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Author name code: alsina-ballester
ADS astronomy entries on 2022-09-14
author:Alsina Ballester, Ernest
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Title: The polarization signals of the solar K I D lines and their
magnetic sensitivity
Authors: Alsina Ballester, Ernest
2022arXiv220811728A Altcode:
This work aims to identify the relevant physical processes in shaping
the intensity and polarization patterns of the solar K I D lines
through spectral syntheses, placing particular emphasis on the D2
line. The theoretical Stokes profiles were obtained by numerically
solving the radiative transfer problem for polarized radiation
considering one-dimensional semi-empirical models of the solar
atmosphere. The calculations account for scattering polarization,
partial frequency redistribution (PRD) effects, hyperfine structure
(HFS), J- and F-state interference, multiple isotopes, and magnetic
fields of arbitrary strength and orientation. The intensity and circular
polarization profiles of both D lines can be suitably modeled while
neglecting both J-state interference and HFS. The magnetograph formula
can be applied to both lines, without including HFS, to estimate weak
longitudinal magnetic fields in the lower chromosphere. By contrast,
modeling their scattering polarization signals requires the inclusion of
HFS. The D2 scattering polarization amplitude is strongly depolarized
by HFS, but it remains measurable. An appreciable error is incurred in
the scattering polarization profile if PRD effects are not taken into
account. Collisions during scattering processes also have an appreciable
depolarizing effect. Finally, the D2 scattering polarization signal
is especially sensitive to magnetic fields with strengths around
10 G and it strongly depends on their orientation. Despite this,
its center-to-limb variation relative to the amplitude at the limb
is largely insensitive to the field strength and orientation. These
findings highlight the value of the K I D2 line polarization for
diagnostics of the solar magnetism, and show that the linear and
circular polarization signals of this line are primarily sensitive
to magnetic fields in the lower chromosphere and upper photosphere,
respectively.
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Title: The transfer of polarized radiation in resonance lines with
partial frequency redistribution, J-state interference, and arbitrary
magnetic fields. A radiative transfer code and useful approximations
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2022A&A...664A..76A Altcode: 2022arXiv220412523A
<BR /> Aims: We present the theoretical framework and numerical
methods we have implemented to solve the problem of the generation and
transfer of polarized radiation in spectral lines without assuming
local thermodynamical equilibrium, while accounting for scattering
polarization, partial frequency redistribution (due to both the
Doppler effect and elastic collisions), J-state interference, and
hyperfine structure. The resulting radiative transfer code allows
one to model the impact of magnetic fields of an arbitrary strength
and orientation through the Hanle, incomplete Paschen-Back, and
magneto-optical effects. We also evaluate the suitability of a series
of approximations for modeling the scattering polarization in the
wings of strong resonance lines at a much lower computational cost,
which is particularly valuable for the numerically intensive case of
three-dimensional radiative transfer. <BR /> Methods: We examine the
suitability of the considered approximations by using our radiative
transfer code to model the Stokes profiles of the Mg II h & k lines
and of the H I Lyman-α line in magnetized one-dimensional models of
the solar atmosphere. <BR /> Results: Neglecting Doppler redistribution
in the scattering processes that are unperturbed by elastic collisions
(i.e., treating them as coherent in the observer's frame) produces a
negligible error in the scattering polarization wings of the Mg II
resonance lines and a minor one in the Lyman-α wings, although it
is unsuitable to model the cores of these lines. For both lines, the
scattering processes that are perturbed by elastic collisions only
give a significant contribution to the intensity component of the
emissivity. Neglecting collisional as well as Doppler redistribution
(so that all scattering processes are coherent) represents a rough
but suitable approximation for the wings of the Mg II resonance lines,
but a very poor one for the Lyman-α wings. The magnetic sensitivity
in the scattering polarization wings of the considered lines can be
modeled by accounting for the magnetic field in only the η<SUB>I</SUB>
and ρ<SUB>V</SUB> coefficients of the Stokes-vector transfer equation
(i.e., using the zero-field expression for the emissivity).
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Title: The polarization angle in the wings of Ca I 4227: A new
observable for diagnosing unresolved photospheric magnetic fields
Authors: Capozzi, Emilia; Alsina Ballester, Ernest; Belluzzi, Luca;
Trujillo Bueno, Javier
2022A&A...657A..44C Altcode: 2021arXiv211108967C
Context. When observed in quiet regions close to the solar limb, many
strong resonance lines show conspicuous linear polarization signals,
produced by scattering processes (i.e., scattering polarization), with
extended wing lobes. Recent studies indicate that, contrary to what
was previously believed, the wing lobes are sensitive to the presence
of relatively weak longitudinal magnetic fields through magneto-optical
(MO) effects. <BR /> Aims: We theoretically investigate the sensitivity
of the scattering polarization wings of the Ca I 4227 Å line to the
MO effects, and we explore its diagnostic potential for inferring
information on the longitudinal component of the photospheric magnetic
field. <BR /> Methods: We calculate the intensity and polarization
profiles of the Ca I 4227 Å line by numerically solving the problem
of the generation and transfer of polarized radiation under non-local
thermodynamic equilibrium conditions in one-dimensional semi-empirical
models of the solar atmosphere, taking into account the joint action
of the Hanle, Zeeman, and MO effects. We consider volume-filling
magnetic fields as well as magnetic fields occupying a fraction of
the resolution element. <BR /> Results: In contrast to the circular
polarization signals produced by the Zeeman effect, we find that the
linear polarization angle in the scattering polarization wings of Ca
I 4227 presents a clear sensitivity, through MO effects, not only to
the flux of the photospheric magnetic field, but also to the fraction
of the resolution element that the magnetic field occupies. <BR />
Conclusions: We identify the linear polarization angle in the wings
of strong resonance lines as a valuable observable for diagnosing
unresolved magnetic fields. Used in combination with observables that
encode information on the magnetic flux and other properties of the
observed atmospheric region (e.g., temperature and density), it can
provide constraints on the filling factor of the magnetic field.
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Title: Demonstration of Chromospheric Magnetic Mapping with CLASP2.1
Authors: McKenzie, David; Ishikawa, Ryohko; Trujillo Bueno, Javier;
Auchere, F.; Kobayashi, Ken; Winebarger, Amy; Kano, Ryouhei; Song,
Donguk; Okamoto, Joten; Rachmeler, Laurel; De Pontieu, Bart; Vigil,
Genevieve; Belluzzi, Luca; Alsina Ballester, Ernest; del Pino Aleman,
Tanausu; Bethge, Christian; Sakao, Taro; Stepan, Jiri
2021AGUFMSH52A..06M Altcode:
Probing the magnetic nature of the Suns atmosphere requires measurement
of the Stokes I, Q, U and V profiles of relevant spectral lines (of
which Q, U and V encode the magnetic field information). Many of the
magnetically sensitive lines formed in the chromosphere and transition
region are in the ultraviolet spectrum, necessitating observations
above the absorbing terrestrial atmosphere. The Chromospheric
Layer Spectro-Polarimeter (CLASP2) sounding rocket was flown
successfully in April 2019, as a follow-on to the successful flight in
September 2015 of the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP). Both projects were funded by NASAs Heliophysics Technology
and Instrument Development for Science (H-TIDeS) program to develop
and test a technique for observing the Sun in ultraviolet light,
and for quantifying the polarization of that light. By demonstrating
successful measurement and interpretation of the polarization in
hydrogen Lyman-alpha and the Mg II h and k spectral lines, the CLASP
and CLASP2 missions are vital first steps towards routine quantitative
characterization of the local thermal and magnetic conditions in the
solar chromosphere. In October of 2021, we re-flew the CLASP2 payload
with a modified observing program to further demonstrate the maturity
of the UV spectropolarimetry techniques, and readiness for development
into a satellite observatory. During the reflight, called CLASP2.1,
the spectrograph slit was scanned across an active region plage to
acquire a two-dimensional map of Stokes V/I, to demonstrate the ability
of UV spectropolarimetry to yield chromospheric magnetic fields over
a large area. This presentation will display preliminary results from
the flight of CLASP2.1.
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Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
2021arXiv211011885J Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present various types of discontinuities. The
radiative transfer is a typical example of such a problem. This
calls for high-order well-behaved techniques to interpolate both
smooth and discontinuous data. Aims. The final aim is to propose
new techniques suitable for applications in the context of numerical
radiative transfer. Methods. We have proposed and tested two different
techniques. Essentially non-oscillatory (ENO) techniques generate
several candidate interpolations based on different substencils. The
smoothest candidate interpolation is determined from a measure for
the local smoothness, thereby enabling the essential non-oscillatory
property. Weighted ENO (WENO) techniques use a convex combination of
all candidate substencils to obtain high-order accuracy in smooth
regions while keeping the essentially non-oscillatory property. In
particular, we have outlined and tested a novel well-performing
fourth-order WENO interpolation technique for both uniform and
nonuniform grids. Results. Numerical tests prove that the fourth-order
WENO interpolation guarantees fourth-order accuracy in smooth regions
of the interpolated functions. In the presence of discontinuities, the
fourth-order WENO interpolation enables the non-oscillatory property,
avoiding oscillations. Unlike Bézier and monotonic high-order Hermite
interpolations, it does not degenerate to a linear interpolation near
smooth extrema of the interpolated function. Conclusions. The novel
fourth-order WENO interpolation guarantees high accuracy in smooth
regions, while effectively handling discontinuities. This interpolation
technique might be particularly suitable for several problems, including
a number of radiative transfer applications such as multidimensional
problems, multigrid methods, and formal solutions.
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Title: Solving the Paradox of the Solar Sodium D<SUB>1</SUB> Line
Polarization
Authors: Alsina Ballester, Ernest; Belluzzi, Luca; Trujillo Bueno,
Javier
2021PhRvL.127h1101A Altcode: 2021arXiv210808334A
Twenty-five years ago, enigmatic linear polarization signals were
discovered in the core of the sodium D<SUB>1</SUB> line. The only
explanation that could be found implied that the solar chromosphere is
practically unmagnetized, in contradiction with other evidences. This
opened a paradox that has challenged physicists for many years. Here
we present its solution, demonstrating that these polarization signals
can be properly explained in the presence of magnetic fields in the
gauss range. This result opens a novel diagnostic window for exploring
the elusive magnetism of the solar chromosphere.
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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.
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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.
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Title: Fast and accurate approximation of the angle-averaged
redistribution function for polarized radiation
Authors: Paganini, A.; Hashemi, B.; Alsina Ballester, E.; Belluzzi, L.
2021A&A...645A...4P Altcode: 2020arXiv201003508P
Context. Modeling spectral line profiles taking frequency redistribution
effects into account is a notoriously challenging problem from the
computational point of view, especially when polarization phenomena
(atomic polarization and polarized radiation) are taken into
account. Frequency redistribution effects are conveniently described
through the redistribution function formalism, and the angle-averaged
approximation is often introduced to simplify the problem. Even in
this case, the evaluation of the emission coefficient for polarized
radiation remains computationally costly, especially when magnetic
fields are present or complex atomic models are considered. <BR /> Aims:
We aim to develop an efficient algorithm to numerically evaluate the
angle-averaged redistribution function for polarized radiation. <BR />
Methods: The proposed approach is based on a low-rank approximation
via trivariate polynomials whose univariate components are represented
in the Chebyshev basis. <BR /> Results: The resulting algorithm is
significantly faster than standard quadrature-based schemes for any
target accuracy in the range [10<SUP>-6</SUP>, 10<SUP>-2</SUP>].
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Title: Magnetic Sensitivity in the Wing Scattering Polarization
Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2019ApJ...880...85A Altcode: 2019arXiv190110994A
The linear polarization produced by scattering processes in the hydrogen
Lyα line of the solar disk radiation is a key observable for probing
the chromosphere-corona transition region (TR) and the underlying
chromospheric plasma. While the line-center signal encodes information
on the magnetic field and the three-dimensional structure of the TR,
the sizable scattering polarization signals that the joint action of
partial frequency redistribution and J-state interference produce in
the Lyα wings have generally been thought to be sensitive only to the
thermal structure of the solar atmosphere. Here we show that the wings
of the Q/I and U/I scattering polarization profiles of this line are
actually sensitive to the presence of chromospheric magnetic fields,
with strengths similar to those that produce the Hanle effect in the
line core (i.e., between 5 and 100 G, approximately). In spite of the
fact that the Zeeman splitting induced by such weak fields is very
small compared to the total width of the line, the magneto-optical
effects that couple the transfer equations for Stokes Q and U are
actually able to produce sizable changes in the Q/I and U/I wings. We
find that magnetic fields with longitudinal components larger than 100
G produce an almost complete depolarization of the wings of the Lyα
Q/I profiles within a ±5 Å spectral range around the line center,
while stronger fields are required for the U/I wing signals to be
depolarized to a similar extent. The theoretical results presented
here further expand the diagnostic content of the unprecedented
spectropolarimetric observations provided by the Chromospheric
Lyman-Alpha Spectro-Polarimeter.
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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.
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Title: The Transfer of Resonance Line Polarization with PRD in the
General Hanle-Zeeman Regime
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2019ASPC..526..119A Altcode:
We present numerical radiative transfer calculations of the four
Stokes parameters of the radiation emerging from one-dimensional model
atmospheres. In this investigation we account for the impact of partial
frequency redistribution (PRD) in scattering and the joint action of
the Hanle and Zeeman effects. Strong resonance lines of interest for
chromospheric magnetic field diagnostics have been considered, namely
the Ca I line at 4227 Å and the Mg II k line at 2795 Å. The Stokes
profiles of these lines have been obtained by considering two-level
atomic models, both in the absence and in the presence of magnetic
fields. We draw attention to the fact that the magneto-optical
terms of the transfer equations for Stokes Q and U are responsible
for an interesting and previously unnoticed magnetic sensitivity of
their scattering polarization profiles beyond the Doppler core. This
important discovery contributes to paint a more detailed picture of the
influence of relatively weak magnetic fields on the observable linear
polarization signals of strong chromospheric lines, highlighting the
importance of a PRD treatment for such lines.
---------------------------------------------------------
Title: A novel fourth-order WENO interpolation technique. A possible
new tool designed for radiative transfer
Authors: Janett, Gioele; Steiner, Oskar; Alsina Ballester, Ernest;
Belluzzi, Luca; Mishra, Siddhartha
2019A&A...624A.104J Altcode:
Context. Several numerical problems require the interpolation of
discrete data that present at the same time (i) complex smooth
structures and (ii) various types of discontinuities. The radiative
transfer in solar and stellar atmospheres is a typical example of such
a problem. This calls for high-order well-behaved techniques that are
able to interpolate both smooth and discontinuous data. <BR /> Aims:
This article expands on different nonlinear interpolation techniques
capable of guaranteeing high-order accuracy and handling discontinuities
in an accurate and non-oscillatory fashion. The final aim is to propose
new techniques which could be suitable for applications in the context
of numerical radiative transfer. <BR /> Methods: We have proposed
and tested two different techniques. Essentially non-oscillatory
(ENO) techniques generate several candidate interpolations based
on different substencils. The smoothest candidate interpolation is
determined from a measure for the local smoothness, thereby enabling the
essentially non-oscillatory property. Weighted ENO (WENO) techniques
use a convex combination of all candidate substencils to obtain
high-order accuracy in smooth regions while keeping the essentially
non-oscillatory property. In particular, we have outlined and tested a
novel well-performing fourth-order WENO interpolation technique for both
uniform and nonuniform grids. <BR /> Results: Numerical tests prove that
the fourth-order WENO interpolation guarantees fourth-order accuracy
in smooth regions of the interpolated functions. In the presence
of discontinuities, the fourth-order WENO interpolation enables the
non-oscillatory property, avoiding oscillations. Unlike Bézier and
monotonic high-order Hermite interpolations, it does not degenerate
to a linear interpolation near smooth extrema of the interpolated
function. Conclusion. The novel fourth-order WENO interpolation
guarantees high accuracy in smooth regions, while effectively handling
discontinuities. This interpolation technique might be particularly
suitable for several problems, including a number of radiative transfer
applications such as multidimensional problems, multigrid methods,
and formal solutions.
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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.
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Title: Magneto-optical Effects in the Scattering Polarization Wings
of the Ca I 4227 Å Resonance Line
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2018ApJ...854..150A Altcode: 2017arXiv171100372A; 2017arXiv171100372B
The linear polarization pattern produced by scattering processes in
the Ca I 4227 Å resonance line is a valuable observable for probing
the solar atmosphere. Via the Hanle effect, the very significant Q/I
and U/I line-center signals are sensitive to the presence of magnetic
fields in the lower chromosphere with strengths between 5 and 125 G,
approximately. On the other hand, partial frequency redistribution
(PRD) produces sizable signals in the wings of the Q/I profile,
which have always been thought to be insensitive to the presence
of magnetic fields. Interestingly, novel observations of this line
revealed a surprising behavior: fully unexpected signals in the
wings of the U/I profile and spatial variability in the wings of
both Q/I and U/I. We show that the magneto-optical (MO) terms of
the Stokes-vector transfer equation produce sizable signals in the
wings of U/I and a clear sensitivity of the Q/I and U/I wings to the
presence of photospheric magnetic fields with strengths similar to
those that produce the Hanle effect in the line core. This radiative
transfer investigation on the joint action of scattering processes and
the Hanle and Zeeman effects in the Ca I 4227 Å line should facilitate
the development of more reliable techniques for exploring the magnetism
of stellar atmospheres. To this end, we can now exploit the circular
polarization produced by the Zeeman effect, the magnetic sensitivity
caused by the above-mentioned MO effects in the Q/I and U/I wings,
and the Hanle effect in the line core.
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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: The Transfer of Resonance Line Polarization with Partial
Frequency Redistribution in the General Hanle-Zeeman Regime
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2017ApJ...836....6A Altcode: 2016arXiv160905723B; 2016arXiv160905723A; 2017ApJ...836....6B
The spectral line polarization encodes a wealth of information about the
thermal and magnetic properties of the solar atmosphere. Modeling the
Stokes profiles of strong resonance lines is, however, a complex problem
both from a theoretical and computational point of view, especially when
partial frequency redistribution (PRD) effects need to be taken into
account. In this work, we consider a two-level atom in the presence
of magnetic fields of arbitrary intensity (Hanle-Zeeman regime) and
orientation, both deterministic and micro-structured. Working within the
framework of a rigorous PRD theoretical approach, we have developed
a numerical code that solves the full non-LTE radiative transfer
problem for polarized radiation, in one-dimensional models of the
solar atmosphere, accounting for the combined action of the Hanle and
Zeeman effects, as well as for PRD phenomena. After briefly discussing
the relevant equations, we describe the iterative method of solution
of the problem and the numerical tools that we have developed and
implemented. We finally present some illustrative applications to two
resonance lines that form at different heights in the solar atmosphere,
and provide a detailed physical interpretation of the calculated Stokes
profiles. We find that magneto-optical effects have a strong impact on
the linear polarization signals that PRD effects produce in the wings of
strong resonance lines. We also show that the weak-field approximation
has to be used with caution when PRD effects are considered.
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Title: The Magnetic Sensitivity of the Mg II k Line to the Joint
Action of Hanle, Zeeman, and Magneto-optical Effects
Authors: Alsina Ballester, E.; Belluzzi, L.; Trujillo Bueno, J.
2016ApJ...831L..15A Altcode: 2016arXiv161000649A
We highlight the main results of a radiative transfer investigation on
the magnetic sensitivity of the solar Mg II k resonance line at 2795.5
Å, accounting for the joint action of the Hanle and Zeeman effects
as well as partial frequency redistribution phenomena. We confirm
that at the line center, the linear polarization signals produced by
scattering processes are measurable, and that they are sensitive, via
the Hanle effect, to magnetic fields with strengths between 5 and 50 G,
approximately. We also show that the Zeeman effect produces conspicuous
circular polarization signals, especially for longitudinal fields
stronger than 50 G, which can be used to estimate the magnetization of
the solar chromosphere via the familiar magnetograph formula. The most
novel result is that magneto-optical effects produce, in the wings of
the line, a decrease of the Q/I scattering polarization pattern and
the appearance of U/I signals (I.e., a rotation of the plane of linear
polarization). This sensitivity of the Q/I and U/I wing signals to
both weak (∼5 G) and stronger magnetic fields expands the scientific
interest of the Mg II k line for probing the chromosphere in quiet
and active regions of the Sun.
