explanation blue bibcodes open ADS page with paths to full text
Author name code: stepan
ADS astronomy entries on 2022-09-14
author:"Stepan, Jiri"
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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.
---------------------------------------------------------
Title: Spectropolarimetric observations of the solar atmosphere in
the Hα 6563 Å line
Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Bianda, M.; Štěpán,
J.; Ramelli, R.
2022A&A...659A.179J Altcode: 2022arXiv220103815J
We present novel spectropolarimetric observations of the hydrogen
Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at
the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno
(IRSOL). The linear polarization is clearly dominated by the scattering
of anisotropic radiation and the Hanle effect, while the circular
polarization is dominated by the Zeeman effect. The observed linear
polarization signals show a rich spatial variability, the interpretation
of which would open a new window for probing the solar chromosphere. We
study their spatial variation within coronal holes, finding a different
behaviour for the U/I signals near the north and south solar poles. We
identify some spatial patterns, which may facilitate the interpretation
of the observations. In close-to-the-limb regions with sizable circular
polarization signals, we find similar asymmetric Q/I profiles. We
also show examples of net circular polarization profiles (NCP), along
with the corresponding linear polarization signals. The application
of the weak field approximation to the observed circular polarization
signals gives 10 G (40-60 G) close to the limb quiet (plage) regions
for the average longitudinal field strength over the spatio-temporal
resolution element.
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Title: Novel framework for the three-dimensional NLTE inverse problem
Authors: Štěpán, Jiří; del Pino Alemán, Tanausú; Trujillo
Bueno, Javier
2022A&A...659A.137S Altcode: 2022arXiv220101504S
The inversion of spectropolarimetric observations of the solar upper
atmosphere is one of the most challenging goals in solar physics. If
we account for all relevant ingredients of the spectral line formation
process, such as the three-dimensional (3D) radiative transfer out
of local thermodynamic equilibrium (NLTE), the task becomes extremely
computationally expensive. Instead of generalizing 1D methods to 3D, we
have developed a new approach to the inverse problem. In our meshfree
method, we do not consider the requirement of 3D NLTE consistency
as an obstacle, but as a natural regularization with respect to the
traditional pixel-by-pixel methods. This leads to more robust and
less ambiguous solutions. We solve the 3D NLTE inverse problem as
an unconstrained global minimization problem that avoids repetitive
evaluations of the Λ operator. Apart from the 3D NLTE consistency,
the method allows us to easily include additional conditions of
physical consistency such as the zero divergence of the magnetic
field. Stochastic ingredients make the method less prone to ending up
within the local minima of the loss function. Our method is capable
of solving the inverse problem faster by several orders of magnitude
than by using grid-based methods. The method can provide accurate and
physically consistent results if sufficient computing time is available,
along with approximate solutions in the case of very complex plasma
structures or limited computing time.
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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.
---------------------------------------------------------
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.
---------------------------------------------------------
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: Evaluating the Reliability of a Simple Method to Map the
Magnetic Field Azimuth in the Solar Chromosphere
Authors: Jurčák, Jan; Štěpán, Jiří; Trujillo Bueno, Javier
2021ApJ...911...23J Altcode: 2021arXiv210202880J
The Zeeman effect is of limited utility for probing the magnetism
of the quiet solar chromosphere. The Hanle effect in some spectral
lines is sensitive to such magnetism, but the interpretation of the
scattering polarization signals requires taking into account that the
chromospheric plasma is highly inhomogeneous and dynamic (i.e., that
the magnetic field is not the only cause of symmetry breaking). Here
we investigate the reliability of a well-known formula for mapping the
azimuth of chromospheric magnetic fields directly from the scattering
polarization observed in the Ca II 8542 Å line, which is typically
in the saturation regime of the Hanle effect. To this end, we use
the Stokes profiles of the Ca II 8542 Å line computed with the PORTA
radiative transfer code in a three-dimensional (3D) model of the solar
chromosphere, degrading them to mimic spectropolarimetric observations
for a range of telescope apertures and noise levels. The simulated
observations are used to obtain the magnetic field azimuth at each
point of the field of view, which we compare with the actual values
within the 3D model. We show that, apart from intrinsic ambiguities,
the method provides solid results. Their accuracy depends more on
the noise level than on the telescope diameter. Large-aperture solar
telescopes, like DKIST and EST, are needed to achieve the required
polarimetric sensitivity using reasonable exposure times.
---------------------------------------------------------
Title: The Effects of Three-dimensional Radiative Transfer on the
Resonance Polarization of the Ca I 4227 Å Line
Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Štěpán, J.; del
Pino Alemán, T.
2021ApJ...909..183J Altcode: 2021arXiv210104421J
The sizable linear polarization signals produced by the scattering of
anisotropic radiation in the core of the Ca I 4227 Å line constitute
an important observable for probing the inhomogeneous and dynamic
plasma of the lower solar chromosphere. Here we show the results
of a three-dimensional (3D) radiative transfer complete frequency
redistribution investigation of the line's scattering polarization
in a magnetohydrodynamical 3D model of the solar atmosphere. We
take into account not only the Hanle effect produced by the model's
magnetic field but also the symmetry breaking caused by the horizontal
inhomogeneities and macroscopic velocity gradients. The spatial
gradients of the horizontal components of the macroscopic velocities
produce very significant forward scattering polarization signals
without the need of magnetic fields, while the Hanle effect tends to
depolarize them at the locations where the model's magnetic field is
stronger than about 5 G. The standard 1.5D approximation is found to
be unsuitable for understanding the line's scattering polarization,
but we introduce a novel improvement to this approximation that
produces results in qualitative agreement with the full 3D results. The
instrumental degradation of the calculated polarization signals is also
investigated, showing what we can expect to observe with the Visible
Spectro-Polarimeter at the upcoming Daniel K. Inouye Solar Telescope.
---------------------------------------------------------
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: Improved near optimal angular quadratures for polarised
radiative transfer in 3D MHD models
Authors: Jaume Bestard, Jaume; Štěpán, Jiří; Trujillo Bueno,
Javier
2021A&A...645A.101J Altcode: 2020arXiv201204981J
Accurate angular quadratures are crucial for the numerical solution
of three-dimensional (3D) radiative transfer problems, especially
when the spectral line polarisation produced by the scattering of
anisotropic radiation is included. There are two requirements for
obtaining an optimal quadrature and they are difficult to satisfy
simultaneously: high accuracy and short computing time. By imposing
certain symmetries, we were recently able to derive a set of near
optimal angular quadratures. Here, we extend our previous investigation
by considering other symmetries. Moreover, we test the performance of
our new quadratures by numerically solving a radiative transfer problem
of resonance line polarisation in a 3D model of the solar atmosphere
resulting from a magneto-hydrodynamical simulation. The new angular
quadratures derived here outperform the previous ones in terms of the
number of rays needed to achieve any given accuracy. <P />The tables
mentioned in Sect. 4 are only available at the CDS via anonymous ftp to
<A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A101">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A101</A>
---------------------------------------------------------
Title: IRIS Mg II Observations and Non-LTE Modeling of Off-limb
Spicules
Authors: Tei, A.; Gunár, S.; Heinzel, P.; Okamoto, T. J.; Štěpán,
J.; Jejčič, S.; Shibata, K.
2020AGUFMSH0010008T Altcode:
We investigated the off-limb spicules observed in the Mg II h and k
spectral lines by Interface Region Imaging Spectrograph (IRIS) in a
solar polar coronal hole. We analyzed the large data set of obtained
spectra to extract quantitative information about the line intensities,
line shifts, and line widths. The observed Mg II line profiles are
broad and double peaked at lower altitudes, broad but flat topped
at middle altitudes, and narrow and single peaked with the largest
Doppler shifts at higher altitudes. We used one-dimensional non-LTE
vertical slab models (i.e., models that consider departures from local
thermodynamic equilibrium) in single-slab and multi-slab configurations
to interpret the observations and to investigate how a superposition
of spicules along a line of sight (LOS) affects the synthetic Mg II
line profiles. The employed multi-slab models are either static,
i.e., without any LOS velocities, or assume randomly assigned LOS
velocities of individual slabs, representing the spicule dynamics. We
performed such single-slab and multi-slab modeling for a broad set of
model input parameters and examined the dependence of the Mg II line
profiles on these parameters. In this presentation, we demonstrate that
the observed line widths of the Mg h and k line profiles are strongly
affected by the presence of multiple spicules along the LOS. We also
show that the profiles obtained at higher altitudes can be reproduced
by single-slab models representing individual spicules. We found that
the multi-slab model with a random distribution of the LOS velocities
ranging from −25 to 25 km/s can well reproduce the width and the
shape of the Mg II profiles observed at middle altitudes.
---------------------------------------------------------
Title: VizieR Online Data Catalog: 3D MHD models angular quadratures
(Jaume Bestard+, 2021)
Authors: Jaume Bestard, J.; Stepan, J.; Trujillo Bueno, J.
2020yCat..36450101J Altcode:
The file name has the following convention: u: quadrature for
unpolarised radiation p: quadrature for polarised radiation lX:
quadrature for the order L=X nY: quadrature with N=Y rays in total. All
the files have the same structure. <P />(28 data files).
---------------------------------------------------------
Title: On the Possibility of Detecting Helium D3 Line Polarization
with Metis
Authors: Heinzel, Petr; Štěpán, Jiři; Bemporad, Alessandro;
Fineschi, Silvano; Jejčič, Sonja; Labrosse, Nicolas; Susino, Roberto
2020ApJ...900....8H Altcode: 2020arXiv200708940H
Metis, the space coronagraph on board the Solar Orbiter, offers us
new capabilities for studying eruptive prominences and coronal mass
ejections (CMEs). Its two spectral channels, hydrogen Lα and visible
light (VL), will provide for the first time coaligned and cotemporal
images to study dynamics and plasma properties of CMEs. Moreover,
with the VL channel (580-640 nm) we find an exciting possibility
to detect the helium D<SUB>3</SUB> line (587.73 nm) and its linear
polarization. The aim of this study is to predict the diagnostic
potential of this line regarding the CME thermal and magnetic
structure. For a grid of models we first compute the intensity of the
D<SUB>3</SUB> line together with VL continuum intensity due to Thomson
scattering on core electrons. We show that the Metis VL channel will
detect a mixture of both, with predominance of the helium emission at
intermediate temperatures between 30 and 50,000 K. Then we use the
code HAZEL to compute the degree of linear polarization detectable
in the VL channel. This is a mixture of D<SUB>3</SUB> scattering
polarization and continuum polarization. The former one is lowered in
the presence of a magnetic field and the polarization axis is rotated
(Hanle effect). Metis has the capability of measuring Q/I and U/I
polarization degrees and we show their dependence on temperature and
magnetic field. At T = 30,000 K we find a significant lowering of
Q/I which is due to strongly enhanced D<SUB>3</SUB> line emission,
while depolarization at 10 G amounts roughly to 10%.
---------------------------------------------------------
Title: The polarization of the Hα line in the quiet solar
chromosphere
Authors: Jaume Bestard, J.; Trujillo Bueno, J.; Štěpán, J.; Bianda,
M.; Ramelli, R.
2020sea..confE.200J Altcode:
One-dimensional radiative transfer (RT) calculations suggested that the
scattering polarization profiles of the Hα line are very sensitive to
the strength and structure of the chromospheric magnetic field. Here
we present unprecedented spectropolarimetric observations of the Hα
line obtained with ZIMPOL-3 at IRSOL. The linear polarization profiles
show a rich variety of shapes and amplitudes, as well as an interesting
spatial variability. We confront them with the theoretical scattering
polarization profiles we have obtained by solving the complex RT problem
of the Hα polarization in a 3D model from MHD simulations, highlighting
the impact produced by the model's magnetic and velocity field. This
investigation reveals the great interest of the Hα polarization in
the present new era of large-aperture solar telescopes.
---------------------------------------------------------
Title: Near optimal angular quadratures for polarised radiative
transfer
Authors: Štěpán, Jiří; Jaume Bestard, Jaume; Trujillo Bueno,
Javier
2020A&A...636A..24S Altcode: 2020arXiv200212736S
In three-dimensional (3D) radiative transfer (RT) problems, the
tensor product quadratures are generally not optimal in terms of
the number of discrete ray directions needed for a given accuracy
of the angular integration of the radiation field. In this paper, we
derive a new set of angular quadrature rules that are more suitable
for solving 3D RT problems with the short- and long-characteristics
formal solvers. These quadratures are more suitable than the currently
used ones for the numerical calculation of the radiation field tensors
that are relevant in the problem of the generation and transfer of
polarised radiation without assuming local thermodynamical equilibrium
(non-LTE). We show that our new quadratures can save up to about 30%
of computing time with respect to the Gaussian-trapezoidal product
quadratures with the same accuracy. <P />The tables mentioned in
Appendix A are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/636/A24">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/636/A24</A>
---------------------------------------------------------
Title: VizieR Online Data Catalog: Polarised radiative transfer
angular quadratures (Stepan+, 2020)
Authors: Stepan, J.; Jaume Bestard, J.; Trujillo Bueno, J.
2020yCat..36360024S Altcode:
Quadrature for unpolarised and polarised radiations. <P />(13 data
files).
---------------------------------------------------------
Title: IRIS Mg II Observations and Non-LTE Modeling of Off-limb
Spicules in a Solar Polar Coronal Hole
Authors: Tei, Akiko; Gunár, Stanislav; Heinzel, Petr; Okamoto,
Takenori J.; Štěpán, Jiří; Jejčič, Sonja; Shibata, Kazunari
2020ApJ...888...42T Altcode: 2019arXiv191112243T
We investigated the off-limb spicules observed in the Mg II h and k
lines by IRIS in a solar polar coronal hole. We analyzed the large data
set of obtained spectra to extract quantitative information about the
line intensities, shifts, and widths. The observed Mg II line profiles
are broad and double peaked at lower altitudes, broad but flat topped
at middle altitudes, and narrow and single peaked with the largest
Doppler shifts at higher altitudes. We use one-dimensional non-LTE
vertical slab models (I.e., models that consider departures from local
thermodynamic equilibrium) in single-slab and multi-slab configurations
to interpret the observations and to investigate how a superposition
of spicules along the line of sight (LOS) affects the synthetic Mg
II line profiles. The used multi-slab models either are static, I.e.,
without any LOS velocities, or assume randomly assigned LOS velocities
of individual slabs, representing the spicule dynamics. We conducted
such single-slab and multi-slab modeling for a broad set of model
input parameters and showed the dependence of the Mg II line profiles
on these parameters. We demonstrated that the observed line widths
of the h and k line profiles are strongly affected by the presence
of multiple spicules along the LOS. We later showed that the profiles
obtained at higher altitudes can be reproduced by single-slab models
representing individual spicules. We found that the multi-slab model
with a random distribution of the LOS velocities ranging from -25 to
25 km s<SUP>-1</SUP> can well reproduce the width and the shape of Mg
II profiles observed at middle altitudes.
---------------------------------------------------------
Title: Solar and Stellar Chromospheres
Authors: Heinzel, Petr; Štěpán, Jiří
2019ASPC..519...59H Altcode:
RADCOSMOS conference was largely devoted to many aspects of stellar
photospheric modeling, and namely a great success of Ivan's code
<tt>TLUSTY</tt> was emphasized. In this short contribution
we focus on models of solar and stellar chromospheres which are not
in radiative equilibrium and thus require the knowledge of various
non-radiative heating mechanisms. We desrcibe both semiempirical,
as well as fully 3D RMHD models and mention the importance of partial
redistribution for a proper line diagnostics. Finally, since RADCOSMOS
was also covering topic of the polarized line transfer, we present
here some of the latests achievements related to the solar chromosphere.
---------------------------------------------------------
Title: Modeling the Scattering Polarization of the Hydrogen Lyα
Line Observed by CLASP in a Filament Channel
Authors: Štěpán, J.; Trujillo Bueno, J.; Gunár, S.; Heinzel, P.;
del Pino Alemán, T.; Kano, R.; Ishikawa, R.; Narukage, N.; Bando,
T.; Winebarger, A.; Kobayashi, K.; Auchère, F.
2019ASPC..526..165S Altcode:
The 400 arcsec spectrograph slit of CLASP crossed mainly quiet
regions of the solar chromosphere, from the limb towards the solar
disk center. Interestingly, in the CLASP slit-jaw images and in the
SDO images of the He II line at 304 Å, we can identify a filament
channel (FC) extending over more than 60 arcsec crossing the slit of
the spectrograph. In order to interpret the peculiar spatial variation
of the Q/I and U/I signals observed by CLASP in the hydrogen Lyα line
(1216 Å), we perform multi-dimensional radiative transfer modeling
in given filament models. In this contribution, we show the first
results of the two-dimensional calculations we have carried out, with
the aim of determining the filament thermal and magnetic structure by
comparing the theoretical and the observed polarization signals. Our
results suggest that the temperature gradients in the filament observed
by CLASP are significantly larger than previously thought.
---------------------------------------------------------
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: 3D Whole-Prominence Fine Structure Model as a Test Case for
Verification and Development of Magnetic Field Inversion Techniques
Authors: Gunár, S.; Mackay, D. H.; Štěpán, J.; Heinzel, P.;
Trujillo Bueno, J.
2019ASPC..526..159G Altcode:
We show the potential of a new 3D whole-prominence fine structure
model to serve as a well-controlled yet complex environment for testing
inversion techniques for the magnetic field inference. The realistic
3D magnetic field and plasma environment provided by the model can
be used for the direct synthesis of spectro-polarimetric data. Such
synthetic data can be analyzed by advanced inversion tools and their
results compared with the known properties provided by the model.
---------------------------------------------------------
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: Comparison of theoretical and observed Ca II 8542 Stokes
profiles in quiet regions at the centre of the solar disc
Authors: Jurčák, J.; Štěpán, J.; Trujillo Bueno, J.; Bianda, M.
2018A&A...619A..60J Altcode: 2018arXiv180809470J
Context. Interpreting the Stokes profiles observed in quiet regions
of the solar chromosphere is a challenging task. The Stokes Q and U
profiles are dominated by the scattering polarisation and the Hanle
effect, and these processes can only be correctly quantified if 3D
radiative transfer effects are taken into account. Forward-modelling
of the intensity and polarisation of spectral lines using a 3D model
atmosphere is a suitable approach in order to statistically compare
the theoretical and observed line profiles. <BR /> Aims: Our aim is to
present novel observations of the Ca II 8542 Å line profiles in a quiet
region at the centre of the solar disc and to quantitatively compare
them with the theoretical Stokes profiles obtained by solving the
problem of the generation and transfer of polarised radiation in a 3D
model atmosphere. We aim at estimating the reliability of the 3D model
atmosphere, excluding its known lack of dynamics and/or insufficient
density, using not only the line intensity but the full vector of
Stokes parameters. <BR /> Methods: We used data obtained with the
ZIMPOL instrument at the Istituto Ricerche Solari Locarno (IRSOL) and
compared the observations with the theoretical profiles computed with
the PORTA radiative transfer code, using as solar model atmosphere a
3D snapshot taken from a radiation-magnetohydrodynamics simulation. The
synthetic profiles were degraded to match the instrument and observing
conditions. <BR /> Results: The degraded theoretical profiles of the
Ca II 8542 line are qualitatively similar to the observed ones. We
confirm that there is a fundamental difference in the widths of all
Stokes profiles: the observed lines are wider than the theoretical
lines. We find that the amplitudes of the observed profiles are larger
than those of the theoretical ones, which suggests that the symmetry
breaking effects in the solar chromosphere are stronger than in the
model atmosphere. This means that the isosurfaces of temperature,
velocity, and magnetic field strength and orientation are more
corrugated in the solar chromosphere than in the currently available
3D radiation-magnetohydrodynamics simulation.
---------------------------------------------------------
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: A Novel Investigation of the Small-scale Magnetic Activity
of the Quiet Sun via the Hanle Effect in the Sr I 4607 Å Line
Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Štěpán, J.;
Shchukina, N.
2018ApJ...863..164D Altcode: 2018arXiv180607293D
One of the key research problems in stellar physics is to decipher the
small-scale magnetic activity of the quiet solar atmosphere. Recent
magneto-convection simulations that account for small-scale dynamo
action have provided three-dimensional (3D) models of the solar
photosphere characterized by a high degree of small-scale magnetic
activity, similar to that found through theoretical interpretation of
the scattering polarization observed in the Sr I 4607 Å line. Here
we present the results of a novel investigation of the Hanle effect
in this resonance line based on 3D radiative transfer calculations
in a high-resolution magneto-convection model having most of
the convection zone magnetized close to the equipartition and a
surface mean field strength < B> ≈ 170 G. The Hanle effect
produced by the model’s magnetic field depolarizes the zero-field
scattering polarization signals significantly, to the extent that the
center-to-limb variation (CLV) of the calculated spatially averaged
polarization amplitudes is compatible with the observations. The
standard deviation of the horizontal fluctuations of the calculated
scattering polarization signals is very sensitive to the model’s
magnetic field, and we find that the predicted spatial variations
are sufficiently sizable so as to be able to detect them, especially
with the next generation of solar telescopes. We find that at all
on-disk positions, the theoretical scattering polarization signals are
anticorrelated with the continuum intensity. To facilitate reaching
new observational breakthroughs, we show how the theoretically
predicted polarization signals and spatial variations are modified
when deteriorating the signal-to-noise ratio and the spectral and
spatial resolutions of the simulated observations.
---------------------------------------------------------
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: Self-consistent multi-dimensional inversion problem
Authors: Stepan, Jiri
2018cosp...42E3246S Altcode:
The main goal of solar spectropolarimetry is to infer reliable physical
information about the plasma structures from the observed Stokes
profiles of spectral lines. In order to solve the general multi-D
inversion problem of optically thick spectral lines, one can take
advantage of sparsity of the observed data. Additional regularization of
the problem imposed by the need to satisfy the requirement of the NLTE
self-consistent solution leads to a robust inference method. Coupling
of the massivelly parallel forward solver taking into account the
scattering polarization, Hanle, and Zeeman effects with a suitable
inversion algorithm allows us to develop efficient inversion procedures
for thermal and magnetic properties of the solar plasma. Here we show
the first results of our numerical experiments.
---------------------------------------------------------
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: 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: 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: Three-dimensional radiative transfer simulations of the
scattering polarization
Authors: Stepan, Jiri
2017psio.confE..26S Altcode:
No abstract at ADS
---------------------------------------------------------
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: 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).
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Title: The Hanle and Zeeman Polarization Signals of the Solar Ca II
8542 Å Line
Authors: Štěpán, Jiří; Trujillo Bueno, Javier
2016ApJ...826L..10S Altcode: 2016arXiv160607741S
We highlight the main results of a three-dimensional (3D) multilevel
radiative transfer investigation about the solar disk-center
polarization of the Ca II 8542 Å line. First, through the use
of a 3D model of the solar atmosphere, we investigate the linear
polarization that occurs due to the atomic level polarization produced
by the absorption and scattering of anisotropic radiation, taking
into account the symmetry-breaking effects caused by its thermal,
dynamic, and magnetic structure. Second, we study the contribution of
the Zeeman effect to the linear and circular polarization. Finally,
we show examples of the Stokes profiles produced by the joint action
of the atomic level polarization and the Hanle and Zeeman effects. We
find that the Zeeman effect tends to dominate the linear polarization
signals only in the localized patches of opposite magnetic polarity,
where the magnetic field is relatively strong and slightly inclined;
outside such very localized patches, the linear polarization is
often dominated by the contribution of atomic level polarization. We
demonstrate that a correct modeling of this last contribution requires
taking into account the symmetry-breaking effects caused by the
thermal, dynamic, and magnetic structure of the solar atmosphere,
and that in the 3D model used the Hanle effect in forward-scattering
geometry (disk-center observation) mainly reduces the polarization
corresponding to the zero-field case. We emphasize that, in general,
a reliable modeling of the linear polarization in the Ca II 8542 Å
line requires taking into account the joint action of atomic level
polarization and the Hanle and Zeeman effects.
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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.
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Title: Three-dimensional simulations of scattering polarization and
the Hanle effect in MHD chromospheric models
Authors: Štěpán, J.
2015IAUS..305..360S Altcode: 2015arXiv151009103S
Scattering line polarization and the Hanle effect are among the
most important mechanisms for diagnostics of the solar and stellar
atmospheres. The fact that real stellar atmospheres are horizontally
inhomogeneous makes the spectral synthesis and interpretation very
challenging because the effect of thermodynamic fluctuations on
spectral line polarization is entangled with the action of magnetic
fields. This applies to the spatially resolved as well as to the
averaged spectra. The necessary step towards the interpretation of such
spectra is to study the line formation in sufficiently realistic 3D
MHD models and compare the synthetic spectra with observations. This
paper gives an overview of recent progress in the field of 3D NLTE
synthesis of polarized spectral lines resulting from investigations
with the radiative transfer code PORTA.
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Title: Formation of polarized spectral lines in atmospheres with
horizontal inhomogeneities
Authors: Tichý, A.; Štěpán, J.; Trujillo Bueno, J.; Kubát, J.
2015IAUS..305..401T Altcode:
We study the problem of the generation and transfer of spectral line
intensity and polarization in models of stellar atmospheres with
horizontal plasma inhomogeneities. We solve the non-LTE radiative
transfer problem in full 3D geometry taking into account resonant
scattering polarization and its modification by magnetic fields
via the Hanle effect. We show that horizontal fluctuations of
the thermodynamical conditions of stellar atmospheres can have a
significant impact on the linear polarization of the emergent spectral
line radiation and its center-to-limb variation.
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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: Three-dimensional Radiative Transfer Simulations of
the Scattering Polarization of the Hydrogen Lyα Line in a
Magnetohydrodynamic Model of the Chromosphere-Corona Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Leenaarts, J.; Carlsson, M.
2015ApJ...803...65S Altcode: 2015arXiv150106382S
Probing the magnetism of the upper solar chromosphere requires measuring
and modeling the scattering polarization produced by anisotropic
radiation pumping in UV spectral lines. Here we apply PORTA (a novel
radiative transfer code) to investigate the hydrogen Lyα line in
a three-dimensional model of the solar atmosphere resulting from a
state of the art magnetohydrodynamic (MHD) simulation. At full spatial
resolution the linear polarization signals are very significant all
over the solar disk, with a large fraction of the field of view (FOV)
showing line-center amplitudes well above the 1% level. Via the Hanle
effect the line-center polarization signals are sensitive to the
magnetic field of the model's transition region, even when its mean
field strength is only 15 G. The breaking of the axial symmetry of the
radiation field produces significant forward-scattering polarization
in Lyα, without the need of an inclined magnetic field. Interestingly,
the Hanle effect tends to decrease such forward-scattering polarization
signals in most of the points of the FOV. When the spatial resolution is
degraded, the line-center polarization of Lyα drops below the 1% level,
reaching values similar to those previously found in one-dimensional
(1D) semi-empirical models (i.e., up to about 0.5 %). The center to
limb variation (CLV) of the spatially averaged polarization signals
is qualitatively similar to that found in 1D models, with the largest
line-center amplitudes at μ =cos θ ≈ 0.4 (θ being the heliocentric
angle). These results are important, both for designing the needed
space-based instrumentation and for a reliable interpretation of future
observations of the Lyα polarization.
---------------------------------------------------------
Title: On the Origin of Linear Polarization in Solar Flares
Authors: Štěpán, J.; Heinzel, P.
2014ASPC..489..133S Altcode:
A significant degree of linear polarization (up to few percent) of
some spectral lines is occasionally reported from the observations of
solar flares. This polarization is often found at the edges of the
flare ribbons and it is usually radial or tangential. The mechanism
usually considered as being responsible for this effect is the
impact polarization by electron and/or proton beams bombarding the
chromosphere. We point out that resonant scattering polarization in a
multi-dimensional geometry of the chromosphere has to be considered as
an important ingredient of the problem. The significant horizontal
inhomogeneities at the boundaries of the flare ribbons causes a
considerable change in the radiation field anisotropy which may lead
to emission of strongly linearly polarized spectral lines. For more
details see Štěpán, & Heinzel (2013)
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Title: PORTA: A Massively Parallel Code for 3D Non-LTE Polarized
Radiative Transfer
Authors: Štěpán, J.
2014ASPC..489..243S Altcode:
The interpretation of the Stokes profiles of the solar (stellar)
spectral line radiation requires solving a non-LTE radiative transfer
problem that can be very complex, especially when the main interest lies
in modeling the linear polarization signals produced by scattering
processes and their modification by the Hanle effect. One of the
main difficulties is due to the fact that the plasma of a stellar
atmosphere can be highly inhomogeneous and dynamic, which implies the
need to solve the non-equilibrium problem of generation and transfer of
polarized radiation in realistic three-dimensional stellar atmospheric
models. Here we present PORTA, a computer program we have developed for
solving, in three-dimensional (3D) models of stellar atmospheres, the
problem of the generation and transfer of spectral line polarization
taking into account anisotropic radiation pumping and the Hanle
and Zeeman effects in multilevel atoms. The numerical method of
solution is based on a highly convergent iterative algorithm, whose
convergence rate is insensitive to the grid size, and on an accurate
short-characteristics formal solver of the Stokes-vector transfer
equation which uses monotonic Bezier interpolation. In addition to
the iterative method and the 3D formal solver, another important
feature of PORTA is a novel parallelization strategy suitable for
taking advantage of massively parallel computers. Linear scaling
of the solution with the number of processors allows to reduce the
solution time by several orders of magnitude. We present useful
benchmarks and a few illustrations of applications using a 3D model
of the solar chromosphere resulting from MHD simulations. Finally,
we present our conclusions with a view to future research. For more
details see Štěpán & Trujillo Bueno (2013).
---------------------------------------------------------
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: 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: Scattering Polarization in Solar Flares
Authors: Štěpán, Jiří; Heinzel, Petr
2013ApJ...778L...6S Altcode: 2013arXiv1310.3284S
There is ongoing debate about the origin and even the very existence
of a high degree of linear polarization of some chromospheric
spectral lines observed in solar flares. The standard explanation
of these measurements is in terms of the impact polarization caused
by non-thermal proton and/or electron beams. In this work, we study
the possible role of resonance line polarization due to radiation
anisotropy in the inhomogeneous medium of the flare ribbons. We
consider a simple two-dimensional model of the flaring chromosphere
and we self-consistently solve the non-LTE problem taking into
account the role of resonant scattering polarization and of the
Hanle effect. Our calculations show that the horizontal plasma
inhomogeneities at the boundary of the flare ribbons can lead to a
significant radiation anisotropy in the line formation region and,
consequently, to a fractional linear polarization of the emergent
radiation of the order of several percent. Neglecting the effects of
impact polarization, our model can provide a clue for resolving some
of the common observational findings, namely: (1) why a high degree
of polarization appears mainly at the edges of the flare ribbons;
(2) why polarization can also be observed during the gradual phase of
a flare; and (3) why polarization is mostly radial or tangential. We
conclude that radiation transfer in realistic multi-dimensional models
of solar flares needs to be considered as an essential ingredient for
understanding the observed spectral line polarization.
---------------------------------------------------------
Title: PORTA: A three-dimensional multilevel radiative transfer code
for modeling the intensity and polarization of spectral lines with
massively parallel computers
Authors: Štěpán, Jiří; Trujillo Bueno, Javier
2013A&A...557A.143S Altcode: 2013arXiv1307.4217S
The interpretation of the intensity and polarization of the spectral
line radiation produced in the atmosphere of the Sun and of other stars
requires solving a radiative transfer problem that can be very complex,
especially when the main interest lies in modeling the spectral line
polarization produced by scattering processes and the Hanle and Zeeman
effects. One of the difficulties is that the plasma of a stellar
atmosphere can be highly inhomogeneous and dynamic, which implies the
need to solve the non-equilibrium problem of the generation and transfer
of polarized radiation in realistic three-dimensional (3D) stellar
atmospheric models. Here we present PORTA, an efficient multilevel
radiative transfer code we have developed for the simulation of the
spectral line polarization caused by scattering processes and the Hanle
and Zeeman effects in 3D models of stellar atmospheres. The numerical
method of solution is based on the non-linear multigrid iterative method
and on a novel short-characteristics formal solver of the Stokes-vector
transfer equation which uses monotonic Bézier interpolation. Therefore,
with PORTA the computing time needed to obtain at each spatial grid
point the self-consistent values of the atomic density matrix (which
quantifies the excitation state of the atomic system) scales linearly
with the total number of grid points. Another crucial feature of PORTA
is its parallelization strategy, which allows us to speed up the
numerical solution of complicated 3D problems by several orders of
magnitude with respect to sequential radiative transfer approaches,
given its excellent linear scaling with the number of available
processors. The PORTA code can also be conveniently applied to solve
the simpler 3D radiative transfer problem of unpolarized radiation in
multilevel systems.
---------------------------------------------------------
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.
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Title: The Hanle Effect of Lyα in a Magnetohydrodynamic Model of
the Solar Transition Region
Authors: Štěpán, J.; Trujillo Bueno, J.; Carlsson, M.; Leenaarts, J.
2012ApJ...758L..43S Altcode: 2012arXiv1208.4929S
In order to understand the heating of the solar corona it is crucial
to obtain empirical information on the magnetic field in its lower
boundary (the transition region). To this end, we need to measure
and model the linear polarization produced by scattering processes in
strong UV lines, such as the hydrogen Lyα line. The interpretation
of the observed Stokes profiles will require taking into account that
the outer solar atmosphere is highly structured and dynamic, and that
the height of the transition region may well vary from one place in
the atmosphere to another. Here, we report on the Lyα scattering
polarization signals we have calculated in a realistic model of an
enhanced network region, resulting from a state-of-the-art radiation
magnetohydrodynamic simulation. This model is characterized by spatially
complex variations of the physical quantities at transition region
heights. The results of our investigation lead us to emphasize that
scattering processes in the upper solar chromosphere should indeed
produce measurable linear polarization in Lyα. More importantly,
we show that via the Hanle effect the model's magnetic field produces
significant changes in the emergent Q/I and U/I profiles. Therefore, we
argue that by measuring the polarization signals produced by scattering
processes and the Hanle effect in Lyα and contrasting them with those
computed in increasingly realistic atmospheric models, we should be
able to decipher the magnetic, thermal, and dynamic structure of the
upper chromosphere and transition region of the Sun.
---------------------------------------------------------
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 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: A 3D Radiative Transfer Code for Modeling the Hanle Effect
in the Lyman α line
Authors: Štepán, J.; Trujillo Bueno, J.
2012ASPC..456...59S Altcode: 2012arXiv1205.2959S
In order to obtain empirical information on the magnetism of the
solar transition region we need to measure and interpret the linear
polarization produced by scattering processes in FUV and EUV spectral
lines. Via the Hanle effect such linear polarization signals are
sensitive to the magnetic fields expected for the quiet and active
regions of the outer solar atmosphere. For example, the Ly$\alpha$ line
of H\,{\sc i} at 1216\,Å is mainly sensitive to magnetic strengths
between 10 and 100 G. The interpretation of the observed spectral
line polarization requires the development of suitable modeling
tools. To this end, we have developed a three-dimensional (3D),
non-LTE multilevel radiative transfer code for modeling the intensity
and linear polarization produced by scattering processes in spectral
lines and its modification by the Hanle effect.
---------------------------------------------------------
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.
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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 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.
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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.
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Title: The Hanle Effect of the Hydrogen Lyα Line for Probing the
Magnetism of the Solar Transition Region
Authors: Trujillo Bueno, Javier; Štěpán, Jiří; Casini, Roberto
2011ApJ...738L..11T Altcode: 2011arXiv1107.4787T
We present some theoretical predictions concerning the amplitude and
magnetic sensitivity of the linear-polarization signals produced by
scattering processes in the hydrogen Lyα line of the solar transition
region. To this end, we have calculated the atomic-level polarization
(population imbalances and quantum coherences) induced by anisotropic
radiation pumping in semiempirical 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 line-center amplitudes of the emergent linear-polarization
signals are found to vary typically between 0.1% and 1%, 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 diagnostic window for magnetic field measurements in
the upper chromosphere and transition region of the Sun.
---------------------------------------------------------
Title: Scattering Polarization of Hydrogen Lines in Weakly Magnetized
Stellar Atmospheres. I. Formulation and Application to Isothermal
Models
Authors: Štěpán, Jiří; Trujillo Bueno, Javier
2011ApJ...732...80S Altcode: 2011arXiv1102.4012S
Although the spectral lines of hydrogen contain valuable information
on the physical properties of a variety of astrophysical plasmas,
including the upper solar chromosphere, relatively little is known
about their scattering polarization signals, whose modification via the
Hanle effect may be exploited for magnetic field diagnostics. Here we
report on a basic theoretical investigation of the linear polarization
produced by scattering processes and the Hanle effect in Lyα, Lyβ,
and Hα taking into account multilevel radiative transfer effects
in an isothermal stellar atmosphere model, the fine-structure of the
hydrogen levels, as well as the impact of collisions with electrons
and protons. The main aim of this first paper is to elucidate the
key physical mechanisms that control the emergent fractional linear
polarization in the three lines, as well as its sensitivity to the
perturbers' density and to the strength and structure of microstructured
and deterministic magnetic fields. To this end, we apply an efficient
radiative transfer code we have developed for performing numerical
simulations of the Hanle effect in multilevel systems with overlapping
line transitions. For low-density plasmas, such as that of the upper
solar chromosphere, collisional depolarization is caused mainly by
collisional transitions between the fine-structure levels of the n =
3 level, so that it is virtually insignificant for Lyα but important
for Lyβ and Hα. We show the impact of the Hanle effect on the three
lines taking into account the radiative transfer coupling between
the different hydrogen line transitions. For example, we demonstrate
that the linear polarization profile of the Hα line is sensitive to
the presence of magnetic field gradients in the line core formation
region, and that in solar-like chromospheres selective absorption
of polarization components does not play any significant role in the
emergent scattering polarization.
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Title: A Sounding Rocket Experiment for Spectropolarimetric
Observations with the Ly<SUB>α</SUB> Line at 121.6 nm (CLASP)
Authors: Ishikawa, R.; Bando, T.; Fujimura, D.; Hara, H.; Kano,
R.; Kobiki, T.; Narukage, N.; Tsuneta, S.; Ueda, K.; Wantanabe,
H.; Kobayashi, K.; Trujillo Bueno, J.; Manso Sainz, R.; Stepan, J.;
de Pontieu, B.; Carlsson, M.; Casini, R.
2011ASPC..437..287I Altcode:
A team consisting of Japan, USA, Spain, and Norway is developing a
high-throughput Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP),
which is proposed to fly with a NASA sounding rocket in 2014. CLASP will
explore the magnetism of the upper solar chromosphere and transition
region via the Hanle effect of the Ly<SUB>α</SUB> line for the first
time. This experiment requires spectropolarimetric observations with
high polarimetric sensitivity (∼0.1%) and wavelength resolution
(0.1 Å). The final spatial resolution (slit width) is being discussed
taking into account the required high signal-to-noise ratio. We have
demonstrated the performance of the Ly<SUB>α</SUB> polarimeter by
extensively using the Ultraviolet Synchrotron ORbital Radiation Facility
(UVSOR) at the Institute for Molecular Sciences. In this contribution,
we report these measurements at UVSOR together with the current status
of the CLASP project.
---------------------------------------------------------
Title: Scattering Polarization and the Hanle Effect in Hα as a
Probe of Chromospheric Magnetism: Modeling vs. Observations
Authors: Štěpán, J.; Trujillo Bueno, J.; Ramelli, R.; Bianda, M.
2011ASPC..437..117S Altcode: 2011arXiv1102.4941S
The Hanle effect in strong spectral lines is the physical mechanism
that should be increasingly exploited for quantitative explorations
of the magnetism of the quiet solar chromospheric plasma. Here we
show, by means of multilevel radiative transfer calculations and new
spectropolarimetric observations, that the amplitude and shape of the
scattering polarization profiles of the Hα line is very sensitive to
the strength and structure of the chromospheric magnetic field. The
interpretation of the observations in terms of one-dimensional radiative
transfer modeling suggests that there is an abrupt magnetization in
the upper chromosphere of the quiet Sun.
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Title: On the Probable Existence of an Abrupt Magnetization in the
Upper Chromosphere of the Quiet Sun
Authors: Štěpán, Jiří; Trujillo Bueno, Javier
2010ApJ...711L.133S Altcode: 2010arXiv1002.1574S
We report on a detailed radiative transfer modeling of the observed
scattering polarization in the Hα line, which allows us to infer
quantitative information on the magnetization of the quiet solar
chromosphere. Our analysis suggests the presence of a magnetic
complexity zone with a mean field strength langBrang > 30 G lying
just below the sudden transition region to the coronal temperatures. The
chromospheric plasma directly underneath is very weakly magnetized,
with langBrang ~ 1 G. The possible existence of this abrupt change in
the degree of magnetization of the upper chromosphere of the quiet Sun
might have large significance for our understanding of chromospheric
(and, therefore, coronal) heating.
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Title: On the sensitivity of the Halpha scattering polarization to
chromospheric magnetism
Authors: Štěpán, Jiří; Trujillo Bueno, Javier
2010MmSAI..81..810S Altcode: 2010arXiv1001.2720S
A particularly interesting line for exploring the physical conditions
of the quiet solar chromosphere is Halpha , but its intensity profile
is magnetically insensitive and the small circular polarization
signatures produced by the longitudinal Zeeman effect come mainly
from the underlying photosphere. Here we show that the Hanle effect in
Halpha provides quantitative information on the magnetism of the quiet
chromosphere. To this end, we calculate the response function of the
emergent scattering polarization to perturbations in the magnetic field.
---------------------------------------------------------
Title: NLTE Effects in the Transfer of Polarized Lines of Multiterm
Atoms
Authors: Štěpán, J.
2009ASPC..405..307S Altcode:
The formation of spectral lines in a magnetized atmosphere is a
complex issue both from the conceptual and computational point of
view. The NLTE effects have been shown to play a significant role
in many astrophysical situations both for unpolarized and polarized
cases. We present a code for the NLTE radiative transfer calculations
in a plane-parallel magnetized atmosphere for the so-called multiterm
picture of atomic levels. We discuss the effects of NLTE radiative
transfer on the polarization state of emergent radiation.
---------------------------------------------------------
Title: Possible creation of net circular polarization and not only
depolarization of spectral lines by isotropic collisions
Authors: Štěpán, J.; Sahal-Bréchot, S.
2008sf2a.conf..573S Altcode: 2008arXiv0811.4573S
We will show that isotropic collisions of electrons and protons with
neutral hydrogen can lead to creation of net orientation of the
atomic levels in the presence of a magnetic field. Consequently,
the emitted Stokes-V profile of the spectral lines can be almost
symmetric in contrast to the typical antisymmetric signature of
the Zeeman effect. Moreover, the amplitude of the symmetric lobe
can be significantly higher than the amplitude of the antisymmetric
components. This mechanism is caused by a ±{M} symmetry breaking of
the collisional transitions between different Zeeman sublevels. We
will show an example of our first results for the Hα line. This
new mechanism could perhaps explain the net circular polarization of
spectral lines observed in some solar limb observations and which are
currently not understood. However, our results are very preliminary
and more developments are needed for going further on.
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Title: Hydrogen Balmer line formation in solar flares affected by
return currents
Authors: Štepán, J. Å.; Kašparová, J.; Karlický, M.; Heinzel, P.
2007A&A...472L..55S Altcode: 2007arXiv0708.0265S
Aims:We investigate the effect of the electric return currents in
solar flares on the profiles of hydrogen Balmer lines. We consider the
monoenergetic approximation for the primary beam and runaway model of
the neutralizing return current. <BR />Methods: Propagation of the
10 keV electron beam from a coronal reconnection site is considered
for the semiempirical chromosphere model F1. We estimate the local
number density of return current using two approximations for beam
energy fluxes between 4 × 10<SUP>11</SUP> and 1 × 10<SUP>12</SUP>
erg cm<SUP>-2</SUP> s<SUP>-1</SUP>. Inelastic collisions of beam and
return-current electrons with hydrogen are included according to their
energy distributions, and the hydrogen Balmer line intensities are
computed using an NLTE radiative transfer approach. <BR />Results:
In comparison to traditional NLTE models of solar flares that neglect
the return-current effects, we found a significant increase emission
in the Balmer line cores due to nonthermal excitation by return
current. Contrary to the model without return current, the line
shapes are sensitive to a beam flux. It is the result of variation
in the return-current energy that is close to the hydrogen excitation
thresholds and the density of return-current electrons.
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Title: Hydrogen Hα line polarization in solar flares. Theoretical
investigation of atomic polarization by proton beams considering
self-consistent NLTE polarized radiative transfer
Authors: Štěpán, J.; Heinzel, P.; Sahal-Bréchot, S.
2007A&A...465..621S Altcode: 2007astro.ph..1617S
Context: We present a theoretical review of the effect of impact
polarization of a hydrogen Hα line due to an expected proton beam
bombardment in solar flares. <BR />Aims: Several observations indicate
the presence of the linear polarization of the hydrogen Hα line
observed near the solar limb above 5% and preferentially in the radial
direction. We theoretically review the problem of deceleration of the
beam originating in the coronal reconnection site due to its interaction
with the chromospheric plasma, and describe the formalism of the density
matrix used in our description of the atomic processes and the treatment
of collisional rates. <BR />Methods: We solve the self-consistent NLTE
radiation transfer problem for the particular semiempirical chromosphere
models for both intensity and linear polarization components of the
radiation field. <BR />Results: In contrast to recent calculations,
our results show that the energy distribution of the proton beam at Hα
formation levels and depolarizing collisions by background electrons
and protons cause a significant reduction of the effect below 0.1%. The
radiation transfer solution shows that tangential resonance-scattering
polarization dominates over the impact polarization effect in all
considered models. <BR />Conclusions: . In the models studied, proton
beams are unlikely to be a satisfying explanation for the observed
linear polarization of the Hα line.
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Title: Polarization diagnostics of proton beams in solar flares
Authors: Štěpán, J.
2007MmSAI..78...83S Altcode: 2007astro.ph..2060S
We review the problem of proton beam bombardment of solar chromosphere
considering the self-consistent NLTE polarized radiation transfer in
hydrogen lines. Several observations indicate a linear polarization
of the Halpha line of the order of 5% or higher and preferentially in
radial direction. This polarization is often explained as anisotropic
collisional excitation of the n=3 level by vertical proton beams. Our
calculations indicate that deceleration of the proton beam with initial
power-law energy distribution together with increased electron and
proton densities in the Halpha forming layers lead to a negligible line
polarization. Thus the proton beams seem not to be a good candidate
for explanation of the observed polarization degree. On the other hand,
the effect of electric return currents could perhaps provide a better
explanation of the observed linear polarization. We report the new
calculations of this effect.
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Title: Multigrid Methods for Polarized Radiative Transfer
Authors: Štěpán, J.
2006ASPC..358..148S Altcode: 2006astro.ph.11112S
A new iterative method for non-LTE multilevel polarized radiative
transfer in hydrogen lines is presented. Iterative methods (such as
the Jacobi method) tend to damp out high-frequency components of the
error fast, but converges poorly due to slow reduction of low-frequency
components. The idea is to use a set of differently coarsed grids
to reduce both the short- and long-period errors. This leads to the
so-called multigrid (MG) methods. For the grid of~N spatial points,
the number of iterations required to solve a non-LTE transfer problem
is of the order of~O(N). This fact could be of great importance for
problems with fine structure and for multi-dimensional models. The
efficiency of the so-called standard MG iteration in comparison to
Jacobi iteration is shown. The formalism of density matrix is applied
to the demonstrative example of~1D, semi-infinite, non-magnetic,
3-principal level hydrogen atmospheric model. The effect of depolarizing
collisions with thermal electrons is taken into account as well as
general treatment of overlapping profiles.
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Title: Polarization Diagnostics of Proton Beams in Solar Flares
Authors: Stepan, J.; Heinzel, P.; Kasparova, J.; Sahal-Brechot, S.
2006IAUJD...1E..55S Altcode:
We review the problem of proton beam bombardment of solar chromosphere
considering the self-consistent NLTE polarized radiation transfer in
hydrogen lines. Several observations indicate a linear polarization
of H-alpha line of the order of 5% or higher and preferentially in
radial direction. This polarization is often explained as anisotropic
collisional excitation of the n= 3 level by vertical proton beams. Our
calculations indicate that deceleration of the proton beam with initial
power-law energy distribution together with increased electron and
proton densities in H-alpha forming layers lead to negligible line
polarization. Thus the proton beams seem not to be a good candidate
for explanation of the observed polarization degree.
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Title: CCD Photometry of the SX Phoenicis Star BL Camelopardalis
Authors: Wolf, M.; Crlikova, M.; Basta, M.; Sarounova, L.; Stepan,
J.; Sveda, L.; Vymetalik, O.
2002IBVS.5317....1W Altcode:
This note presents 14 new times of maximum light for BL Cam collected
at Ondrejov observatory in 2001/2002.
