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Author name code: belluzzi
ADS astronomy entries on 2022-09-14
author:"Belluzzi, Luca"
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Title: Quiet Sun Center to Limb Variation of the Linear Polarization
Observed by CLASP2 Across the Mg II h and k Lines
Authors: Rachmeler, L. A.; Bueno, J. Trujillo; McKenzie, D. E.;
Ishikawa, R.; Auchère, F.; Kobayashi, K.; Kano, R.; Okamoto,
T. J.; Bethge, C. W.; Song, D.; Ballester, E. Alsina; Belluzzi,
L.; Pino Alemán, T. del; Ramos, A. Asensio; Yoshida, M.; Shimizu,
T.; Winebarger, A.; Kobelski, A. R.; Vigil, G. D.; Pontieu, B. De;
Narukage, N.; Kubo, M.; Sakao, T.; Hara, H.; Suematsu, Y.; Štěpán,
J.; Carlsson, M.; Leenaarts, J.
2022ApJ...936...67R Altcode: 2022arXiv220701788R
The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket
mission was launched on 2019 April 11. CLASP2 measured the four Stokes
parameters of the Mg II h and k spectral region around 2800 Å along a
200″ slit at three locations on the solar disk, achieving the first
spatially and spectrally resolved observations of the solar polarization
in this near-ultraviolet region. The focus of the work presented here
is the center-to-limb variation of the linear polarization across these
resonance lines, which is produced by the scattering of anisotropic
radiation in the solar atmosphere. The linear polarization signals of
the Mg II h and k lines are sensitive to the magnetic field from the
low to the upper chromosphere through the Hanle and magneto-optical
effects. We compare the observations to theoretical predictions
from radiative transfer calculations in unmagnetized semiempirical
models, arguing that magnetic fields and horizontal inhomogeneities
are needed to explain the observed polarization signals and spatial
variations. This comparison is an important step in both validating and
refining our understanding of the physical origin of these polarization
signatures, and also in paving the way toward future space telescopes
for probing the magnetic fields of the solar upper atmosphere via
ultraviolet spectropolarimetry.
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Title: Numerical solutions to linear transfer problems of polarized
radiation. III. Parallel preconditioned Krylov solver tailored for
modeling PRD effects
Authors: Benedusi, Pietro; Janett, Gioele; Riva, Simone; Krause,
Rolf; Belluzzi, Luca
2022A&A...664A.197B Altcode:
Context. The polarization signals produced by the scattering of
anistropic radiation in strong resonance lines encode important
information about the elusive magnetic fields in the outer layers
of the solar atmosphere. An accurate modeling of these signals is
a very challenging problem from the computational point of view,
in particular when partial frequency redistribution (PRD) effects in
scattering processes are accounted for with a general angle-dependent
treatment. <BR /> Aims: We aim at solving the radiative transfer
problem for polarized radiation in nonlocal thermodynamic equilibrium
conditions, taking angle-dependent PRD effects into account. The
problem is formulated for a two-level atomic model in the presence of
arbitrary magnetic and bulk velocity fields. The polarization produced
by scattering processes and the Zeeman effect is considered. <BR />
Methods: The proposed solution strategy is based on an algebraic
formulation of the problem and relies on a convenient physical
assumption, which allows its linearization. We applied a nested
matrix-free GMRES iterative method. Effective preconditioning
is obtained in a multifidelity framework by considering the
light-weight description of scattering processes in the limit of
complete frequency redistribution (CRD). <BR /> Results: Numerical
experiments for a one-dimensional (1D) atmospheric model show near
optimal strong and weak scaling of the proposed CRD-preconditioned
GMRES method, which converges in few iterations, independently of
the discretization parameters. A suitable parallelization strategy
and high-performance computing tools lead to competitive run times,
providing accurate solutions in a few minutes. <BR /> Conclusions:
The proposed solution strategy allows the fast systematic modeling of
the scattering polarization signals of strong resonance lines, taking
angle-dependent PRD effects into account together with the impact of
arbitrary magnetic and bulk velocity fields. Almost optimal strong
and weak scaling results suggest that this strategy is applicable
to realistic 3D models. Moreover, the proposed strategy is general,
and applications to more complex atomic models are possible.
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Title: The transfer of polarized radiation in resonance lines with
partial frequency redistribution, J-state interference, and arbitrary
magnetic fields. A radiative transfer code and useful approximations
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2022A&A...664A..76A Altcode: 2022arXiv220412523A
<BR /> Aims: We present the theoretical framework and numerical
methods we have implemented to solve the problem of the generation and
transfer of polarized radiation in spectral lines without assuming
local thermodynamical equilibrium, while accounting for scattering
polarization, partial frequency redistribution (due to both the
Doppler effect and elastic collisions), J-state interference, and
hyperfine structure. The resulting radiative transfer code allows
one to model the impact of magnetic fields of an arbitrary strength
and orientation through the Hanle, incomplete Paschen-Back, and
magneto-optical effects. We also evaluate the suitability of a series
of approximations for modeling the scattering polarization in the
wings of strong resonance lines at a much lower computational cost,
which is particularly valuable for the numerically intensive case of
three-dimensional radiative transfer. <BR /> Methods: We examine the
suitability of the considered approximations by using our radiative
transfer code to model the Stokes profiles of the Mg II h & k lines
and of the H I Lyman-α line in magnetized one-dimensional models of
the solar atmosphere. <BR /> Results: Neglecting Doppler redistribution
in the scattering processes that are unperturbed by elastic collisions
(i.e., treating them as coherent in the observer's frame) produces a
negligible error in the scattering polarization wings of the Mg II
resonance lines and a minor one in the Lyman-α wings, although it
is unsuitable to model the cores of these lines. For both lines, the
scattering processes that are perturbed by elastic collisions only
give a significant contribution to the intensity component of the
emissivity. Neglecting collisional as well as Doppler redistribution
(so that all scattering processes are coherent) represents a rough
but suitable approximation for the wings of the Mg II resonance lines,
but a very poor one for the Lyman-α wings. The magnetic sensitivity
in the scattering polarization wings of the considered lines can be
modeled by accounting for the magnetic field in only the η<SUB>I</SUB>
and ρ<SUB>V</SUB> coefficients of the Stokes-vector transfer equation
(i.e., using the zero-field expression for the emissivity).
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Title: Hanle rotation signatures in Sr I 4607 Å
Authors: Zeuner, F.; Belluzzi, L.; Guerreiro, N.; Ramelli, R.;
Bianda, M.
2022A&A...662A..46Z Altcode: 2022arXiv220208659Z
Context. Measuring small-scale magnetic fields and constraining
their role in energy transport and dynamics in the solar atmosphere
are crucial, albeit challenging, tasks in solar physics. To this aim,
observations of scattering polarization and the Hanle effect in various
spectral lines are increasingly used to complement traditional magnetic
field determination techniques. <BR /> Aims: One of the strongest
scattering polarization signals in the photosphere is measured in the
Sr I line at 4607.3 Å when observed close to the solar limb. Here,
we present the first observational evidence of Hanle rotation in the
linearly polarized spectrum of this line at several limb distances. <BR
/> Methods: We used the Zurich IMaging POLarimeter, ZIMPOL at the
IRSOL observatory, with exceptionally good seeing conditions and long
integration times. We combined the fast-modulating polarimeter with a
slow modulator installed in front of the telescope. This combination
allows for a high level of precision and unprecedented accuracy in
the measurement of spectropolarimetric data. <BR /> Results: Fixing
the reference direction for positive Stokes Q parallel to the limb,
we detected singly peaked U/I signals well above the noise level. We
can exclude any instrumental origins for such U/I signals. These
signatures are exclusively found in the Sr I line, but not in the
adjoining Fe I line, therefore eliminating the Zeeman effect as the
mechanism responsible for their appearance. However, we find a clear
spatial correlation between the circular polarization produced by the
Zeeman effect and the U/I amplitudes. This suggests that the detected
U/I signals are the signatures of Hanle rotation caused by a spatially
resolved magnetic field. <BR /> Conclusions: A novel measurement
technique allows for determining the absolute level of polarization
with unprecedented precision. Using this technique, high-precision
spectropolarimetric observations reveal, for the first time, unambiguous
U/I signals attributed to Hanle rotation in the Sr I line.
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Title: The polarization angle in the wings of Ca I 4227: A new
observable for diagnosing unresolved photospheric magnetic fields
Authors: Capozzi, Emilia; Alsina Ballester, Ernest; Belluzzi, Luca;
Trujillo Bueno, Javier
2022A&A...657A..44C Altcode: 2021arXiv211108967C
Context. When observed in quiet regions close to the solar limb, many
strong resonance lines show conspicuous linear polarization signals,
produced by scattering processes (i.e., scattering polarization), with
extended wing lobes. Recent studies indicate that, contrary to what
was previously believed, the wing lobes are sensitive to the presence
of relatively weak longitudinal magnetic fields through magneto-optical
(MO) effects. <BR /> Aims: We theoretically investigate the sensitivity
of the scattering polarization wings of the Ca I 4227 Å line to the
MO effects, and we explore its diagnostic potential for inferring
information on the longitudinal component of the photospheric magnetic
field. <BR /> Methods: We calculate the intensity and polarization
profiles of the Ca I 4227 Å line by numerically solving the problem
of the generation and transfer of polarized radiation under non-local
thermodynamic equilibrium conditions in one-dimensional semi-empirical
models of the solar atmosphere, taking into account the joint action
of the Hanle, Zeeman, and MO effects. We consider volume-filling
magnetic fields as well as magnetic fields occupying a fraction of
the resolution element. <BR /> Results: In contrast to the circular
polarization signals produced by the Zeeman effect, we find that the
linear polarization angle in the scattering polarization wings of Ca
I 4227 presents a clear sensitivity, through MO effects, not only to
the flux of the photospheric magnetic field, but also to the fraction
of the resolution element that the magnetic field occupies. <BR />
Conclusions: We identify the linear polarization angle in the wings
of strong resonance lines as a valuable observable for diagnosing
unresolved magnetic fields. Used in combination with observables that
encode information on the magnetic flux and other properties of the
observed atmospheric region (e.g., temperature and density), it can
provide constraints on the filling factor of the magnetic field.
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Title: Demonstration of Chromospheric Magnetic Mapping with CLASP2.1
Authors: McKenzie, David; Ishikawa, Ryohko; Trujillo Bueno, Javier;
Auchere, F.; Kobayashi, Ken; Winebarger, Amy; Kano, Ryouhei; Song,
Donguk; Okamoto, Joten; Rachmeler, Laurel; De Pontieu, Bart; Vigil,
Genevieve; Belluzzi, Luca; Alsina Ballester, Ernest; del Pino Aleman,
Tanausu; Bethge, Christian; Sakao, Taro; Stepan, Jiri
2021AGUFMSH52A..06M Altcode:
Probing the magnetic nature of the Suns atmosphere requires measurement
of the Stokes I, Q, U and V profiles of relevant spectral lines (of
which Q, U and V encode the magnetic field information). Many of the
magnetically sensitive lines formed in the chromosphere and transition
region are in the ultraviolet spectrum, necessitating observations
above the absorbing terrestrial atmosphere. The Chromospheric
Layer Spectro-Polarimeter (CLASP2) sounding rocket was flown
successfully in April 2019, as a follow-on to the successful flight in
September 2015 of the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP). Both projects were funded by NASAs Heliophysics Technology
and Instrument Development for Science (H-TIDeS) program to develop
and test a technique for observing the Sun in ultraviolet light,
and for quantifying the polarization of that light. By demonstrating
successful measurement and interpretation of the polarization in
hydrogen Lyman-alpha and the Mg II h and k spectral lines, the CLASP
and CLASP2 missions are vital first steps towards routine quantitative
characterization of the local thermal and magnetic conditions in the
solar chromosphere. In October of 2021, we re-flew the CLASP2 payload
with a modified observing program to further demonstrate the maturity
of the UV spectropolarimetry techniques, and readiness for development
into a satellite observatory. During the reflight, called CLASP2.1,
the spectrograph slit was scanned across an active region plage to
acquire a two-dimensional map of Stokes V/I, to demonstrate the ability
of UV spectropolarimetry to yield chromospheric magnetic fields over
a large area. This presentation will display preliminary results from
the flight of CLASP2.1.
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Title: Numerical solutions to linear transfer problems of polarized
radiation. I. Algebraic formulation and stationary iterative methods
Authors: Janett, Gioele; Benedusi, Pietro; Belluzzi, Luca; Krause, Rolf
2021A&A...655A..87J Altcode: 2021arXiv211011861J
Context. The numerical modeling of the generation and transfer
of polarized radiation is a key task in solar and stellar physics
research and has led to a relevant class of discrete problems that
can be reframed as linear systems. In order to solve such problems, it
is common to rely on efficient stationary iterative methods. However,
the convergence properties of these methods are problem-dependent, and
a rigorous investigation of their convergence conditions, when applied
to transfer problems of polarized radiation, is still lacking. <BR />
Aims: After summarizing the most widely employed iterative methods used
in the numerical transfer of polarized radiation, this article aims to
clarify how the convergence of these methods depends on different design
elements, such as the choice of the formal solver, the discretization
of the problem, or the use of damping factors. The main goal is to
highlight advantages and disadvantages of the different iterative
methods in terms of stability and rate of convergence. <BR /> Methods:
We first introduce an algebraic formulation of the radiative transfer
problem. This formulation allows us to explicitly assemble the iteration
matrices arising from different stationary iterative methods, compute
their spectral radii and derive their convergence rates, and test
the impact of different discretization settings, problem parameters,
and damping factors. <BR /> Results: Numerical analysis shows that
the choice of the formal solver significantly affects, and can even
prevent, the convergence of an iterative method. Moreover, the use of
a suitable damping factor can both enforce stability and increase the
convergence rate. <BR /> Conclusions: The general methodology used
in this article, based on a fully algebraic formulation of linear
transfer problems of polarized radiation, provides useful estimates
of the convergence rates of various iterative schemes. Additionally,
it can lead to novel solution approaches as well as analyses for a
wider range of settings, including the unpolarized case.
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Title: Modeling the scattering polarization of the solar Ca I 4227
Å line with angle-dependent partial frequency redistribution
Authors: Janett, Gioele; Ballester, Ernest Alsina; Guerreiro, Nuno;
Riva, Simone; Belluzzi, Luca; del Pino Alemán, Tanausú; Bueno,
Javier Trujillo
2021A&A...655A..13J Altcode: 2021arXiv211011990J
Context. The correct modeling of the scattering polarization signals
observed in several strong resonance lines requires taking partial
frequency redistribution (PRD) phenomena into account. Modeling
scattering polarization with PRD effects is very computationally
demanding and the simplifying angle-averaged (AA) approximation is
therefore commonly applied. <BR /> Aims: This work aims to assess the
impact and the range of validity of the AA approximation with respect
to the general angle-dependent (AD) treatment of PRD effects in the
modeling of scattering polarization in strong resonance lines, with a
focus on the solar Ca I 4227 Å line. <BR /> Methods: Spectral line
polarization was modeled by solving the radiative transfer problem
for polarized radiation, under nonlocal thermodynamic equilibrium
conditions, taking PRD effects into account in static one-dimensional
semi-empirical atmospheric models presenting arbitrary magnetic
fields. The problem was solved through a two-step approach. In step 1,
the problem was solved for the intensity only, considering a multilevel
atom. In step 2, the problem was solved including polarization,
considering a two-level atom with an unpolarized and infinitely sharp
lower level, and fixing the lower level population calculated at
step 1. <BR /> Results: The results for the Ca I 4227 Å line show
a good agreement between the AA and AD calculations for the Q/I and
U/I wings' signals. However, AA calculations reveal an artificial
trough in the line-core peak of the linear polarization profiles,
whereas AD calculations show a sharper peak in agreement with the
observations. <BR /> Conclusions: An AD treatment of PRD effects is
essential to correctly model the line-core peak of the scattering
polarization signal of the Ca I 4227 Å line. By contrast, in the
considered static case, the AA approximation seems to be suitable
to model the wing scattering polarization lobes and their magnetic
sensitivity through magneto-optical effects.
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Title: Numerical solutions to linear transfer problems of polarized
radiation. II. Krylov methods and matrix-free implementation
Authors: Benedusi, Pietro; Janett, Gioele; Belluzzi, Luca; Krause, Rolf
2021A&A...655A..88B Altcode: 2021arXiv211011873B
Context. Numerical solutions to transfer problems of polarized radiation
in solar and stellar atmospheres commonly rely on stationary iterative
methods, which often perform poorly when applied to large problems. In
recent times, stationary iterative methods have been replaced by
state-of-the-art preconditioned Krylov iterative methods for many
applications. However, a general description and a convergence analysis
of Krylov methods in the polarized radiative transfer context are
still lacking. <BR /> Aims: We describe the practical application of
preconditioned Krylov methods to linear transfer problems of polarized
radiation, possibly in a matrix-free context. The main aim is to
clarify the advantages and drawbacks of various Krylov accelerators
with respect to stationary iterative methods and direct solution
strategies. <BR /> Methods: After a brief introduction to the concept
of Krylov methods, we report the convergence rate and the run time of
various Krylov-accelerated techniques combined with different formal
solvers when applied to a 1D benchmark transfer problem of polarized
radiation. In particular, we analyze the GMRES, BICGSTAB, and CGS
Krylov methods, preconditioned with Jacobi, (S)SOR, or an incomplete
LU factorization. Furthermore, specific numerical tests were performed
to study the robustness of the various methods as the problem size
grew. <BR /> Results: Krylov methods accelerate the convergence, reduce
the run time, and improve the robustness (with respect to the problem
size) of standard stationary iterative methods. Jacobi-preconditioned
Krylov methods outperform SOR-preconditioned stationary iterations in
all respects. In particular, the Jacobi-GMRES method offers the best
overall performance for the problem setting in use. <BR /> Conclusions:
Krylov methods can be more challenging to implement than stationary
iterative methods. However, an algebraic formulation of the radiative
transfer problem allows one to apply and study Krylov acceleration
strategies with little effort. Furthermore, many available numerical
libraries implement matrix-free Krylov routines, enabling an almost
effortless transition to Krylov methods.
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Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
2021arXiv211011885J Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present various types of discontinuities. The
radiative transfer is a typical example of such a problem. This
calls for high-order well-behaved techniques to interpolate both
smooth and discontinuous data. Aims. The final aim is to propose
new techniques suitable for applications in the context of numerical
radiative transfer. Methods. We have proposed and tested two different
techniques. Essentially non-oscillatory (ENO) techniques generate
several candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure for
the local smoothness, thereby enabling the essential non-oscillatory
property. Weighted ENO (WENO) techniques use a convex combination of
all candidate substencils to obtain high-order accuracy in smooth
regions while keeping the essentially non-oscillatory property. In
particular, we have outlined and tested a novel well-performing
fourth-order WENO interpolation technique for both uniform and
nonuniform grids. Results. Numerical tests prove that the fourth-order
WENO interpolation guarantees fourth-order accuracy in smooth regions
of the interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory property,
avoiding oscillations. Unlike Bézier and monotonic high-order Hermite
interpolations, it does not degenerate to a linear interpolation near
smooth extrema of the interpolated function. Conclusions. The novel
fourth-order WENO interpolation guarantees high accuracy in smooth
regions, while effectively handling discontinuities. This interpolation
technique might be particularly suitable for several problems, including
a number of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.
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Title: Magnetic imaging of the outer solar atmosphere (MImOSA)
Authors: Peter, H.; Ballester, E. Alsina; Andretta, V.; Auchère, F.;
Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
Chitta, L. P.; Dalmasse, K.; Alemán, T. del Pino; Feller, A.; Froment,
C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
D.; Solanki, S. K.; Štěpán, J.; Teriaca, L.; Bueno, J. Trujillo
2021ExA...tmp...95P Altcode:
The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the habitability of
planets orbiting these host stars. Although the magnetic field at the
surface of the Sun is reasonably well characterised by observations,
the information on the magnetic field in the higher atmospheric layers
is mainly indirect. This lack of information hampers our progress in
understanding solar magnetic activity. Overcoming this limitation would
allow us to address four paramount long-standing questions: (1) How
does the magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic field
structure, drive and interact with the plasma in the chromosphere and
upper atmosphere? (3) How does the magnetic field destabilise the outer
solar atmosphere and thus affect the interplanetary environment? (4)
How do magnetic processes accelerate particles to high energies? New
ground-breaking observations are needed to address these science
questions. We suggest a suite of three instruments that far exceed
current capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-IR
telescope of the 1-3 m class aimed mainly to measure the magnetic
field in the chromosphere by combining high spatial resolution
and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is
designed to measure the large-scale magnetic field in the corona with
an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter
based on a 30 cm telescope that combines high throughput in the
extreme UV with polarimetry to connect the magnetic measurements
of the other two instruments. Placed in a near-Earth orbit, the data
downlink would be maximised, while a location at L4 or L5 would provide
stereoscopic observations of the Sun in combination with Earth-based
observatories. This mission to measure the magnetic field will finally
unlock the driver of the dynamics in the outer solar atmosphere and
thereby will greatly advance our understanding of the Sun and the
heliosphere.
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Title: Solving the Paradox of the Solar Sodium D<SUB>1</SUB> Line
Polarization
Authors: Alsina Ballester, Ernest; Belluzzi, Luca; Trujillo Bueno,
Javier
2021PhRvL.127h1101A Altcode: 2021arXiv210808334A
Twenty-five years ago, enigmatic linear polarization signals were
discovered in the core of the sodium D<SUB>1</SUB> line. The only
explanation that could be found implied that the solar chromosphere is
practically unmagnetized, in contradiction with other evidences. This
opened a paradox that has challenged physicists for many years. Here
we present its solution, demonstrating that these polarization signals
can be properly explained in the presence of magnetic fields in the
gauss range. This result opens a novel diagnostic window for exploring
the elusive magnetism of the solar chromosphere.
---------------------------------------------------------
Title: Mapping of Solar Magnetic Fields from the Photosphere to the
Top of the Chromosphere with CLASP2
Authors: McKenzie, D.; Ishikawa, R.; Trujillo Bueno, J.; Auchere, F.;
del Pino Aleman, T.; Okamoto, T.; Kano, R.; Song, D.; Yoshida, M.;
Rachmeler, L.; Kobayashi, K.; Narukage, N.; Kubo, M.; Ishikawa, S.;
Hara, H.; Suematsu, Y.; Sakao, T.; Bethge, C.; De Pontieu, B.; Vigil,
G.; Winebarger, A.; Alsina Ballester, E.; Belluzzi, L.; Stepan, J.;
Asensio Ramos, A.; Carlsson, M.; Leenaarts, J.
2021AAS...23810603M Altcode:
Coronal heating, chromospheric heating, and the heating &
acceleration of the solar wind, are well-known problems in solar
physics. Additionally, knowledge of the magnetic energy that
powers solar flares and coronal mass ejections, important drivers
of space weather, is handicapped by imperfect determination of the
magnetic field in the sun's atmosphere. Extrapolation of photospheric
magnetic measurements into the corona is fraught with difficulties and
uncertainties, partly due to the vastly different plasma beta between
the photosphere and the corona. Better results in understanding
the coronal magnetic field should be derived from measurements of
the magnetic field in the chromosphere. To that end, we are pursuing
quantitative determination of the magnetic field in the chromosphere,
where plasma beta transitions from greater than unity to less than
unity, via ultraviolet spectropolarimetry. The CLASP2 mission, flown
on a sounding rocket in April 2019, succeeded in measuring all four
Stokes polarization parameters in UV spectral lines formed by singly
ionized Magnesium and neutral Manganese. Because these ions produce
spectral lines under different conditions, CLASP2 thus was able to
quantify the magnetic field properties at multiple heights in the
chromosphere simultaneously, as shown in the recent paper by Ishikawa
et al. In this presentation we will report the findings of CLASP2,
demonstrating the variation of magnetic fields along a track on
the solar surface and as a function of height in the chromosphere;
and we will illustrate what is next for the CLASP missions and the
demonstration of UV spectropolarimetry in the solar chromosphere.
---------------------------------------------------------
Title: Mapping solar magnetic fields from the photosphere to the
base of the corona
Authors: Ishikawa, Ryohko; Bueno, Javier Trujillo; del Pino Alemán,
Tanausú; Okamoto, Takenori J.; McKenzie, David E.; Auchère,
Frédéric; Kano, Ryouhei; Song, Donguk; Yoshida, Masaki; Rachmeler,
Laurel A.; Kobayashi, Ken; Hara, Hirohisa; Kubo, Masahito; Narukage,
Noriyuki; Sakao, Taro; Shimizu, Toshifumi; Suematsu, Yoshinori; Bethge,
Christian; De Pontieu, Bart; Dalda, Alberto Sainz; Vigil, Genevieve D.;
Winebarger, Amy; Ballester, Ernest Alsina; Belluzzi, Luca; Štěpán,
Jiří; Ramos, Andrés Asensio; Carlsson, Mats; Leenaarts, Jorrit
2021SciA....7.8406I Altcode: 2021arXiv210301583I
Routine ultraviolet imaging of the Sun's upper atmosphere shows the
spectacular manifestation of solar activity; yet we remain blind to
its main driver, the magnetic field. Here we report unprecedented
spectropolarimetric observations of an active region plage and
its surrounding enhanced network, showing circular polarization in
ultraviolet (Mg II $h$ & $k$ and Mn I) and visible (Fe I) lines. We
infer the longitudinal magnetic field from the photosphere to the
very upper chromosphere. At the top of the plage chromosphere the
field strengths reach more than 300 gauss, strongly correlated with
the Mg II $k$ line core intensity and the electron pressure. This
unique mapping shows how the magnetic field couples the different
atmospheric layers and reveals the magnetic origin of the heating in
the plage chromosphere.
---------------------------------------------------------
Title: Magnetic Imaging of the Outer Solar Atmosphere (MImOSA):
Unlocking the driver of the dynamics in the upper solar atmosphere
Authors: Peter, H.; Alsina Ballester, E.; Andretta, V.; Auchere, F.;
Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
Chitta, L. P.; Dalmasse, K.; del Pino Aleman, T.; Feller, A.; Froment,
C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
D.; Solanki, S. K.; Stepan, J.; Teriaca, L.; Trujillo Bueno, J.
2021arXiv210101566P Altcode:
The magnetic activity of the Sun directly impacts the Earth and human
life. Likewise, other stars will have an impact on the habitability
of planets orbiting these host stars. The lack of information on the
magnetic field in the higher atmospheric layers hampers our progress in
understanding solar magnetic activity. Overcoming this limitation would
allow us to address four paramount long-standing questions: (1) How
does the magnetic field couple the different layers of the atmosphere,
and how does it transport energy? (2) How does the magnetic field
structure, drive and interact with the plasma in the chromosphere and
upper atmosphere? (3) How does the magnetic field destabilise the outer
solar atmosphere and thus affect the interplanetary environment? (4)
How do magnetic processes accelerate particles to high energies? New
ground-breaking observations are needed to address these science
questions. We suggest a suite of three instruments that far exceed
current capabilities in terms of spatial resolution, light-gathering
power, and polarimetric performance: (a) A large-aperture UV-to-IR
telescope of the 1-3 m class aimed mainly to measure the magnetic
field in the chromosphere by combining high spatial resolution and high
sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to
measure the large-scale magnetic field in the corona with an aperture
of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30
cm telescope that combines high throughput in the extreme UV with
polarimetry to connect the magnetic measurements of the other two
instruments. This mission to measure the magnetic field will unlock
the driver of the dynamics in the outer solar atmosphere and thereby
greatly advance our understanding of the Sun and the heliosphere.
---------------------------------------------------------
Title: Fast and accurate approximation of the angle-averaged
redistribution function for polarized radiation
Authors: Paganini, A.; Hashemi, B.; Alsina Ballester, E.; Belluzzi, L.
2021A&A...645A...4P Altcode: 2020arXiv201003508P
Context. Modeling spectral line profiles taking frequency redistribution
effects into account is a notoriously challenging problem from the
computational point of view, especially when polarization phenomena
(atomic polarization and polarized radiation) are taken into
account. Frequency redistribution effects are conveniently described
through the redistribution function formalism, and the angle-averaged
approximation is often introduced to simplify the problem. Even in
this case, the evaluation of the emission coefficient for polarized
radiation remains computationally costly, especially when magnetic
fields are present or complex atomic models are considered. <BR /> Aims:
We aim to develop an efficient algorithm to numerically evaluate the
angle-averaged redistribution function for polarized radiation. <BR />
Methods: The proposed approach is based on a low-rank approximation
via trivariate polynomials whose univariate components are represented
in the Chebyshev basis. <BR /> Results: The resulting algorithm is
significantly faster than standard quadrature-based schemes for any
target accuracy in the range [10<SUP>-6</SUP>, 10<SUP>-2</SUP>].
---------------------------------------------------------
Title: Imaging spectropolarimetry for magnetic field diagnostics in
solar prominences
Authors: Di Campli, R.; Ramelli, R.; Bianda, M.; Furno, I.; Kumar
Dhara, S.; Belluzzi, L.
2020A&A...644A..89D Altcode:
Context. Narrowband imaging spectropolarimetry is one of the most
powerful tools available to infer information about the intensity and
topology of the magnetic fields present in extended plasma structures
in the solar atmosphere. <BR /> Aims: We describe the instrumental
set-up and the observing procedure that we have developed and optimized
at the Istituto Ricerche Solari Locarno in order to perform imaging
spectropolarimetry. A measurement that highlights the potential of the
ensuing observations for magnetic field diagnostics in solar prominences
is presented. <BR /> Methods: Monochromatic images of solar prominences
were obtained by combining a tunable narrowband filter, based on two
Fabry-Perot etalons, with a Czerny-Turner spectrograph. Linear and
circular polarization were measured at every pixel of the monochromatic
image with the Zurich Imaging Polarimeter, ZIMPOL. A wavelength scan was
performed across the profile of the considered spectral line. The HAZEL
inversion code was applied to the observed Stokes profiles to infer a
series of physical properties of the observed structure. <BR /> Results:
We carried out a spectropolarimetric observation of a prominence,
consisting of a set of quasi-monochromatic images across the He I
D<SUB>3</SUB> line at 5876 Å in the four Stokes parameters. The map
of observed Stokes profiles was inverted with HAZEL, finding magnetic
fields with intensities between 15 and 30 G and directed along the
spine of the prominence, which is in agreement with the results of
previous works.
---------------------------------------------------------
Title: Observational indications of magneto-optical effects in the
scattering polarization wings of the Ca I 4227 Å line
Authors: Capozzi, Emilia; Ballester, Ernest Alsina; Belluzzi, Luca;
Bianda, Michele; Dhara, Sajal Kumar; Ramelli, Renzo
2020A&A...641A..63C Altcode: 2020arXiv200613653C
Context. Several strong resonance lines, such as H I Ly-α, Mg II
k, Ca II K, and Ca I 4227 Å, are characterized by deep and broad
absorption profiles in the solar intensity spectrum. These resonance
lines show conspicuous linear scattering polarization signals when
observed in quiet regions close to the solar limb. Such signals show
a characteristic triplet-peak structure with a sharp peak in the line
core and extended wing lobes. The line core peak is sensitive to the
presence of magnetic fields through the Hanle effect, which however
is known not to operate in the line wings. Recent theoretical studies
indicate that, contrary to what was previously believed, the wing linear
polarization signals are also sensitive to the magnetic field through
magneto-optical (MO) effects. <BR /> Aims: We search for observational
indications of this recently discovered physical mechanism in the
scattering polarization wings of the Ca I 4227 Å line. <BR /> Methods:
We performed a series of spectropolarimetric observations of this
line using the Zurich IMaging POLarimeter camera at the Gregory-Coudé
telescope at Istituto Ricerche Solari Locarno in Switzerland and at the
GREGOR telescope in Tenerife (Spain). <BR /> Results: Spatial variations
of the total linear polarization degree and linear polarization angle
are clearly appreciable in the wings of the observed line. We provide
a detailed discussion of our observational results, showing that the
detected variations always take place in regions in which longitudinal
magnetic fields are present, thus supporting the theoretical prediction
that they are produced by MO effects.
---------------------------------------------------------
Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
2019arXiv191208650S Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
---------------------------------------------------------
Title: Observations on spatial variations of the Sr I 4607 Å
scattering polarization signals at different limb distances with
ZIMPOL
Authors: Dhara, Sajal Kumar; Capozzi, Emilia; Gisler, Daniel; Bianda,
Michele; Ramelli, Renzo; Berdyugina, Svetlana; Alsina, Ernest;
Belluzzi, Luca
2019A&A...630A..67D Altcode: 2019arXiv190803366D
Context. The Sr I 4607 Å spectral line shows one of the strongest
scattering polarization signals in the visible solar spectrum. The
amplitude of this polarization signal is expected to vary at granular
spatial scales, due to the combined action of the Hanle effect and the
local anisotropy of the radiation field. Observing these variations
would be of great interest because it would provide precious information
on the small-scale activity of the solar photosphere. At present, few
detections of such spatial variations have been reported. This is due
to the difficulty of these measurements, which require combining high
spatial (∼0.1″), spectral (≤20 mÅ), and temporal resolution (<
1 min) with increased polarimetric sensitivity (∼10<SUP>-4</SUP>). <BR
/> Aims: We aim to detect spatial variations at granular scales of the
scattering polarization peak of the Sr I 4607 Å line at different
limb distances, and to study the correlation with the continuum
intensity. <BR /> Methods: Using the Zurich IMaging POLarimeter
(ZIMPOL) system mounted at the GREGOR telescope and spectrograph in
Tenerife, Spain, we carried out spectro-polarimetric measurements
to obtain the four Stokes parameters in the Sr I line at different
limb distances, from μ = 0.2 to μ = 0.8, on the solar disk. <BR />
Results: Spatial variations of the scattering polarization signal in
the Sr I 4607 Å line, with a spatial resolution of about 0.66″, are
clearly observed at every μ. The spatial scale of these variations is
comparable to the granular size. A statistical analysis reveals that
the linear scattering polarization amplitude in this Sr I spectral
line is positively correlated with the intensity in the continuum,
corresponding to the granules, at every μ.
---------------------------------------------------------
Title: Magnetic Sensitivity in the Wings of the Linear Polarization
Profiles of the Hydrogen Lyman-α Line
Authors: Ballester, E. A.; Belluzzi, L.; Bueno, J. T.
2019spw..confE..11B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Sensitivity in the Wing Scattering Polarization
Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2019ApJ...880...85A Altcode: 2019arXiv190110994A
The linear polarization produced by scattering processes in the hydrogen
Lyα line of the solar disk radiation is a key observable for probing
the chromosphere-corona transition region (TR) and the underlying
chromospheric plasma. While the line-center signal encodes information
on the magnetic field and the three-dimensional structure of the TR,
the sizable scattering polarization signals that the joint action of
partial frequency redistribution and J-state interference produce in
the Lyα wings have generally been thought to be sensitive only to the
thermal structure of the solar atmosphere. Here we show that the wings
of the Q/I and U/I scattering polarization profiles of this line are
actually sensitive to the presence of chromospheric magnetic fields,
with strengths similar to those that produce the Hanle effect in the
line core (i.e., between 5 and 100 G, approximately). In spite of the
fact that the Zeeman splitting induced by such weak fields is very
small compared to the total width of the line, the magneto-optical
effects that couple the transfer equations for Stokes Q and U are
actually able to produce sizable changes in the Q/I and U/I wings. We
find that magnetic fields with longitudinal components larger than 100
G produce an almost complete depolarization of the wings of the Lyα
Q/I profiles within a ±5 Å spectral range around the line center,
while stronger fields are required for the U/I wing signals to be
depolarized to a similar extent. The theoretical results presented
here further expand the diagnostic content of the unprecedented
spectropolarimetric observations provided by the Chromospheric
Lyman-Alpha Spectro-Polarimeter.
---------------------------------------------------------
Title: Measurement of the Evolution of the Magnetic Field of the
Quiet Photosphere over a Solar Cycle
Authors: Ramelli, R.; Bianda, M.; Berdyugina, S.; Belluzzi, L.;
Kleint, L.
2019ASPC..526..283R Altcode:
The solar photosphere is filled by magnetic fields tangled at scales
much smaller than the resolution capability of solar telescopes. These
hidden magnetic fields can be investigated via the Hanle effect. In
2007, we started a synoptic program to explore whether the magnetic flux
of the quiet photosphere varies with the solar cycle. For this purpose
we applied a differential Hanle effect technique based on observations
of scattering polarization in C<SUB>2</SUB> molecular lines around
514.0 nm, taken with a cadence of approximately one month. Our results
now span almost one complete solar cycle.
---------------------------------------------------------
Title: Numerical Methods for the Radiative Transfer Equation of
Polarized Light
Authors: Janett, G.; Steiner, O.; Belluzzi, L.
2019ASPC..526..133J Altcode:
The quest of the "best formal solver" available is still open and the
lack of a clear comparison between the different numerical methods
proposed by the community does not facilitate a conclusion. This work
presents a reference paradigm for the characterization of formal
solvers, based on the concepts of order of accuracy, stability and
computational cost. This overview aims to facilitate the comprehension
of the advantages and weaknesses of the already existing formal solvers
and of those yet to come.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: McKenzie, D. E.; Ishikawa, R.; Trujillo Bueno, J.; Auchére,
F.; Rachmeler, L. A.; Kubo, M.; Kobayashi, K.; Winebarger, A. R.;
Bethge, C. W.; Narukage, N.; Kano, R.; Ishikawa, S.; de Pontieu,
B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Štěpán, J.; del Pino
Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A.
2019ASPC..526..361M Altcode:
The hydrogen Lyman-α line at 121.6 nm and the Mg k line at 279.5
nm are especially relevant for deciphering the magnetic structure
of the chromosphere since their line-center signals are formed in
the chromosphere and transition region, with unique sensitivities to
magnetic fields. We propose the Chromospheric LAyer Spectro-Polarimeter
(CLASP2), to build upon the success of the first CLASP flight, which
measured the linear polarization in H I Lyman-α. The existing CLASP
instrument will be refitted to measure all four Stokes parameters in
the 280 nm range, including variations due to the anisotropic radiation
pumping, the Hanle effect, and the Zeeman effect.
---------------------------------------------------------
Title: Relaxation Phenomena Due to Collisions with Neutral Perturbers
in Hyperfine Structure Multiplets
Authors: Landi Degl'Innocenti, E.; Belluzzi, L.
2019ASPC..526...29L Altcode:
A connection is established between the depolarizing rates, due to
collisions with neutral perturbers, of the statistical tensors of a
hyperfine structure multiplet and those of the statistical tensors
of the parent fine structure level. This connection is described
by a simple equation involving a 12-j symbol of the second kind. An
application is performed to derive an analytical expression for the
scattering matrix of a hyperfine structured two-level atom in the
presence of collisions and a magnetic field. Numerical results are
presented.
---------------------------------------------------------
Title: The Transfer of Resonance Line Polarization with PRD in the
General Hanle-Zeeman Regime
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2019ASPC..526..119A Altcode:
We present numerical radiative transfer calculations of the four
Stokes parameters of the radiation emerging from one-dimensional model
atmospheres. In this investigation we account for the impact of partial
frequency redistribution (PRD) in scattering and the joint action of
the Hanle and Zeeman effects. Strong resonance lines of interest for
chromospheric magnetic field diagnostics have been considered, namely
the Ca I line at 4227 Å and the Mg II k line at 2795 Å. The Stokes
profiles of these lines have been obtained by considering two-level
atomic models, both in the absence and in the presence of magnetic
fields. We draw attention to the fact that the magneto-optical
terms of the transfer equations for Stokes Q and U are responsible
for an interesting and previously unnoticed magnetic sensitivity of
their scattering polarization profiles beyond the Doppler core. This
important discovery contributes to paint a more detailed picture of the
influence of relatively weak magnetic fields on the observable linear
polarization signals of strong chromospheric lines, highlighting the
importance of a PRD treatment for such lines.
---------------------------------------------------------
Title: Solar Polarization Workshop 8
Authors: Belluzzi, L.; Casini, R.; Romoli, M.; Trujillo Bueno, J.
2019ASPC..526.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comparison of Scattering Polarization Signals Observed by
CLASP: Possible Indication of the Hanle Effect
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pomtieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2019ASPC..526..305I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et
al. 2012; Kobayashi et al. 2012; Kubo et al. 2014) observed, for the
first time, the linear polarization produced by scattering processes
in the hydrogen Lyman-α (121.57 nm) and Si III (120.56 nm) lines of
the solar disk radiation. The complexity of the observed scattering
polarization (i.e., conspicuous spatial variations in Q/I and U/I
at spatial scales of 10″-20″ and the absence of center-to- limb
variation at the Lyman-α center; see Kano et al. 2017) motivated us
to search for possible hints of the operation of the Hanle effect by
comparing: (a) the Lyman-α line center signal, for which the critical
field strength (B<SUB>H</SUB>) for the onset of the Hanle effect is
53 G, (b) the Lyman-α wing, which is insensitive to the Hanle effect,
and (c) the Si III line, whose B<SUB>H</SUB> = 290 G. We focus on four
regions with different total unsigned photospheric magnetic fluxes
(estimated from SDO/HMI observations), and compare the corresponding
U/I spatial variations in the Lyman-α wing, Lyman-α center, and Si III
line. The U/I signal in the Lyman-α wing shows an antisymmetric spatial
distribution, which is caused by the presence of a bright structure in
all the selected regions, regardless of the total unsigned photospheric
magnetic flux. In an internetwork region, the Lyman-α center shows an
antisymmetric spatial variation across the selected bright structure,
but it does not show it in other more magnetized regions. In the Si III
line, the spatial variation of U/I deviates from the above-mentioned
antisymmetric shape as the total unsigned photospheric magnetic flux
increases. We argue that a plausible explanation of this differential
behavior is the operation of the Hanle effect. <P />This work, presented
in an oral contribution at this Workshop, has been published on The
Astrophysical Journal (Ishikawa et al. 2017).
---------------------------------------------------------
Title: Second Solar Spectrum Observations with ZIMPOL
Authors: Bianda, M.; Ramelli, R.; Gisler, D.; Belluzzi, L.; Carlin,
E. S.
2019ASPC..526..223B Altcode:
The observational activity of IRSOL takes advantage of the
high-precision instrumentation available, and of the possibility
to carry out programs requiring observing time spread over weeks or
months. Particular attention is paid to scattering polarization signals
modified by the Hanle effect in spectral lines. Several programs at
IRSOL are also aimed at getting the technical know-how necessary to
exploit the potentialities of the new generation of large-aperture
solar telescopes, which will provide observations with unprecedented
spatial and temporal resolution without sacrificing spectropolarimetric
sensitivity. In this contribution, we provide a brief description of
some projects that are presently carried out at IRSOL.
---------------------------------------------------------
Title: Spatial variations of the SrI 4607Åscattering polarization
signals at subgranular scale observed with ZIMPOL at GREGOR telescope
Authors: Dhara, Sajal Kumar; Capozzi, Emilia; Gisler, Daniel; Bianda,
Michele; Ramelli, Renzo; Berdyugina, Svetlana; Alsina, Ernest;
Belluzzi, Luca
2019arXiv190403986D Altcode:
Sr I 4607Åspectral line shows one of the strongest scattering
polarization signals in the visible solar spectrum. The amplitudes of
these signals are expected to vary at granular spatial scales. This
variation can be due to changes in the magnetic field intensity
and orientation (Hanle effect) as well as due to spatial and
temporal variations in the plasma properties. Measuring the spatial
variation of such polarization signal would allow us to study the
properties of the magnetic fields at subgranular region. But, the
observations are challenging since both high spatial resolution and
high spectropolarimetric sensitivity are required at the same time. To
the aim of measuring these spatial variations at granular scale, we
carried out a spectro-polarimetric measurement with the Zurich IMaging
POLarimeter (ZIMPOL), at the GREGOR solar telescope at different
limb distances on solar disk. Our results show a spatial variation
of scattering linear polarization signals in Sr I 4607Åline at the
granular scale at every $\mu$, starting from 0.2 to 0.8. The correlation
between the polarization signal amplitude and the continuum intensity
imply statistically that the scattering polarization is higher at the
granular regions than in the intergranular lanes.
---------------------------------------------------------
Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
2019A&A...624A.104J Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The radiative
transfer in solar and stellar atmospheres is a typical example of such
a problem. This calls for high-order well-behaved techniques that are
able to interpolate both smooth and discontinuous data. <BR /> Aims:
This article expands on different nonlinear interpolation techniques
capable of guaranteeing high-order accuracy and handling discontinuities
in an accurate and non-oscillatory fashion. The final aim is to propose
new techniques which could be suitable for applications in the context
of numerical radiative transfer. <BR /> Methods: We have proposed
and tested two different techniques. Essentially non-oscillatory
(ENO) techniques generate several candidate interpolations based
on different substencils. The smoothest candidate interpolation is
determined from a measure for the local smoothness, thereby enabling the
essentially non-oscillatory property. Weighted ENO (WENO) techniques
use a convex combination of all candidate substencils to obtain
high-order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and tested a
novel well-performing fourth-order WENO interpolation technique for both
uniform and nonuniform grids. <BR /> Results: Numerical tests prove that
the fourth-order WENO interpolation guarantees fourth-order accuracy
in smooth regions of the interpolated functions. In the presence
of discontinuities, the fourth-order WENO interpolation enables the
non-oscillatory property, avoiding oscillations. Unlike Bézier and
monotonic high-order Hermite interpolations, it does not degenerate
to a linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively handling
discontinuities. This interpolation technique might be particularly
suitable for several problems, including a number of radiative transfer
applications such as multidimensional problems, multigrid methods,
and formal solutions.
---------------------------------------------------------
Title: Spatial variations of the Sr I 4607 Åscattering polarization
signals at subgranular scale observed with ZIMPOL at the GREGOR
telescope
Authors: Kumar Dhara, Sajal; Capozzi, Emilia; Gisler, Daniel; Bianda,
Michele; Ramelli, Renzo; Berdyugina, Svetlana; Alsina, Ernest;
Belluzzi, Luca
2019NCimC..42....6K Altcode:
Sr I 4607 Åspectral line shows one of the strongest scattering
polarization signals in the visible solar spectrum. The amplitudes of
these signals are expected to vary at granular spatial scales. This
variation can be due to changes in the magnetic field intensity and
orientation (Hanle effect) as well as due to spatial and temporal
variations in the plasma properties. Measuring the spatial variation
of such polarization signal are challenging since both high spatial
resolution and high spectropolarimetric sensitivity are required at
the same time. To the aim of measuring these spatial variations at
granular scale, we carried out a spectro-polarimetric measurement
with the Zurich IMaging POLarimeter (ZIMPOL), at the GREGOR solar
telescope at different limb distances on solar disk. Our results show
a spatial variation of scattering linear polarization signals in Sr
I 4607 Åline at the granular scale at every μ, starting from 0.2
to 0.8. The correlation between the scattering linear polarization
signal amplitude and the continuum intensity imply statistically that
the scattering polarization is higher at the granular regions than in
the intergranular lanes.
---------------------------------------------------------
Title: CLASP Constraints on the Magnetization and Geometrical
Complexity of the Chromosphere-Corona Transition Region
Authors: Trujillo Bueno, J.; Štěpán, J.; Belluzzi, L.; Asensio
Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Ishikawa,
R.; Kano, R.; Winebarger, A.; Auchère, F.; Narukage, N.; Kobayashi,
K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara,
H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.;
Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M.
2018ApJ...866L..15T Altcode: 2018arXiv180908865T
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a
suborbital rocket experiment that on 2015 September 3 measured
the linear polarization produced by scattering processes in the
hydrogen Lyα line of the solar disk radiation. The line-center
photons of this spectral line radiation mostly stem from the
chromosphere-corona transition region (TR). These unprecedented
spectropolarimetric observations revealed an interesting surprise,
namely that there is practically no center-to-limb variation (CLV) in
the Q/I line-center signals. Using an analytical model, we first show
that the geometric complexity of the corrugated surface that delineates
the TR has a crucial impact on the CLV of the Q/I and U/I line-center
signals. Second, we introduce a statistical description of the solar
atmosphere based on a 3D model derived from a state-of-the-art radiation
magnetohydrodynamic simulation. Each realization of the statistical
ensemble is a 3D model characterized by a given degree of magnetization
and corrugation of the TR, and for each such realization we solve the
full 3D radiative transfer problem taking into account the impact
of the CLASP instrument degradation on the calculated polarization
signals. Finally, we apply the statistical inference method presented
in a previous paper to show that the TR of the 3D model that produces
the best agreement with the CLASP observations has a relatively weak
magnetic field and a relatively high degree of corrugation. We emphasize
that a suitable way to validate or refute numerical models of the upper
solar chromosphere is by confronting calculations and observations
of the scattering polarization in ultraviolet lines sensitive to the
Hanle effect.
---------------------------------------------------------
Title: A Statistical Inference Method for Interpreting the CLASP
Observations
Authors: Štěpán, J.; Trujillo Bueno, J.; Belluzzi, L.; Asensio
Ramos, A.; Manso Sainz, R.; del Pino Alemán, T.; Casini, R.; Kano, R.;
Winebarger, A.; Auchère, F.; Ishikawa, R.; Narukage, N.; Kobayashi,
K.; Bando, T.; Katsukawa, Y.; Kubo, M.; Ishikawa, S.; Giono, G.; Hara,
H.; Suematsu, Y.; Shimizu, T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.;
Cirtain, J.; Champey, P.; De Pontieu, B.; Carlsson, M.
2018ApJ...865...48S Altcode: 2018arXiv180802725S
On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP) successfully measured the linear polarization produced by
scattering processes in the hydrogen Lyα line of the solar disk
radiation, revealing conspicuous spatial variations in the Q/I and U/I
signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes
encode information on the magnetic field of the chromosphere-corona
transition region, but they are also sensitive to the three-dimensional
structure of this corrugated interface region. With the help of a simple
line-formation model, here we propose a statistical inference method
for interpreting the Lyα line-center polarization observed by CLASP.
---------------------------------------------------------
Title: Formal Solutions for Polarized Radiative
Transfer. IV. Numerical Performances in Practical Problems
Authors: Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
2018ApJ...865...16J Altcode: 2018arXiv180906604J
The numerical computation of reliable and accurate Stokes profiles
is of great relevance in solar physics. In the synthesis process,
many actors play a relevant role: among them the formal solver, the
discrete atmospheric model, and the spectral line. This paper tests
the performances of different numerical schemes in the synthesis
of polarized spectra for different spectral lines and atmospheric
models. The hierarchy between formal solvers is enforced, stressing the
peculiarities of high-order and low-order formal solvers. The density
of grid points necessary for reaching a given accuracy requirement is
quantitatively described for specific situations.
---------------------------------------------------------
Title: Current State of UV Spectro-Polarimetry and its Future
Direction
Authors: Ishikawa, Ryohko; Sakao, Taro; Katsukawa, Yukio; Hara,
Hirohisa; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Kubo, Masahito;
Auchere, Frederic; De Pontieu, Bart; Winebarger, Amy; Kobayashi,
. Ken; Kano, Ryouhei; Narukage, Noriyuki; Trujillo Bueno, Javier;
Song, Dong-uk; Manso Sainz, Rafael; Asensio Ramos, Andres; Leenaarts,
Jorritt; Carlsson, Mats; Bando, Takamasa; Ishikawa, Shin-nosuke;
Tsuneta, Saku; Belluzzi, Luca; Suematsu, Yoshinori; Giono, Gabriel;
Yoshida, Masaki; Goto, Motoshi; Del Pino Aleman, Tanausu; Stepan,
Jiri; Okamoto, Joten; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Champey,
Patrick; Alsina Ballester, Ernest; Casini, Roberto; McKenzie, David;
Rachmeler, Laurel; Bethge, Christian
2018cosp...42E1564I Altcode:
To obtain quantitative information on the magnetic field in low beta
regions (i.e., upper chromosphere and above) has been increasingly
important to understand the energetic phenomena of the outer
solar atmosphere such as flare, coronal heating, and the solar wind
acceleration. In the UV range, there are abundant spectral lines that
originate in the upper chromosphere and transition region. However,
the Zeeman effect in these spectral lines does not give rise to easily
measurable polarization signals because of the weak magnetic field
strength and the larger Doppler broadening compared with the Zeeman
effect. Instead, the Hanle effect in UV lines is expected to be a
suitable diagnostic tool of the magnetic field in the upper atmospheric
layers. To investigate the validity of UV spectro-polarimetry and
the Hanle effect, the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP), which is a NASA sounding- rocket experiment, was launched at
White Sands in US on September 3, 2015. During its 5 minutes ballistic
flight, it successfully performed spectro-polarimetric observations
of the hydrogen Lyman-alpha line (121.57 nm) with an unprecedentedly
high polarization sensitivity of 0.1% in this wavelength range. CLASP
observed the linear polarization produced by scattering process in VUV
lines for the first time and detected the polarization signals which
indicate the operation of the Hanle effect. Following the success
of CLASP, we are confident that UV spectro-polarimetry is the way
to proceed, and we are planning the second flight of CLASP (CLASP2:
Chromospheric LAyer SpectroPolarimeter 2). For this second flight we
will carry out spectro-polarimetry in the Mg II h and k lines around
280 nm, with minimum modifications of the CLASP1 instrument. The linear
polarization in the Mg II k line is induced by scattering processes and
the Hanle effect, being sensitive to magnetic field strengths of 5 to 50
G. In addition, the circular polarizations in the Mg II h and k lines
induced by the Zeeman effect can be measurable in at least plage and
active regions. The combination of the Hanle and Zeeman effects could
help us to more reliably infer the magnetic fields of the upper solar
chromosphere. CLASP2 was selected for flight and is being developed for
launch in the spring of 2019.Based on these sounding rocket experiments
(CLASP1 and 2), we aim at establishing the strategy and refining the
instrument concept for future space missions to explore the enigmatic
atmospheric layers via UV spectro-polarimetry.
---------------------------------------------------------
Title: Structure of the Balmer jump. The isolated hydrogen atom
Authors: Calvo, F.; Belluzzi, L.; Steiner, O.
2018A&A...613A..55C Altcode: 2019arXiv190110241C
Context. The spectrum of the hydrogen atom was explained by Bohr
more than one century ago. We revisit here some of the aspects of the
underlying quantum structure, with a modern formalism, focusing on the
limit of the Balmer series. <BR /> Aims: We investigate the behaviour
of the absorption coefficient of the isolated hydrogen atom in the
neighbourhood of the Balmer limit. <BR /> Methods: We analytically
computed the total cross-section arising from bound-bound and bound-free
transitions in the isolated hydrogen atom at the Balmer limit, and
established a simplified semi-analytical model for the surroundings of
that limit. We worked within the framework of the formalism of Landi
Degl'Innocenti & Landolfi (2004, Astrophys. Space Sci. Lib.,
307), which permits an almost straight-forward generalization of
our results to other atoms and molecules, and which is perfectly
suitable for including polarization phenomena in the problem. <BR />
Results: We analytically show that there is no discontinuity at the
Balmer limit, even though the concept of a "Balmer jump" is still
meaningful. Furthermore, we give a possible definition of the location
of the Balmer jump, and we check that this location is dependent
on the broadening mechanisms. At the Balmer limit, we compute the
cross-section in a fully analytical way. <BR /> Conclusions: The Balmer
jump is produced by a rapid drop of the total Balmer cross-section,
yet this variation is smooth and continuous when both bound-bound and
bound-free processes are taken into account, and its shape and location
is dependent on the broadening mechanisms.
---------------------------------------------------------
Title: Spatial variations of the Sr I 4607 Å scattering polarization
peak
Authors: Bianda, M.; Berdyugina, S.; Gisler, D.; Ramelli, R.; Belluzzi,
L.; Carlin, E. S.; Stenflo, J. O.; Berkefeld, T.
2018A&A...614A..89B Altcode: 2018arXiv180303531B
Context. The scattering polarization signal observed in the
photospheric Sr I 4607 Å line is expected to vary at granular
spatial scales. This variation can be due to changes in the magnetic
field intensity and orientation (Hanle effect), but also to spatial
and temporal variations in the plasma properties. Measuring the
spatial variation of such polarization signal would allow us to
study the properties of the magnetic fields at subgranular scales,
but observations are challenging since both high spatial resolution
and high spectropolarimetric sensitivity are required. <BR /> Aims:
We aim to provide observational evidence of the polarization peak
spatial variations, and to analyze the correlation they might have
with granulation. <BR /> Methods: Observations conjugating high spatial
resolution and high spectropolarimetric precision were performed with
the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope,
taking advantage of the adaptive optics system and the newly installed
image derotator. <BR /> Results: Spatial variations of the scattering
polarization in the Sr I 4607 Å line are clearly observed. The
spatial scale of these variations is comparable with the granular
size. Small correlations between the polarization signal amplitude
and the continuum intensity indicate that the polarization is higher
at the center of granules than in the intergranular lanes.
---------------------------------------------------------
Title: Magneto-optical Effects in the Scattering Polarization Wings
of the Ca I 4227 Å Resonance Line
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2018ApJ...854..150A Altcode: 2017arXiv171100372A; 2017arXiv171100372B
The linear polarization pattern produced by scattering processes in
the Ca I 4227 Å resonance line is a valuable observable for probing
the solar atmosphere. Via the Hanle effect, the very significant Q/I
and U/I line-center signals are sensitive to the presence of magnetic
fields in the lower chromosphere with strengths between 5 and 125 G,
approximately. On the other hand, partial frequency redistribution
(PRD) produces sizable signals in the wings of the Q/I profile,
which have always been thought to be insensitive to the presence
of magnetic fields. Interestingly, novel observations of this line
revealed a surprising behavior: fully unexpected signals in the
wings of the U/I profile and spatial variability in the wings of
both Q/I and U/I. We show that the magneto-optical (MO) terms of
the Stokes-vector transfer equation produce sizable signals in the
wings of U/I and a clear sensitivity of the Q/I and U/I wings to the
presence of photospheric magnetic fields with strengths similar to
those that produce the Hanle effect in the line core. This radiative
transfer investigation on the joint action of scattering processes and
the Hanle and Zeeman effects in the Ca I 4227 Å line should facilitate
the development of more reliable techniques for exploring the magnetism
of stellar atmospheres. To this end, we can now exploit the circular
polarization produced by the Zeeman effect, the magnetic sensitivity
caused by the above-mentioned MO effects in the Q/I and U/I wings,
and the Hanle effect in the line core.
---------------------------------------------------------
Title: The Physics and Diagnostic Potential of Ultraviolet
Spectropolarimetry
Authors: Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio;
Belluzzi, Luca
2018smf..book..183T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Physics and Diagnostic Potential of Ultraviolet
Spectropolarimetry
Authors: Trujillo Bueno, Javier; Landi Degl'Innocenti, Egidio;
Belluzzi, Luca
2017SSRv..210..183T Altcode: 2017SSRv..tmp...15T
The empirical investigation of the magnetic field in the outer solar
atmosphere is a very important challenge in astrophysics. To this end,
we need to identify, measure and interpret observable quantities
sensitive to the magnetism of the upper chromosphere, transition
region and corona. This paper provides an overview of the physics
and diagnostic potential of spectropolarimetry in permitted spectral
lines of the ultraviolet solar spectrum, such as the Mg ii h and k
lines around 2800 Å, the hydrogen Lyman-α line at 1216 Å, and the
Lyman-α line of He ii at 304 Å. The outer solar atmosphere is an
optically pumped vapor and the linear polarization of such spectral
lines is dominated by the atomic level polarization produced by the
absorption and scattering of anisotropic radiation. Its modification
by the action of the Hanle and Zeeman effects in the inhomogeneous
and dynamic solar atmosphere needs to be carefully understood because
it encodes the magnetic field information. The circular polarization
induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii
h & k) is also of diagnostic interest, especially for probing
the outer solar atmosphere in plages and more active regions. The few
(pioneering) observational attempts carried out so far to measure the
ultraviolet spectral line polarization produced by optically pumped
atoms in the upper chromosphere, transition region and corona are also
discussed. We emphasize that ultraviolet spectropolarimetry is a key
gateway to the outer atmosphere of the Sun and of other stars.
---------------------------------------------------------
Title: Measurement of the evolution of the magnetic field of the
quiet photosphere during a solar cycle
Authors: Ramelli, Renzo; Bianda, Michele; Berdyugina, Svetlana;
Belluzzi, Luca; Kleint, Lucia
2017arXiv170803287R Altcode:
The solar photosphere is filled by a magnetic field which is tangled
on scales much smaller than the resolution capability of solar
telescopes. This hidden magnetic field can be investigated via the
Hanle effect. In 2007 we started a synoptic program to explore if the
magnetic flux of the quiet photosphere varies with the solar cycle. For
this purpose we applied a differential Hanle effect technique based
on observations of scattering polarization in C$_2$ molecular lines
around 514.0 nm, taken generally every month. Our results now span
almost one complete solar cycle.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: Rachmeler, Laurel; E McKenzie, David; Ishikawa, Ryohko;
Trujillo Bueno, Javier; Auchère, Frédéric; Kobayashi, Ken;
Winebarger, Amy; Bethge, Christian; Kano, Ryouhei; Kubo, Masahito;
Song, Donguk; Narukage, Noriyuki; Ishikawa, Shin-nosuke; De Pontieu,
Bart; Carlsson, Mats; Yoshida, Masaki; Belluzzi, Luca; Stepan, Jiri;
del Pino Alemná, Tanausú; Ballester, Ernest Alsina; Asensio Ramos,
Andres
2017SPD....4811010R Altcode:
We present the instrument, science case, and timeline of the CLASP2
sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha
Spectro-Polarimeter) sounding rocket flight in 2015 resulted in
the first-ever linear polarization measurements of solar hydrogen
Lyman-alpha line, which is sensitive to the Hanle effect and can be used
to constrain the magnetic field and geometric complexity of the upper
chromosphere. Ly-alpha is one of several upper chromospheric lines that
contain magnetic information. In the spring of 2019, we will re-fly
the modified CLASP telescope to measure the full Stokes profile of Mg
II h & k near 280 nm. This set of lines is sensitive to the upper
chromospheric magnetic field via both the Hanle and the Zeeman effects.
---------------------------------------------------------
Title: Formal Solutions for Polarized Radiative
Transfer. II. High-order Methods
Authors: Janett, Gioele; Steiner, Oskar; Belluzzi, Luca
2017ApJ...845..104J Altcode: 2017arXiv170901280J
When integrating the radiative transfer equation for polarized light,
the necessity of high-order numerical methods is well known. In fact,
well-performing high-order formal solvers enable higher accuracy and
the use of coarser spatial grids. Aiming to provide a clear comparison
between formal solvers, this work presents different high-order
numerical schemes and applies the systematic analysis proposed by
Janett et al., emphasizing their advantages and drawbacks in terms of
order of accuracy, stability, and computational cost.
---------------------------------------------------------
Title: CLASP2: The Chromospheric LAyer Spectro-Polarimeter
Authors: Rachmeler, Laurel A.; McKenzie, D. E.; Ishikawa, R.;
Trujillo-Bueno, J.; Auchere, F.; Kobayashi, K.; Winebarger, A.;
Bethge, C.; Kano, R.; Kubo, M.; Song, D.; Narukage, N.; Ishikawa, S.;
De Pontieu, B.; Carlsson, M.; Yoshida, M.; Belluzzi, L.; Stepan, J.;
del Pino Alemán, T.; Alsina Ballester, E.; Asensio Ramos, A.
2017shin.confE..79R Altcode:
We present the instrument, science case, and timeline of the CLASP2
sounding rocket mission. The successful CLASP (Chromospheric Lyman-Alpha
Spectro-Polarimeter) sounding rocket flight in 2015 resulted in
the first-ever linear polarization measurements of solar hydrogen
Lyman-alpha line, which is sensitive to the Hanle effect and can be used
to constrain the magnetic field and geometric complexity of the upper
chromosphere. Ly-alpha is one of several upper chromospheric lines that
contain magnetic information. In the spring of 2019, we will re-fly
the modified CLASP telescope to measure the full Stokes profile of Mg
II h & k near 280 nm. This set of lines is sensitive to the upper
chromospheric magnetic field via both the Hanle and the Zeeman effects.
---------------------------------------------------------
Title: Indication of the Hanle Effect by Comparing the Scattering
Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2017ApJ...841...31I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter is a sounding
rocket experiment that has provided the first successful measurement
of the linear polarization produced by scattering processes in
the hydrogen Lyα line (121.57 nm) radiation of the solar disk. In
this paper, we report that the Si III line at 120.65 nm also shows
scattering polarization and we compare the scattering polarization
signals observed in the Lyα and Si III lines in order to search for
observational signatures of the Hanle effect. We focus on four selected
bright structures and investigate how the U/I spatial variations vary
between the Lyα wing, the Lyα core, and the Si III line as a function
of the total unsigned photospheric magnetic flux estimated from Solar
Dynamics Observatory/Helioseismic and Magnetic Imager observations. In
an internetwork region, the Lyα core shows an antisymmetric spatial
variation across the selected bright structure, but it does not show
it in other more magnetized regions. In the Si III line, the spatial
variation of U/I deviates from the above-mentioned antisymmetric
shape as the total unsigned photospheric magnetic flux increases. A
plausible explanation of this difference is the operation of the Hanle
effect. We argue that diagnostic techniques based on the scattering
polarization observed simultaneously in two spectral lines with very
different sensitivities to the Hanle effect, like Lyα and Si III,
are of great potential interest for exploring the magnetism of the
upper solar chromosphere and transition region.
---------------------------------------------------------
Title: Formal Solutions for Polarized Radiative Transfer. I. The
DELO Family
Authors: Janett, Gioele; Carlin, Edgar S.; Steiner, Oskar; Belluzzi,
Luca
2017ApJ...840..107J Altcode: 2017arXiv170901274J
The discussion regarding the numerical integration of the polarized
radiative transfer equation is still open and the comparison between
the different numerical schemes proposed by different authors in the
past is not fully clear. Aiming at facilitating the comprehension of
the advantages and drawbacks of the different formal solvers, this
work presents a reference paradigm for their characterization based
on the concepts of order of accuracy, stability, and computational
cost. Special attention is paid to understand the numerical methods
belonging to the Diagonal Element Lambda Operator family, in an attempt
to highlight their specificities.
---------------------------------------------------------
Title: Polarization Calibration of the Chromospheric Lyman-Alpha
SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV
Range. Part II: In-Flight Calibration
Authors: Giono, G.; Ishikawa, R.; Narukage, N.; Kano, R.; Katsukawa,
Y.; Kubo, M.; Ishikawa, S.; Bando, T.; Hara, H.; Suematsu, Y.;
Winebarger, A.; Kobayashi, K.; Auchère, F.; Trujillo Bueno, J.;
Tsuneta, S.; Shimizu, T.; Sakao, T.; Cirtain, J.; Champey, P.; Asensio
Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu,
B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M.
2017SoPh..292...57G Altcode:
The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding
rocket instrument designed to measure for the first time the linear
polarization of the hydrogen Lyman-α line (121.6 nm). The instrument
was successfully launched on 3 September 2015 and observations were
conducted at the solar disc center and close to the limb during the
five-minutes flight. In this article, the disc center observations are
used to provide an in-flight calibration of the instrument spurious
polarization. The derived in-flight spurious polarization is consistent
with the spurious polarization levels determined during the pre-flight
calibration and a statistical analysis of the polarization fluctuations
from solar origin is conducted to ensure a 0.014% precision on the
spurious polarization. The combination of the pre-flight and the
in-flight polarization calibrations provides a complete picture of
the instrument response matrix, and a proper error transfer method
is used to confirm the achieved polarization accuracy. As a result,
the unprecedented 0.1% polarization accuracy of the instrument in the
vacuum ultraviolet is ensured by the polarization calibration.
---------------------------------------------------------
Title: Discovery of Scattering Polarization in the Hydrogen Lyα
Line of the Solar Disk Radiation
Authors: Kano, R.; Trujillo Bueno, J.; Winebarger, A.; Auchère, F.;
Narukage, N.; Ishikawa, R.; Kobayashi, K.; Bando, T.; Katsukawa, Y.;
Kubo, M.; Ishikawa, S.; Giono, G.; Hara, H.; Suematsu, Y.; Shimizu,
T.; Sakao, T.; Tsuneta, S.; Ichimoto, K.; Goto, M.; Belluzzi, L.;
Štěpán, J.; Asensio Ramos, A.; Manso Sainz, R.; Champey, P.;
Cirtain, J.; De Pontieu, B.; Casini, R.; Carlsson, M.
2017ApJ...839L..10K Altcode: 2017arXiv170403228K
There is a thin transition region (TR) in the solar atmosphere where
the temperature rises from 10,000 K in the chromosphere to millions
of degrees in the corona. Little is known about the mechanisms that
dominate this enigmatic region other than the magnetic field plays a
key role. The magnetism of the TR can only be detected by polarimetric
measurements of a few ultraviolet (UV) spectral lines, the Lyα line
of neutral hydrogen at 121.6 nm (the strongest line of the solar UV
spectrum) being of particular interest given its sensitivity to the
Hanle effect (the magnetic-field-induced modification of the scattering
line polarization). We report the discovery of linear polarization
produced by scattering processes in the Lyα line, obtained with
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket
experiment. The Stokes profiles observed by CLASP in quiet regions of
the solar disk show that the Q/I and U/I linear polarization signals are
of the order of 0.1% in the line core and up to a few percent in the
nearby wings, and that both have conspicuous spatial variations with
scales of ∼10 arcsec. These observations help constrain theoretical
models of the chromosphere-corona TR and extrapolations of the
magnetic field from photospheric magnetograms. In fact, the observed
spatial variation from disk to limb of polarization at the line core
and wings already challenge the predictions from three-dimensional
magnetohydrodynamical models of the upper solar chromosphere.
---------------------------------------------------------
Title: The Transfer of Resonance Line Polarization with Partial
Frequency Redistribution in the General Hanle-Zeeman Regime
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2017ApJ...836....6A Altcode: 2016arXiv160905723B; 2016arXiv160905723A; 2017ApJ...836....6B
The spectral line polarization encodes a wealth of information about the
thermal and magnetic properties of the solar atmosphere. Modeling the
Stokes profiles of strong resonance lines is, however, a complex problem
both from a theoretical and computational point of view, especially when
partial frequency redistribution (PRD) effects need to be taken into
account. In this work, we consider a two-level atom in the presence
of magnetic fields of arbitrary intensity (Hanle-Zeeman regime) and
orientation, both deterministic and micro-structured. Working within the
framework of a rigorous PRD theoretical approach, we have developed
a numerical code that solves the full non-LTE radiative transfer
problem for polarized radiation, in one-dimensional models of the
solar atmosphere, accounting for the combined action of the Hanle and
Zeeman effects, as well as for PRD phenomena. After briefly discussing
the relevant equations, we describe the iterative method of solution
of the problem and the numerical tools that we have developed and
implemented. We finally present some illustrative applications to two
resonance lines that form at different heights in the solar atmosphere,
and provide a detailed physical interpretation of the calculated Stokes
profiles. We find that magneto-optical effects have a strong impact on
the linear polarization signals that PRD effects produce in the wings of
strong resonance lines. We also show that the weak-field approximation
has to be used with caution when PRD effects are considered.
---------------------------------------------------------
Title: Discovery of Ubiquitous Fast-Propagating Intensity Disturbances
by the Chromospheric Lyman Alpha Spectropolarimeter (CLASP)
Authors: Kubo, M.; Katsukawa, Y.; Suematsu, Y.; Kano, R.; Bando,
T.; Narukage, N.; Ishikawa, R.; Hara, H.; Giono, G.; Tsuneta, S.;
Ishikawa, S.; Shimizu, T.; Sakao, T.; Winebarger, A.; Kobayashi, K.;
Cirtain, J.; Champey, P.; Auchère, F.; Trujillo Bueno, J.; Asensio
Ramos, A.; Štěpán, J.; Belluzzi, L.; Manso Sainz, R.; De Pontieu,
B.; Ichimoto, K.; Carlsson, M.; Casini, R.; Goto, M.
2016ApJ...832..141K Altcode:
High-cadence observations by the slit-jaw (SJ) optics system of the
sounding rocket experiment known as the Chromospheric Lyman Alpha
Spectropolarimeter (CLASP) reveal ubiquitous intensity disturbances
that recurrently propagate in either the chromosphere or the transition
region or both at a speed much higher than the speed of sound. The
CLASP/SJ instrument provides a time series of two-dimensional images
taken with broadband filters centered on the Lyα line at a 0.6 s
cadence. The multiple fast-propagating intensity disturbances appear in
the quiet Sun and in an active region, and they are clearly detected in
at least 20 areas in a field of view of 527″ × 527″ during the 5
minute observing time. The apparent speeds of the intensity disturbances
range from 150 to 350 km s<SUP>-1</SUP>, and they are comparable
to the local Alfvén speed in the transition region. The intensity
disturbances tend to propagate along bright elongated structures away
from areas with strong photospheric magnetic fields. This suggests
that the observed fast-propagating intensity disturbances are related
to the magnetic canopy structures. The maximum distance traveled by
the intensity disturbances is about 10″, and the widths are a few
arcseconds, which are almost determined by a pixel size of 1.″03. The
timescale of each intensity pulse is shorter than 30 s. One possible
explanation for the fast-propagating intensity disturbances observed
by CLASP is magnetohydrodynamic fast-mode waves.
---------------------------------------------------------
Title: The Magnetic Sensitivity of the Mg II k Line to the Joint
Action of Hanle, Zeeman, and Magneto-optical Effects
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2016ApJ...831L..15A Altcode: 2016arXiv161000649A
We highlight the main results of a radiative transfer investigation on
the magnetic sensitivity of the solar Mg II k resonance line at 2795.5
Å, accounting for the joint action of the Hanle and Zeeman effects
as well as partial frequency redistribution phenomena. We confirm
that at the line center, the linear polarization signals produced by
scattering processes are measurable, and that they are sensitive, via
the Hanle effect, to magnetic fields with strengths between 5 and 50 G,
approximately. We also show that the Zeeman effect produces conspicuous
circular polarization signals, especially for longitudinal fields
stronger than 50 G, which can be used to estimate the magnetization of
the solar chromosphere via the familiar magnetograph formula. The most
novel result is that magneto-optical effects produce, in the wings of
the line, a decrease of the Q/I scattering polarization pattern and
the appearance of U/I signals (I.e., a rotation of the plane of linear
polarization). This sensitivity of the Q/I and U/I wing signals to
both weak (∼5 G) and stronger magnetic fields expands the scientific
interest of the Mg II k line for probing the chromosphere in quiet
and active regions of the Sun.
---------------------------------------------------------
Title: Chromospheric LAyer SpectroPolarimeter (CLASP2)
Authors: Narukage, Noriyuki; McKenzie, David E.; Ishikawa, Ryoko;
Trujillo-Bueno, Javier; De Pontieu, Bart; Kubo, Masahito; Ishikawa,
Shin-nosuke; Kano, Ryouhei; Suematsu, Yoshinori; Yoshida, Masaki;
Rachmeler, Laurel A.; Kobayashi, Ken; Cirtain, Jonathan W.; Winebarger,
Amy R.; Asensio Ramos, Andres; del Pino Aleman, Tanausu; Štępán,
Jiri; Belluzzi, Luca; Larruquert, Juan Ignacio; Auchère, Frédéric;
Leenaarts, Jorrit; Carlsson, Mattias J. L.
2016SPIE.9905E..08N Altcode:
The sounding rocket Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP) was launched on September 3rd, 2015, and successfully detected
(with a polarization accuracy of 0.1 %) the linear polarization signals
(Stokes Q and U) that scattering processes were predicted to produce
in the hydrogen Lyman-alpha line (Lyα 121.567 nm). Via the Hanle
effect, this unique data set may provide novel information about the
magnetic structure and energetics in the upper solar chromosphere. The
CLASP instrument was safely recovered without any damage and we have
recently proposed to dedicate its second flight to observe the four
Stokes profiles in the spectral region of the Mg II h and k lines
around 280 nm; in these lines the polarization signals result from
scattering processes and the Hanle and Zeeman effects. Here we describe
the modifications needed to develop this new instrument called the
"Chromospheric LAyer SpectroPolarimeter" (CLASP2).
---------------------------------------------------------
Title: Spectro-polarimetric observation in UV with CLASP to probe
the chromosphere and transition region
Authors: Kano, Ryouhei; Ishikawa, Ryohko; Winebarger, Amy R.; Auchère,
Frédéric; Trujillo Bueno, Javier; Narukage, Noriyuki; Kobayashi,
Ken; Bando, Takamasa; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-Nosuke; Giono, Gabriel; Hara, Hirohisa; Suematsu, Yoshinori;
Shimizu, Toshifumi; Sakao, Taro; Tsuneta, Saku; Ichimoto, Kiyoshi;
Goto, Motoshi; Cirtain, Jonathan W.; De Pontieu, Bart; Casini, Roberto;
Manso Sainz, Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi,
Luca; Carlsson, Mats
2016SPD....4710107K Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a NASA
sounding-rocket experiment that was performed in White Sands in
the US on September 3, 2015. During its 5-minute ballistic flight,
CLASP successfully made the first spectro-polarimetric observation in
the Lyman-alpha line (121.57 nm) originating in the chromosphere and
transition region. Since the Lyman-alpha polarization is sensitive
to magnetic field of 10-100 G by the Hanle effect, we aim to infer
the magnetic field information in such upper solar atmosphere with
this experiment.The obtained CLASP data showed that the Lyman-alpha
scattering polarization is about a few percent in the wings and
the order of 0.1% in the core near the solar limb, as it had been
theoretically predicted, and that both polarization signals have a
conspicuous spatio-temporal variability. CLASP also observed another
upper-chromospheric line, Si III (120.65 nm), whose critical field
strength for the Hanle effect is 290 G, and showed a measurable
scattering polarization of a few % in this line. The polarization
properties of the Si III line could facilitate the interpretation of
the scattering polarization observed in the Lyman-alpha line.In this
presentation, we would like to show how the upper chromosphere and
transition region are seen in the polarization of these UV lines and
discuss the possible source of these complicated polarization signals.
---------------------------------------------------------
Title: Polarized radiative transfer in discontinuous media
Authors: Steiner, O.; Züger, F.; Belluzzi, L.
2016A&A...586A..42S Altcode:
Context. Observations of the solar atmosphere of ever increasing
spatial resolution reveal steep gradients in the magnetic field
and in thermal states. Likewise, numerical simulations of the solar
atmosphere show contact discontinuities and shock fronts. This asks
for the development of robust methods for computing the radiative
transfer of polarized light in discontinuous media. <BR /> Aims:
Here, we propose a new concept for dealing with discontinuities in
the radiative transfer of polarized light and carry out a few basic
test calculations. While in the past, the focus was on interpolating
the source function with ever-increasing accuracy and smoothness,
we propose to take the opposite approach by reconstructing it with
piecewise continuous functions, taking discontinuities on purpose into
account. This concept is known from computational fluid dynamics. <BR />
Methods: Test calculations were carried out for (I) a Milne-Eddington
atmosphere; (II) an atmosphere featuring a single discontinuity that
is shifted across one grid cell; and (III) a two-layered atmosphere
with discontinuities in the source function, the velocity, and the
magnetic field. <BR /> Results: It is shown that the method of piecewise
continuous reconstruction is a viable approach to solving the radiative
transfer equation for polarized light. In the special case where a
discontinuity coincides with a computational cell interface, the method
is capable of producing the exact solution. Overall, the assessment
of the piecewise continuous reconstruction method turns out to be
cautiously positive, but it does not lead to an order-of-magnitude
improvement in accuracy over conventional methods for the examples
considered here. More realistic model atmospheres need to be considered
for judging practical applicability.
---------------------------------------------------------
Title: Radiative Transfer Modeling of the Enigmatic Scattering
Polarization in the Solar Na I D1 Line
Authors: Belluzzi, Luca; Trujillo Bueno, Javier; Landi Degl'Innocenti,
Egidio
2015ApJ...814..116B Altcode: 2015arXiv151105801B
The modeling of the peculiar scattering polarization signals
observed in some diagnostically important solar resonance lines
requires the consideration of the detailed spectral structure of the
incident radiation field as well as the possibility of ground level
polarization, along with the atom's hyperfine structure and quantum
interference between hyperfine F-levels pertaining either to the same
fine structure J-level, or to different J-levels of the same term. Here
we present a theoretical and numerical approach suitable for solving
this complex non-LTE radiative transfer problem. This approach is based
on the density-matrix metalevel theory (where each level is viewed as
a continuous distribution of sublevels) and on accurate formal solvers
of the transfer equations and efficient iterative methods. We show an
application to the D-lines of Na i, with emphasis on the enigmatic
D<SUB>1</SUB> line, pointing out the observable signatures of the
various physical mechanisms considered. We demonstrate that the linear
polarization observed in the core of the D<SUB>1</SUB> line may be
explained by the effect that one gets when the detailed spectral
structure of the anisotropic radiation responsible for the optical
pumping is taken into account. This physical ingredient is capable of
introducing significant scattering polarization in the core of the Na
i D<SUB>1</SUB> line without the need for ground-level polarization.
---------------------------------------------------------
Title: Atomic Scattering Polarization. Observations, Modeling,
Predictions
Authors: Trujillo Bueno, J.; Del Pino Alemán, T.; Belluzzi, L.
2015IAUS..305..127T Altcode: 2015IAUS..305..127B
This paper highlights very recent advances concerning the identification
of new mechanisms that introduce polarization in spectral lines,
which turn out to be key for understanding some of the most enigmatic
scattering polarization signals of the solar visible spectrum. We also
show a radiative transfer prediction on the scattering polarization
pattern across the Mg ii h & k lines, whose radiation can only be
observed from space.
---------------------------------------------------------
Title: Isotropic Inelastic Collisions in a Multiterm Atom with
Hyperfine Structure
Authors: Belluzzi, Luca; Landi Degl'Innocenti, Egidio; Trujillo
Bueno, Javier
2015ApJ...812...73B Altcode:
A correct modeling of the scattering polarization profiles observed
in some spectral lines of diagnostic interest, the sodium doublet
being one of the most important examples, requires taking hyperfine
structure (HFS) and quantum interference between different J-levels
into account. An atomic model suitable for taking these physical
ingredients into account is the so-called multiterm atom with HFS. In
this work, we introduce and study the transfer and relaxation rates
due to isotropic inelastic collisions with electrons, which enter the
statistical equilibrium equations (SEE) for the atomic density matrix
of this atomic model. Under the hypothesis that the electron-atom
interaction is described by a dipolar operator, we provide useful
relations between the rates describing the transfer and relaxation of
quantum interference between different levels (whose numerical values
are in most cases unknown) and the usual rates for the atomic level
populations, for which experimental data and/or approximate theoretical
expressions are generally available. For the particular case of a
two-term atom with HFS, we present an analytical solution of the SEE
for the spherical statistical tensors of the upper term, including both
radiative and collisional processes, and we derive the expression of
the emission coefficient in the four Stokes parameters. Finally, an
illustrative application to the Na i D<SUB>1</SUB> and D<SUB>2</SUB>
lines is presented.
---------------------------------------------------------
Title: Synoptic program to measure the evolution of the photospheric
magnetic field during a solar cycle
Authors: Ramelli, Renzo; Bianda, Michele; Berdyugina, Svetlana;
Stenflo, Jan Olof; Belluzzi, Luca
2015IAUGA..2257074R Altcode:
The solar photosphere is seething with a vast amount of magnetic flux
tangled on scales much smaller than the resolution scale of solar
telescopes that can be investigated by considering the Hanle effect. In
2007, near a minimum of the solar cycle, we started a synoptic program
to explore possible variations of such hidden magnetic flux with
the solar cycle, through the application of a differential Hanle
effect technique on observations of scattering polarization in C2
molecular lines in the region around 514.0 nm. The observing program
is still ongoing generally with the cadence of about one month. The
observations obtained up to now, which include the recent maximum
of the solar activity, don't show large variations of the turbulent
unresolved magnetic field. If the apparent constancy is confirmed
through the current cycle, than it will have important implications,
since it provides hints on the existence of a local dynamo effect at
granular and sub-granular scale, uncorrelated with the global magnetic
field varying with the solar cycle.
---------------------------------------------------------
Title: CLASP: A UV Spectropolarimeter on a Sounding Rocket for
Probing theChromosphere-Corona Transition Regio
Authors: Ishikawa, Ryohko; Kano, Ryouhei; Winebarger, Amy; Auchere,
Frederic; Trujillo Bueno, Javier; Bando, Takamasa; Narukage,
Noriyuki; Kobayashi, Ken; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-nosuke; Giono, Gabriel; Tsuneta, Saku; Hara, Hirohisa; Suematsu,
Yoshinori; Shimizu, Toshifumi; Sakao, Taro; Ichimoto, Kiyoshi;
Cirtain, Jonathan; De Pontieu, Bart; Casini, Roberto; Manso Sainz,
Rafael; Asensio Ramos, Andres; Stepan, Jiri; Belluzzi, Luca
2015IAUGA..2254536I Altcode:
The wish to understand the energetic phenomena of the outer solar
atmosphere makes it increasingly important to achieve quantitative
information on the magnetic field in the chromosphere-corona
transition region. To this end, we need to measure and model the
linear polarization produced by scattering processes and the Hanle
effect in strong UV resonance lines, such as the hydrogen Lyman-alpha
line. A team consisting of Japan, USA, Spain, France, and Norway has
been developing a sounding rocket experiment called the Chromospheric
Lyman-alpha Spectro-Polarimeter (CLASP). The aim is to detect the
scattering polarization produced by anisotropic radiation pumping in
the hydrogen Lyman-alpha line (121.6 nm), and via the Hanle effect to
try to constrain the magnetic field vector in the upper chromosphere
and transition region. In this talk, we will present an overview
of our CLASP mission, its scientific objectives, ground tests made,
and the latest information on the launch planned for the Summer of 2015.
---------------------------------------------------------
Title: A Sounding Rocket Experiment for the Chromospheric Lyman-Alpha
Spectro-Polarimeter (CLASP)
Authors: Kubo, M.; Kano, R.; Kobayashi, K.; Bando, T.; Narukage, N.;
Ishikawa, R.; Tsuneta, S.; Katsukawa, Y.; Ishikawa, S.; Suematsu, Y.;
Hara, H.; Shimizu, T.; Sakao, T.; Ichimoto, K.; Goto, M.; Holloway,
T.; Winebarger, A.; Cirtain, J.; De Pontieu, B.; Casini, R.; Auchère,
F.; Trujillo Bueno, J.; Manso Sainz, R.; Belluzzi, L.; Asensio Ramos,
A.; Štěpán, J.; Carlsson, M.
2014ASPC..489..307K Altcode:
A sounding-rocket experiment called the Chromospheric Lyman-Alpha
Spectro-Polarimeter (CLASP) is presently under development to measure
the linear polarization profiles in the hydrogen Lyman-alpha (Lyα)
line at 121.567 nm. CLASP is a vacuum-UV (VUV) spectropolarimeter to aim
for first detection of the linear polarizations caused by scattering
processes and the Hanle effect in the Lyα line with high accuracy
(0.1%). This is a fist step for exploration of magnetic fields in
the upper chromosphere and transition region of the Sun. Accurate
measurements of the linear polarization signals caused by scattering
processes and the Hanle effect in strong UV lines like Lyα are
essential to explore with future solar telescopes the strength
and structures of the magnetic field in the upper chromosphere and
transition region of the Sun. The CLASP proposal has been accepted by
NASA in 2012, and the flight is planned in 2015.
---------------------------------------------------------
Title: Theoretical Schemes for the Interpretation of Solar
Polarimetric Observations: An Overview and Some New Ideas
Authors: Landi Degl'Innocenti, E.; Belluzzi, L.
2014ASPC..489...99L Altcode:
The theoretical scheme based on the density matrix formalism,
and self-consistently derived from the principles of Quantum
Electrodynamics, is presently one of the most solid frameworks
for the interpretation of solar polarimetric observations. This
scheme has been highly successful, mainly for the interpretation of
polarimetric signals that can be described in the so-called limit of
complete frequency redistribution (CRD). However, it suffers from
the severe limitation that partial frequency redistribution (PRD)
effects cannot be accounted for. The metalevel theory can handle
PRD effects, but all the attempts for its generalization to account
consistently for collisions and lower-level coherences have failed. The
redistribution matrix approach is very suitable for taking PRD effects
into account. However, it can consistently describe only two-level
atoms with unpolarized lower level. New heuristic approaches have been
proposed for treating two-term atoms, but they are based on rather
crude approximations and their validity and physical consistency are
far from being firmly established. Some few reflections that may lead to
the establishment of a self-consistent set of equations, fully derived
from first principles, are put forward. These reflections are based
on the introduction of the Fourier transform of the density matrix.
---------------------------------------------------------
Title: On the Inversion of the Scattering Polarization and the Hanle
Effect Signals in the Hydrogen Lyα Line
Authors: Ishikawa, R.; Asensio Ramos, A.; Belluzzi, L.; Manso Sainz,
R.; Štěpán, J.; Trujillo Bueno, J.; Goto, M.; Tsuneta, S.
2014ApJ...787..159I Altcode: 2014arXiv1404.0786I
Magnetic field measurements in the upper chromosphere and above,
where the gas-to-magnetic pressure ratio β is lower than unity,
are essential for understanding the thermal structure and dynamical
activity of the solar atmosphere. Recent developments in the theory and
numerical modeling of polarization in spectral lines have suggested
that information on the magnetic field of the chromosphere-corona
transition region could be obtained by measuring the linear polarization
of the solar disk radiation at the core of the hydrogen Lyα line at
121.6 nm, which is produced by scattering processes and the Hanle
effect. The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)
sounding rocket experiment aims to measure the intensity (Stokes I)
and the linear polarization profiles (Q/I and U/I) of the hydrogen
Lyα line. In this paper, we clarify the information that the Hanle
effect can provide by applying a Stokes inversion technique based on
a database search. The database contains all theoretical Q/I and U/I
profiles calculated in a one-dimensional semi-empirical model of the
solar atmosphere for all possible values of the strength, inclination,
and azimuth of the magnetic field vector, though this atmospheric
region is highly inhomogeneous and dynamic. We focus on understanding
the sensitivity of the inversion results to the noise and spectral
resolution of the synthetic observations as well as the ambiguities and
limitation inherent to the Hanle effect when only the hydrogen Lyα is
used. We conclude that spectropolarimetric observations with CLASP can
indeed be a suitable diagnostic tool for probing the magnetism of the
transition region, especially when complemented with information on
the magnetic field azimuth that can be obtained from other instruments.
---------------------------------------------------------
Title: The transfer of resonance line polarization with partial
frequency redistribution and J-state interference. Theoretical
approach and numerical methods
Authors: Belluzzi, L.; Trujillo Bueno, J.
2014A&A...564A..16B Altcode: 2014arXiv1403.1701B
The linear polarization signals produced by scattering processes in
strong resonance lines are rich in information on the magnetic and
thermal structure of the chromosphere and transition region of the
Sun and of other stars. A correct modeling of these signals requires
accounting for partial frequency redistribution effects, as well
as for the impact of quantum interference between different fine
structure levels (J-state interference). In this paper, we present a
theoretical approach suitable for modeling the transfer of resonance
line polarization when taking these effects into account, along with an
accurate numerical method of solution of the problem's equations. We
consider a two-term atom with unpolarized lower term and infinitely
sharp lower levels, in the absence of magnetic fields. We show that
by making simple formal substitutions on the quantum numbers, the
theoretical approach derived here for a two-term atom can also be
applied to describe a two-level atom with hyperfine structure. An
illustrative application to the Mg ii doublet around 2800 Å is
presented.
---------------------------------------------------------
Title: A Key Physical Mechanism for Understanding the Enigmatic
Linear Polarization of the Solar Ba II and Na I D<SUB>1</SUB> Lines
Authors: Belluzzi, Luca; Trujillo Bueno, Javier
2013ApJ...774L..28B Altcode: 2013arXiv1308.5422B
The linearly polarized spectrum of the solar limb radiation produced by
scattering processes is of great diagnostic potential for exploring the
magnetism of the solar atmosphere. This spectrum shows an impressive
richness of spectral details and enigmatic Q/I signals, whose physical
origin must be clearly understood before they can be exploited for
diagnostic purposes. The most enduring enigma is represented by the
polarization signals observed in the D<SUB>1</SUB> resonance lines
of Na I (5896 Å) and Ba II (4934 Å), which were expected to be
intrinsically unpolarizable. The totality of sodium and 18% of barium
have hyperfine structure (HFS), and it has been argued that the only way
to produce a scattering polarization signal in such lines is through
the presence of a substantial amount of atomic polarization in their
lower HFS levels. The strong sensitivity of these long-lived levels
to depolarizing mechanisms led to the paradoxical conclusion that
the observed D<SUB>1</SUB>-line polarization is incompatible with the
presence in the lower solar chromosphere of inclined magnetic fields
sensibly stronger than 0.01 G. Here we show that by properly taking
into account the fact that the solar D<SUB>1</SUB>-line radiation has
a non-negligible spectral structure over the short frequency interval
spanned by the HFS transitions, it is possible to produce scattering
polarization signals in the D<SUB>1</SUB> lines of Na I and Ba II
without the need of ground-level polarization. The resulting linear
polarization is not so easily destroyed by elastic collisions and/or
magnetic fields.
---------------------------------------------------------
Title: Chromospheric Lyman Alpha SpectroPolarimeter: CLASP
Authors: Kobayashi, Ken; Kano, R.; Trujillo Bueno, J.; Winebarger,
A. R.; Cirtain, J. W.; Bando, T.; De Pontieu, B.; Ishikawa, R.;
Katsukawa, Y.; Kubo, M.; Narukage, N.; Sakao, T.; Tsuneta, S.;
Auchère, F.; Asensio Ramos, A.; Belluzzi, L.; Carlsson, M.; Casini,
R.; Hara, H.; Ichimoto, K.; Manso Sainz, R.; Shimizu, T.; Stepan,
J.; Suematsu, Y.; Holloway, T.
2013SPD....44..142K Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a VUV
spectropolarimeter optimized for measuring the linear polarization of
the Lyman-alpha line (121.6 nm). The Lyman-alpha line is predicted to
show linear polarization caused by atomic scattering in the chromosphere
and modified by the magnetic field through the Hanle effect. The
Hanle effect is sensitive to weaker magnetic fields than Zeeman
effect, and is not canceled by opposing fields, making it sensitive
to tangled or unresolved magnetic field structures. These factors make
the Hanle effect a valuable tool for probing the magnetic field in the
chromosphere above the quiet sun. To meet this goal, CLASP is designed
to measure linear polarization with 0.1% polarization sensitivity
at 0.01 nm spectral resolution and 10" spatial resolution. CLASP is
scheduled to be launched in 2015.
---------------------------------------------------------
Title: Theoretical formulation of Doppler redistribution in scattering
polarization within the framework of the velocity-space density
matrix formalism
Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.
2013A&A...552A..72B Altcode: 2013arXiv1302.2887B
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations
represent an important physical aspect that, however, is often neglected
in practical applications when adopting the simplifying approximation
of complete redistribution on velocity. In this paper, we present an
application of the non-LTE problem for polarized radiation taking
such correlations into account through the velocity-space density
matrix formalism. We consider a two-level atom with infinitely sharp
upper and lower levels, and we derive the corresponding statistical
equilibrium equations, neglecting the contribution of velocity-changing
collisions. Coupling such equations with the radiative transfer
equations for polarized radiation, we derive a set of coupled equations
for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The
equations presented in this paper provide a complete and solid
description of the physics of pure Doppler redistribution, a phenomenon
generally described within the framework of the redistribution matrix
formalism. The redistribution matrix corresponding to this problem
(generally referred to as R<SUB>I</SUB>) is derived starting from the
statistical equilibrium equations for the velocity-space density matrix
and from the radiative transfer equations for polarized radiation,
thus showing the equivalence of the two approaches.
---------------------------------------------------------
Title: Isotropic inelastic and superelastic collisional rates in a
multiterm atom
Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.
2013A&A...551A..84B Altcode: 2013arXiv1302.0990B
The spectral line polarization of the radiation emerging from a
magnetized astrophysical plasma depends on the state of the atoms
within the medium, whose determination requires considering the
interactions between the atoms and the magnetic field, between the
atoms and photons (radiative transitions), and between the atoms and
other material particles (collisional transitions). In applications
within the framework of the multiterm model atom (which accounts for
quantum interference between magnetic sublevels pertaining either
to the same J-level or to different J-levels within the same term)
collisional processes are generally neglected when solving the master
equation for the atomic density matrix. This is partly due to the lack
of experimental data and/or of approximate theoretical expressions for
calculating the collisional transfer and relaxation rates (in particular
the rates for interference between sublevels pertaining to different
J-levels, and the depolarizing rates due to elastic collisions). In this
paper we formally define and investigate the transfer and relaxation
rates due to isotropic inelastic and superelastic collisions that enter
the statistical equilibrium equations for the atomic density matrix of
a multiterm atom. Under the hypothesis that the interaction between
the collider and the atom can be described by a dipolar operator, we
provide expressions that relate the collisional rates for interference
between different J-levels to the usual collisional rates for J-level
populations, for which experimental data or approximate theoretical
expressions are generally available. We show that the rates for
populations and interference within the same J-level reduce to those
previously obtained for the multilevel model atom (where quantum
interference is assumed to be present only between magnetic sublevels
pertaining to any given J-level). Finally, we apply the general
equations to the case of a two-term atom with unpolarized lower term,
illustrating the impact of inelastic and superelastic collisions on the
scattering line polarization through radiative transfer calculations
in a slab of stellar atmospheric plasma anisotropically illuminated
by the photospheric radiation field.
---------------------------------------------------------
Title: Anomalous Circular Polarization Profiles in the He I 1083.0
nm Multiplet from Solar Spicules
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Beck, C.; Belluzzi, L.
2012ApJ...759...16M Altcode: 2012arXiv1209.2589M
We report Stokes vector observations of solar spicules and a prominence
in the He I 1083 nm multiplet carried out with the Tenerife Infrared
Polarimeter. The observations show linear polarization profiles that
are produced by scattering processes in the presence of a magnetic
field. After a careful data reduction, we demonstrate the existence of
extremely asymmetric Stokes V profiles in the spicular material that
we are able to model with two magnetic components along the line of
sight, and under the presence of atomic orientation in the energy
levels that give rise to the multiplet. We discuss some possible
scenarios that can generate the atomic orientation in spicules. We
stress the importance of spectropolarimetric observations across the
limb to distinguish such signals from observational artifacts.
---------------------------------------------------------
Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP)
Authors: Kano, Ryouhei; Bando, Takamasa; Narukage, Noriyuki; Ishikawa,
Ryoko; Tsuneta, Saku; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-nosuke; Hara, Hirohisa; Shimizu, Toshifumi; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Sakao, Taro; Goto, Motoshi; Kato, Yoshiaki; Imada,
Shinsuke; Kobayashi, Ken; Holloway, Todd; Winebarger, Amy; Cirtain,
Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier;
Štepán, Jiří; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos,
Andres; Auchère, Frédéric; Carlsson, Mats
2012SPIE.8443E..4FK Altcode:
One of the biggest challenges in heliophysics is to decipher the
magnetic structure of the solar chromosphere. The importance of
measuring the chromospheric magnetic field is due to both the key role
the chromosphere plays in energizing and structuring the outer solar
atmosphere and the inability of extrapolation of photospheric fields to
adequately describe this key boundary region. Over the last few years,
significant progress has been made in the spectral line formation
of UV lines as well as the MHD modeling of the solar atmosphere. It
is found that the Hanle effect in the Lyman-alpha line (121.567 nm)
is a most promising diagnostic tool for weaker magnetic fields in
the chromosphere and transition region. Based on this groundbreaking
research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP) to NASA as a sounding rocket experiment, for making the first
measurement of the linear polarization produced by scattering processes
and the Hanle effect in the Lyman-alpha line (121.567 nm), and making
the first exploration of the magnetic field in the upper chromosphere
and transition region of the Sun. The CLASP instrument consists
of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam
spectrograph assembly with a grating working as a beam splitter, and
an identical pair of reflective polarization analyzers each equipped
with a CCD camera. We propose to launch CLASP in December 2014.
---------------------------------------------------------
Title: The Scattering Polarization of the Lyα Lines of H I and He
II Taking into Account Partial Frequency Redistribution and J-state
Interference Effects
Authors: Belluzzi, Luca; Trujillo Bueno, Javier; Štěpán, Jiří
2012ApJ...755L...2B Altcode: 2012arXiv1207.0415B
Recent theoretical investigations have pointed out that the cores of
the Lyα lines of H I and He II should show measurable scattering
polarization signals when observing the solar disk, and that the
magnetic sensitivity, through the Hanle effect, of such linear
polarization signals is suitable for exploring the magnetism of the
solar transition region. Such investigations were carried out in the
limit of complete frequency redistribution (CRD) and neglecting quantum
interference between the two upper J-levels of each line. Here we relax
both approximations and show that the joint action of partial frequency
redistribution and J-state interference produces much more complex
fractional linear polarization (Q/I) profiles, with large amplitudes
in their wings. Such wing polarization signals turn out to be very
sensitive to the temperature structure of the atmospheric model, so
that they can be exploited for constraining the thermal properties of
the solar chromosphere. Finally, we show that the approximation of CRD
without J-state interference is however suitable for estimating the
amplitude of the linear polarization signals in the core of the lines,
where the Hanle effect operates.
---------------------------------------------------------
Title: The Chromospheric Lyman-Alpha SpectroPolarimeter: CLASP
Authors: Kobayashi, K.; Kano, R.; Trujillo-Bueno, J.; Asensio Ramos,
A.; Bando, T.; Belluzzi, L.; Carlsson, M.; De Pontieu, R. C. B.; Hara,
H.; Ichimoto, K.; Ishikawa, R.; Katsukawa, Y.; Kubo, M.; Manso Sainz,
R.; Narukage, N.; Sakao, T.; Stepan, J.; Suematsu, Y.; Tsuneta, S.;
Watanabe, H.; Winebarger, A.
2012ASPC..456..233K Altcode:
The magnetic field plays a crucial role in the chromosphere and the
transition region, and our poor empirical knowledge of the magnetic
field in the upper chromosphere and transition region is a major
impediment to advancing the understanding of the solar atmosphere. The
Hanle effect promises to be a valuable alternative to Zeeman effect
as a method of measuring the magnetic field in the chromosphere and
transition region; it is sensitive to weaker magnetic fields, and
also sensitive to tangled, unresolved field structures. <P />CLASP
is a sounding rocket experiment that aims to observe the Hanle effect
polarization of the Lyman α (1215.67Å) line in the solar chromosphere
and transition region, and prove the usefulness of this technique in
placing constraints on the magnetic field strength and orientation
in the low plasma-β region of the solar atmosphere. The Ly-α line
has been chosen because it is a chromospheric/transition-region line,
and because the Hanle effect polarization of this line is predicted to
be sensitive to 10-250 Gauss, encompassing the range of interest. The
CLASP instrument is designed to measure linear polarization in the
Ly-α line with a polarization sensitivity of 0.1%. The instrument is
currently funded for development. The optical design of the instrument
has been finalized, and an extensive series of component-level tests
are underway to validate the design.
---------------------------------------------------------
Title: The Polarization of the Solar Mg II h and k Lines
Authors: Belluzzi, Luca; Trujillo Bueno, Javier
2012ApJ...750L..11B Altcode: 2012arXiv1203.4351B
Although the h and k lines of Mg II are expected to be of great interest
for probing the upper solar chromosphere, relatively little is known
about their polarization properties which encode the information on the
magnetic field. Here we report the first results of an investigation
whose main goal is to understand the physical mechanisms that control
the scattering polarization across these resonance lines and to
achieve a realistic radiative transfer modeling in the presence of
arbitrary magnetic fields. We show that the joint action of partial
frequency redistribution (PRD) and quantum interference between
the upper J-levels of the two lines produces a complex fractional
linear polarization (Q/I) pattern with large polarization amplitudes
in the blue and red wings, and a negative feature in the spectral
region between the two lines. Another remarkable peculiarity of the
Q/I profile is a conspicuous antisymmetric signal around the center
of the h line, which cannot be obtained unless both PRD and J-state
interference effects are taken into account. In the core of the k line,
PRD effects alone produce a triplet peak structure in the Q/I profile,
the modeling of which can also be achieved via the two-level atom
approximation. In addition to the Hanle effect in the core of the
k line, we also emphasize the diagnostic potential of the circular
polarization produced by the Zeeman effect in the h and k lines,
as well as in other Mg II lines located in their wings.
---------------------------------------------------------
Title: The Hanle Effect in the Lyα Lines of H I and He II for
Measuring the Magnetic Fields of the Solar Transition Region
Authors: Trujillo Bueno, J.; Štepán, J.; Belluzzi, L.
2012ASPC..456..225T Altcode: 2012ASPC..456..225B
The Ly α lines of H I and He II are two of the spectral lines of choice
for FUV and EUV channels of narrowband imagers on board sounding rockets
and space telescopes, which provide spectacular intensity images of the
outer solar atmosphere. Since the magnetic field information is encoded
in the polarization of the spectral line radiation, it is important
to investigate whether the ensuing Ly α radiation from the solar disk
can be polarized, along with its magnetic sensitivity. Here we present
some theoretical predictions concerning the amplitudes and magnetic
sensitivities of the linear polarization signals produced by scattering
processes in these strong emission lines of the solar transition region,
taking into account radiative transfer and the Hanle effect caused by
the presence of organized and random magnetic fields. We find that the
line-center amplitudes of the fractional polarization signals vary
typically between a fraction of a percent and ∼1%, depending on
the Ly α line under consideration, the scattering geometry and the
strength and orientation of the magnetic field. Interestingly, while
the Ly α line of He II starts to be sensitive to the Hanle effect
for magnetic strengths B>̰100 G the hydrogen Lyα line is mainly
sensitive to magnetic strengths between 10 and 100 G. These results
encourage the development of FUV and EUV polarimeters for sounding
rockets and space telescopes with the aim of opening up a diagnostic
window for magnetic field measurements in the upper chromosphere and
transition region of the Sun.
---------------------------------------------------------
Title: The Lyα Lines of H I and He II: A Differential Hanle Effect
for Exploring the Magnetism of the Solar Transition Region
Authors: Trujillo Bueno, Javier; Štěpán, Jiří; Belluzzi, Luca
2012ApJ...746L...9T Altcode: 2011arXiv1112.4746T
The Lyα line of He II at 304 Å is one of the spectral lines
of choice for EUV channels of narrowband imagers on board space
telescopes, which provide spectacular intensity images of the outer
solar atmosphere. Since the magnetic field information is encoded in
the polarization of the spectral line radiation, it is important to
investigate whether the He II line radiation from the solar disk can
be polarized, along with its magnetic sensitivity. Here we report some
theoretical predictions concerning the linear polarization signals
produced by scattering processes in this strong emission line of
the solar transition region, taking into account radiative transfer
and the Hanle effect caused by the presence of organized and random
magnetic fields. We find that the fractional polarization amplitudes
are significant (~1%), even when considering the wavelength-integrated
signals. Interestingly, the scattering polarization of the Lyα line
of He II starts to be sensitive to the Hanle effect for magnetic
strengths B >~ 100 G (i.e., for magnetic strengths of the order of
and larger than the Hanle saturation field of the hydrogen Lyα line
at 1216 Å). We therefore propose simultaneous observations of the
scattering polarization in both Lyα lines to facilitate magnetic field
measurements in the upper solar chromosphere. Even the development
of a narrowband imaging polarimeter for the He II 304 Å line alone
would be already of great diagnostic value for probing the solar
transition region.
---------------------------------------------------------
Title: Analytical maximum likelihood estimation of stellar magnetic
fields
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Belluzzi, L.
2012MNRAS.419..153M Altcode: 2011MNRAS.tmp.1583M; 2011arXiv1108.4366M
The polarized spectrum of stellar radiation encodes valuable information
on the conditions of stellar atmospheres and the magnetic fields
that permeate them. In this paper, we give explicit expressions to
estimate the magnetic field vector and its associated error from the
observed Stokes parameters. We study the solar case where specific
intensities are observed and then the stellar case, where we receive
the polarized flux. In the second case, we concentrate on the explicit
expression for the case of a slow rotator with a dipolar magnetic
field geometry. Moreover, we also give explicit formulae to retrieve
the magnetic field vector from the least-squares deconvolution (LSD)
profiles without assuming mean values for the LSD artificial spectral
line. The formulae have been obtained assuming that the spectral lines
can be described in the weak-field regime and using a maximum likelihood
approach. The errors are recovered by means of the Hermitian matrix. The
bias of the estimators is analysed in depth.
---------------------------------------------------------
Title: The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP)j
Authors: Kobayashi, K.; Tsuneta, S.; Trujillo Bueno, J.; Bando, T.;
Belluzzi, L.; Casini, R.; Carlsson, M.; Cirtain, J. W.; De Pontieu,
B.; Hara, H.; Ichimoto, K.; Ishikawa, R.; Kano, R.; Katsukawa, Y.;
Kim, T.; Kubo, M.; Manso Sainz, R.; Narukage, N.; Asensio Ramos,
A.; Robinson, B.; Sakao, T.; Shimizu, T.; Stepan, J.; Suematsu, Y.;
Watanabe, H.; West, E.; Winebarger, A. R.
2011AGUFM.P14C..05K Altcode:
We present an overview of the Chromospheric Lyman-Alpha
SpectroPolarimeter (CLASP) program. CLASP is a proposed sounding rocket
experiment currently under development as collaboration between Japan,
USA and Spain. The aim is to achieve the first measurement of magnetic
field in the upper chromosphere and transition region of the Sun
through the detection and measurement of Hanle effect polarization
of the Lyman alpha line. The Hanle effect (i.e. the magnetic field
induced modification of the linear polarization due to scattering
processes in spectral lines) is believed to be a powerful tool for
measuring the magnetic field in the upper chromosphere, as it is more
sensitive to weaker magnetic fields than the Zeeman effect, and also
sensitive to magnetic fields tangled at spatial scales too small to be
resolved. The Lyman-alpha (121.567 nm) line has been chosen because
it is a chromospheric/transition-region line, and because the Hanle
effect polarization of the Lyman-alpha line is predicted to be sensitive
to 10-250 Gauss, encompassing the range of interest. Hanle effect is
predicted to be observable as linear polarization or depolarization,
depending on the geometry, with a fractional polarization amplitude
varying between 0.1% and 1% depending on the strength and orientation of
the magnetic field. This quantification of the chromospheric magnetic
field requires a highly sensitive polarization measurement. The
CLASP instrument consists of a large aperture (287 mm) Cassegrain
telescope mated to a polarizing beamsplitter and a matched pair
of grating spectrographs. The polarizing beamsplitter consists
of a continuously rotating waveplate and a linear beamsplitter,
allowing simultaneous measurement of orthogonal polarizations and
in-flight self-calibration. Development of the instrument is underway,
and prototypes of all optical components have been tested using a
synchrotron beamline. The experiment is proposed for flight in 2014.
---------------------------------------------------------
Title: The Impact of Quantum Interference between Different J-levels
on Scattering Polarization in Spectral Lines
Authors: Belluzzi, Luca; Trujillo Bueno, Javier
2011ApJ...743....3B Altcode: 2011arXiv1109.0424B
The spectral line polarization produced by optically pumped atoms
contains a wealth of information on the thermal and magnetic structure
of a variety of astrophysical plasmas, including that of the solar
atmosphere. A correct decoding of such information from the observed
Stokes profiles requires a clear understanding of the effects that
radiatively induced quantum interference (or coherence) between pairs
of magnetic sublevels produces on these observables, in the absence of
and in the presence of magnetic fields of arbitrary strength. Here we
present a detailed theoretical investigation of the role of coherence
between pairs of sublevels pertaining to different fine-structure
J-levels, clarifying when it can be neglected for facilitating the
modeling of the linear polarization produced by scattering processes
in spectral lines. To this end, we apply the quantum theory of spectral
line polarization and calculate the linear polarization patterns of the
radiation scattered at 90° by a slab of stellar atmospheric plasma,
both taking into account and neglecting the above-mentioned quantum
interference. Particular attention is given to the <SUP>2</SUP> S -
<SUP>2</SUP> P, <SUP>5</SUP> S - <SUP>5</SUP> P, and <SUP>3</SUP> P -
<SUP>3</SUP> S multiplets. We point out the observational signatures
of this kind of interference and analyze its sensitivity to the energy
separation between the interfering levels, to the amount of emissivity
in the background continuum radiation, to lower-level polarization,
and to the presence of a magnetic field. Some interesting applications
to the following spectral lines are also presented: Ca II H and K,
Mg II h and k, Na I D<SUB>1</SUB> and D<SUB>2</SUB>, the Ba II 4554
Å and 4934 Å resonance lines, the Cr I triplet at 5207 Å, the O I
triplet at 7773 Å, the Mg I b-lines, and the Hα and Lyα lines of H I.
---------------------------------------------------------
Title: The Hanle Effect from Space for Measuring the Magnetic Fields
of the Upper Solar Chromosphere
Authors: Trujillo Bueno, J.; Stepan, J.; Belluzzi, L.; Manso Sainz, R.
2011AGUFM.P11F1626T Altcode:
We present some theoretical predictions concerning the amplitudes and
magnetic sensitivities of the linear polarization signals produced
by scattering processes in some UV and FUV spectral lines of the
upper chromosphere and transition region, such as Ly-alpha and Mg
II k. To this end, we have calculated the atomic level polarization
(population imbalances and quantum coherences) induced by anisotropic
radiation pumping in semi-empirical and hydrodynamical models of
the solar atmosphere, taking into account radiative transfer and the
Hanle effect caused by the presence of organized and random magnetic
fields. The amplitudes of the emergent linear polarization signals
are found to vary typically between a fraction of a percent and a few
percent, depending on the scattering geometry and the strength and
orientation of the magnetic field. The results shown here encourage the
development of UV polarimeters for sounding rockets and space telescopes
with the aim of opening up a true diagnostic window for magnetic field
measurements in the upper chromosphere and transition region of the Sun.
---------------------------------------------------------
Title: Overview of Chromospheric Lyman-Alpha SpectroPolarimeter
(CLASP)
Authors: Narukage, Noriyuki; Tsuneta, Saku; Bando, Takamasa; Kano,
Ryouhei; Kubo, Masahito; Ishikawa, Ryoko; Hara, Hirohisa; Suematsu,
Yoshinori; Katsukawa, Yukio; Watanabe, Hiroko; Ichimoto, Kiyoshi;
Sakao, Taro; Shimizu, Toshifumi; Kobayashi, Ken; Robinson, Brian; Kim,
Tony; Winebarger, Amy; West, Edward; Cirtain, Jonathan; De Pontieu,
Bart; Casini, Roberto; Trujillo Bueno, Javier; Stepan, Jiri; Manso
Sainz, Rafael; Belluzzi, Luca; Asensio Ramos, Andres; Carlsson, Mats
2011SPIE.8148E..0HN Altcode: 2011SPIE.8148E..16N
The solar chromosphere is an important boundary, through which all of
the plasma, magnetic fields and energy in the corona and solar wind
are supplied. Since the Zeeman splitting is typically smaller than
the Doppler line broadening in the chromosphere and transition region,
it is not effective to explore weak magnetic fields. However, this is
not the case for the Hanle effect, when we have an instrument with
high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha
SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect
linear polarization produced by the Hanle effect in Lyman-alpha line
(121.567 nm) and to make the first direct measurement of magnetic
fields in the upper chromosphere and lower transition region. To
achieve the high sensitivity of ~ 0.1% within a rocket flight (5
minutes) in Lyman-alpha line, which is easily absorbed by materials,
we design the optical system mainly with reflections. The CLASP
consists of a classical Cassegrain telescope, a polarimeter and a
spectrometer. The polarimeter consists of a rotating 1/2-wave plate
and two reflecting polarization analyzers. One of the analyzer also
works as a polarization beam splitter to give us two orthogonal linear
polarizations simultaneously. The CLASP is planned to be launched in
2014 summer.
---------------------------------------------------------
Title: Spectropolarimetric forward modelling of the lines of the
Lyman-series using a self-consistent, global, solar coronal model
Authors: Khan, A.; Belluzzi, L.; Landi Degl'Innocenti, E.; Fineschi,
S.; Romoli, M.
2011A&A...529A..12K Altcode:
Context. The presence and importance of the coronal magnetic field
is illustrated by a wide range of phenomena, such as the abnormally
high temperatures of the coronal plasma, the existence of a slow and
fast solar wind, the triggering of explosive events such as flares
and CMEs. <BR /> Aims: We investigate the possibility of using the
Hanle effect to diagnose the coronal magnetic field by analysing
its influence on the linear polarisation, i.e. the rotation of
the plane of polarisation and depolarisation. <BR /> Methods: We
analyse the polarisation characteristics of the first three lines
of the hydrogen Lyman-series using an axisymmetric, self-consistent,
minimum-corona MHD model with relatively low values of the magnetic
field (a few Gauss). <BR /> Results: We find that the Hanle effect
in the above-mentioned lines indeed seems to be a valuable tool for
analysing the coronal magnetic field. However, great care must be
taken when analysing the spectropolarimetry of the Lα line, given
that a non-radial solar wind and active regions on the solar disk
can mimic the effects of the magnetic field, and, in some cases, even
mask them. Similar drawbacks are not found for the Lβ and Lγ lines
because they are more sensitive to the magnetic field. We also briefly
consider the instrumental requirements needed to perform polarimetric
observations for diagnosing the coronal magnetic fields. <BR />
Conclusions: The combined analysis of the three aforementioned lines
could provide an important step towards better constrainting the value
of solar coronal magnetic fields.
---------------------------------------------------------
Title: Polarized Radiative Transfer: from Solar Applications to
Laboratory Experiments
Authors: Landi Degl'Innocenti, E.; Belluzzi, L.; Bommier, V.
2011ASPC..437...45L Altcode:
The theory of radiative transfer for polarized radiation, developed from
Quantum Electrodynamics for the interpretation of solar observations,
predicts the existence of a variety of physical phenomena that, in
many cases, would deserve being directly tested through laboratory
experiments, also in view of possible practical applications. In this
report we will focus on the description of some of these atomic-physics
phenomena that have been disregarded, or overlooked, in terrestrial
laboratories.
---------------------------------------------------------
Title: Theoretical Approaches to the Physics of Spectral Line
Polarization
Authors: Belluzzi, L.
2011ASPC..437...29B Altcode: 2011arXiv1103.5394B
Due to the continuous developments in polarimetric instrumentation,
which will become even more dramatic in the near future with
the availability of new generation solar telescopes, we are now
severely confronted with a variety of new detailed observations of
high diagnostic potential, whose interpretation requires a firmly
established theoretical framework. In this contribution, I review
the fundamental physical processes that underlie the generation
and transfer of polarized radiation in stellar atmospheres, and I
discuss the present status of the theoretical schemes now available,
pointing out their main successes and limitations. I also present some
ideas about the theoretical improvements that I consider necessary to
achieve a correct interpretation of the complex phenomenology shown by
polarimetric observations, focusing particularly on the second solar
spectrum, which can be considered as one of the most important test
benches of the theory.
---------------------------------------------------------
Title: Magnetic field diagnostics through the second solar spectrum:
a spectroscopic analysis of the most polarizing atomic lines
Authors: Belluzzi, L.; Landi Degl'Innocenti, E.
2010MmSAI..81..710B Altcode:
Detailed observations of the second solar spectrum have shown a complex
phenomenology whose detailed interpretation is still in a preliminary
phase. The only certain thing is that the observed polarization is due
to the phenomenon of resonance scattering. It is well known that such
polarization can be strongly modified by the presence of a magnetic
field through the so-called Hanle effect. It then follows that the
second solar spectrum is, at least potentially, a fundamental instrument
for diagnosing the intensity and topology of the weak magnetic fields
lying in the higher layers of the solar atmosphere. Obviously, to
fully exploit such potential, it is absolutely necessary to arrive
at the correct theoretical interpretation, a task that has been
revealed to involve many difficulties. We present an analysis of the
second solar spectrum, carried out with the aim of investigating its
general properties. The most interesting results concern an empirical
classification of the wide variety of polarization signals that are
observed, and the formulation of three empirical laws describing a
series of properties common to the strongest signals.
---------------------------------------------------------
Title: On the physical origin of the second solar spectrum of the
Sc II line at 4247 Å
Authors: Belluzzi, L.
2009A&A...508..933B Altcode: 2009arXiv0909.5552B
Context: The peculiar three-peak structure of the linear polarization
profile shown in the second solar spectrum by the Ba ii line at 4554
Å has been interpreted as the result of the different contributions
coming from the barium isotopes with and without hyperfine structure. In
the same spectrum, a triple peak polarization signal is also observed
in the Sc ii line at 4247 Å. Scandium has a single stable isotope
(<SUP>45</SUP>Sc), which shows hyperfine structure due to a nuclear spin
I = 7/2.<BR /> Aims: We investigate the possibility of interpreting
the linear polarization profile shown in the second solar spectrum
by this Sc ii line in terms of hyperfine structure.<BR /> Methods:
A two-level model atom with hyperfine structure is assumed. Adopting
an optically thin slab model, the role of atomic polarization and of
hyperfine structure is investigated, avoiding the complications caused
by radiative transfer effects. The slab is assumed to be illuminated
from below by the photospheric continuum, and the polarization of
the radiation scattered at 90° is investigated.<BR /> Results: The
three-peak structure of the scattering polarization profile observed
in this Sc ii line cannot be fully explained in terms of hyperfine
structure.<BR /> Conclusions: Given the similarities between the Sc
ii line at 4247 Å and the Ba ii line at 4554 Å, it is not clear
why, within the same modeling assumptions, only the three-peak
Q/I profile of the barium line can be fully interpreted in terms of
hyperfine structure. The failure to interpret this Sc ii polarization
signal raises important questions, whose resolution might lead to
significant improvements in our understanding of the second solar
spectrum. In particular, if the three-peak structure of the Sc ii
signal is actually produced by a physical mechanism neglected within
the approach considered here, it will be extremely interesting not
only to identify this mechanism, but also to understand why it seems
to be less important in the case of the barium line.
---------------------------------------------------------
Title: The Physical Origin and the Diagnostic Potential of the
Scattering Polarization in the Li I Resonance Doublet at 6708 Å
Authors: Belluzzi, Luca; Landi Degl'Innocenti, Egidio; Trujillo
Bueno, Javier
2009ApJ...705..218B Altcode: 2009arXiv0909.3748B
High-sensitivity measurements of the linearly polarized solar limb
spectrum produced by scattering processes in quiet regions of the solar
atmosphere showed that the Q/I profile of the lithium doublet at 6708 Å
has an amplitude ~10<SUP>-4</SUP> and a curious three-peak structure,
qualitatively similar to that found and confirmed by many observers in
the Na I D<SUB>2</SUB> line. Given that a precise measurement of the
scattering polarization profile of the lithium doublet lies at the
limit of the present observational possibilities, it is worthwhile
to clarify the physical origin of the observed polarization, its
diagnostic potential, and what kind of Q/I shapes can be expected from
theory. To this end, we have applied the quantum theory of atomic
level polarization taking into account the hyperfine structure of
the two stable isotopes of lithium, as well as the Hanle effect
of a microturbulent magnetic field of arbitrary strength. We find
that quantum interferences between the sublevels pertaining to the
upper levels of the D<SUB>2</SUB> and D<SUB>1</SUB> line transitions
of lithium do not cause any observable effect on the emergent Q/I
profile. Our theoretical calculations show that only two Q/I peaks
can be expected, with the strongest one caused by the D<SUB>2</SUB>
line of <SUP>7</SUP>Li I and the weakest one due to the D<SUB>2</SUB>
line of <SUP>6</SUP>Li I. Interestingly, we find that these two peaks
in the theoretical Q/I profile stand out clearly only when the kinetic
temperature of the thin atmospheric region that produces the emergent
spectral line radiation is lower than 4000 K. The fact that such a thin
atmospheric region is located around a height of 200 km in standard
semi-empirical models, where the kinetic temperature is about 5000 K,
leads us to suggest that the most likely Q/I profile produced by the
Sun in the lithium doublet should be slightly asymmetric and dominated
by the <SUP>7</SUP>Li I peak.
---------------------------------------------------------
Title: The Sensitivity of the Second Solar Spectrum of the Li I 6708
Å Doublet to the Lithium Isotopic Abundance and to a Microturbulent
Magnetic Field
Authors: Belluzzi, L.; Landi Degl'Innocenti, E.; Trujillo Bueno, J.
2009ASPC..405..107B Altcode:
The second solar spectrum offers a novel diagnostic window not only to
explore the magnetic field of our nearest star, but also to determine
the atomic and isotopic abundances in the solar atmosphere. A very
interesting example, concerning this second aspect, is the Li I
resonance doublet at 6708~Å. Here we present a detailed theoretical
investigation, carried out within the framework of the density matrix
formalism, on the atomic physics that is involved in the generation
of the observed Q/I profile, which is the result of a complex blend
of the D_1 and D_2 lines of the two stable isotopes of lithium (^6Li
and ^7Li), both showing hyperfine structure. We discuss the diagnostic
potential of this linear polarization profile for the determination of
the lithium isotopic abundance in the quiet Sun, and we investigate the
magnetic sensitivity of this profile to a microturbulent magnetic field.
---------------------------------------------------------
Title: Observations of the Joint Action of the Hanle and Zeeman
Effects in the D_2 Line of Ba II
Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Belluzzi, L.;
Landi Degl'Innocenti, E.
2009ASPC..405...41R Altcode: 2009arXiv0906.2320R
We show a selection of high-sensitivity spectropolarimetric observations
obtained over the last few years in the Ba II D_2-line with the Zürich
Imaging Polarimeter (ZIMPOL) attached to the Gregory Coudé Telescope
of IRSOL. The measurements were collected close to the solar limb, in
several regions with varying degree of magnetic activity. The Stokes
profiles we have observed show clear signatures of the joint action
of the Hanle and Zeeman effects, in very good qualitative agreement
with the theoretical expectations. Polarimetric measurements of this
line show to be very well suited for magnetic field diagnostics of
the lower solar chromosphere, from regions with field intensities as
low as 1 gauss to strongly magnetized ones having kG field strengths.
---------------------------------------------------------
Title: A spectroscopic analysis of the most polarizing atomic lines
of the second solar spectrum
Authors: Belluzzi, L.; Landi Degl'Innocenti, E.
2009A&A...495..577B Altcode:
We present an analysis of the second solar spectrum, as plotted in
graphical form in the three volumes of the atlas “The Second Solar
Spectrum”, in the wavelength range between 3160 Å and 6995 Å. All
the strongest positive linear polarization signals produced by atomic
spectral lines are identified, and their amplitudes are listed, along
with the most relevant spectroscopic properties of the lines. The
various signals are divided into five classes according to the shape
of their Q/I profiles. Three empirical rules are formulated from the
analysis of the amplitude and shape of the various signals, and of
the spectroscopic properties of the spectral lines. <P />Tables 1 and
2 are only available in electronic form at http://www.aanda.org
