studied for
resonance lines formed in finite slabs. The emphasis is on testing the
validity (for polarization calculations) of various approximate forms
of the so-called R<SUB>II</SUB> frequency redistribution function
which describes the formation of resonance lines formed in dilute
media. Polarization profiles obtained with the angle-dependent
R<SUB>II</SUB> are compared to profiles calculated with the
angle-averaged form of R<SUB>II</SUB> and its approximate versions
proposed by Kneer and Ayres. A comparison with complete frequency
redistribution polarization profiles is also presented.
---------------------------------------------------------
Title: The Lyman α line in the solar chromosphere: penetrative
model andapproximations.
Authors: Faurobert, M.; Frisch, H.; Skumanich, A.
1987PAICz..66..205F Altcode: 1987eram....1..205F
The authors present a model for the formation of Lyman α in the
chromosphere below the creation region. The hydrogen atom is treated
as a two-level atom and the chromosphere as a semi-infinite medium
free of primary sources and illuminated by an isotropic and frequency
independent radiation field at r<SUB>0</SUB> ≅ 10<SUP>2</SUP>.
---------------------------------------------------------
Title: A numerical investigation of approximation procedures for
optically thick resonance lines.
Authors: Faurobert, M.
1986A&A...158..191F Altcode:
This paper concerns approximation procedures which can replace
the full transfer equation with R<SUB>II</SUB> partial frequency
redistribution. Existing asymptotic analyses of the formation of
optically thick resonance lines in dilute media show that the interior
radiation field (away from boundaries) can be approximated by the
solution of a space and frequency diffusion equation and provide an
expression of the emergent radiation field, away from line center,
in terms of the interior solution. Here an approximation for the
angle-averaged intensity, to be used at all optical depths, is
constructed by adding the solution of the interior diffusion equation
to the surface value of the angle-averaged intensity. The validity
of this approximation is tested by comparison with the solution
of the full transfer equation, in the case where the probability
of collisional deexcitation and the Doppler width of the line are
uniform. The diffusion equation has then an analytical solution. The
tests are performed first for a two-level atom, considering a wide
range of optical thicknesses and densities, and also for the Lyα line
under physical conditions typical of the quasar broad emission line
regions. In the latter case, the validity of a mean escape probability
approximation has also been examined.
---------------------------------------------------------
Title: Line transfer with complete frequency redistribution in an
absorbing medium - Scaling laws and approximation
Authors: Faurobert, M.; Frisch, H.
1985A&A...149..372F Altcode:
This paper discusses the influence of a continuous absorption
produced by dust grains or photoionization on subordinate or weak
resonance lines. The lines are being formed with complete Doppler or
Voigt frequency redistribution in a one dimensional slab of finite
thickness. An asymptotic analysis in the limit of small β (ratio
of continuum to line opacity coefficient) shows that the large scale
behaviour of the radiation field is described by a singular integral
equation identical to that obtained by Frisch and Frisch (1977) for
collisional destruction, but with an exponentially decreasing kernel
in the case of the Voigt profile. Asymptotic scaling laws for the mean
path length, the mean number of scatterings and the fraction of created
photons which escapes the medium are given in the limits of weak and
strong absorption. A first order escape probability method is used to
evaluate net radiative brackets and line cooling rates, taking into
account a possible emission by the continuum in the frequency domain
of the line. Finally, a definition of the mean number of scatterings
properly incorporating this emission term is proposed.
---------------------------------------------------------
Title: Boundary layer conditions for the transport of radiation
in stars
Authors: Frisch, H.; Faurobert, M.
1984A&A...140...57F Altcode:
An expansion technique used by Larsen et al. (1983) for
describing coherent scattering is applied to characterizing an
outer thermal boundary condition for radiation transport in stellar
interiors. Modifications are introduced to cover conditions of non-gray
radiative equilibrium, which is analyzed asymptotically. An expansion
parameter is defined in terms of coupled transfer and energy equations
and carried out to first order to obtain a ratio of the photon free
path to the stellar characteristic dimension. The ratio provides a
scale for the space variable with which a singular perturbation problem
is solved for interior and boundary conditions, the latter being in
the stellar atmosphere and the sum of the interior solution and the
boundary layer correction. A Robin boundary condition results which
is only 5 percent higher in temperature than the interior in the case
of a gray opacity coefficient.