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Author name code: borrero
ADS astronomy entries on 2022-09-14
author:"Borrero, Juan Manuel"
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Title: Inference of electric currents in the solar photosphere
Authors: Pastor Yabar, A.; Borrero, J. M.; Quintero Noda, C.; Ruiz
Cobo, B.
2021A&A...656L..20P Altcode: 2021arXiv211204356P
Context. Despite their importance, routine and direct measurements
of electric currents, j, in the solar atmosphere have generally not
been possible. <BR /> Aims: We aim at demonstrating the capabilities
of a newly developed method for determining electric currents in
the solar photosphere. <BR /> Methods: We employ three-dimensional
radiative magneto-hydrodynamic (MHD) simulations to produce synthetic
Stokes profiles in several spectral lines with a spatial resolution
similar to what the newly operational 4-meter Daniel K. Inouye Solar
Telescope solar telescope should achieve. We apply a newly developed
inversion method of the polarized radiative transfer equation with
magneto-hydrostatic (MHS) constraints to infer the magnetic field
vector in the three-dimensional Cartesian domain, B(x, y, z), from the
synthetic Stokes profiles. We then apply Ampere's law to determine
the electric currents, j, from the inferred magnetic field, B(x, y,
z), and compare the results with the electric currents present in
the original MHD simulation. <BR /> Results: We show that the method
employed here is able to attain reasonable reliability (close to 50%
of the cases are within a factor of two, and this increases to 60%-70%
for pixels with B ≥ 300 G) in the inference of electric currents for
low atmospheric heights (optical depths at 500 nm τ<SUB>5</SUB>∈[1,
0.1]) regardless of whether a small or large number of spectral lines
are inverted. Above these photospheric layers, the method's accuracy
strongly deteriorates as magnetic fields become weaker and as the MHS
approximation becomes less accurate. We also find that the inferred
electric currents have a floor value that is related to low-magnetized
plasma, where the uncertainty in the magnetic field inference prevents
a sufficiently accurate determination of the spatial derivatives. <BR />
Conclusions: We present a method that allows the inference of the three
components of the electric current vector at deep atmospheric layers
(photospheric layers) from spectropolarimetric observations.
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Title: Combining magneto-hydrostatic constraints with Stokes profiles
inversions. II. Application to Hinode/SP observations
Authors: Borrero, J. M.; Pastor Yabar, A.; Ruiz Cobo, B.
2021A&A...647A.190B Altcode: 2021arXiv210104394B
Context. Inversion techniques applied to the radiative transfer equation
for polarized light are capable of inferring the physical parameters in
the solar atmosphere (temperature T, magnetic field B, and line-of-sight
velocity v<SUB>los</SUB>) from observations of the Stokes vector
(i.e., spectropolarimetric observations) in spectral lines. Inferences
are usually performed in the (x, y, τ<SUB>c</SUB>) domain, where
τ<SUB>c</SUB> refers to the optical-depth scale. Generally, their
determination in the (x, y, z) volume is not possible due to the lack
of a reliable estimation of the gas pressure, particularly in regions
of the solar surface harboring strong magnetic fields. <BR /> Aims:
We aim to develop a new inversion code capable of reliably inferring
the physical parameters in the (x, y, z) domain. <BR /> Methods: We
combine, in a self-consistent way, an inverse solver for the radiative
transfer equation (Firtez-DZ) with a solver for the magneto-hydrostatic
equilibrium, which derives realistic values of the gas pressure
by taking the magnetic pressure and tension into account. <BR />
Results: We test the correct behavior of the newly developed code with
spectropolarimetric observations of two sunspots recorded with the
spectropolarimeter (SP) instrument on board the Hinode spacecraft,
and we show how the physical parameters are inferred in the (x, y,
z) domain, with the Wilson depression of the sunspots arising as a
natural consequence of the force balance. In particular, our approach
significantly improves upon previous determinations that were based
on semiempirical models. <BR /> Conclusions: Our results open the door
for the possibility of calculating reliable electric currents in three
dimensions, j(x, y, z), in the solar photosphere. Further consistency
checks would include a comparison with other methods that have recently
been proposed and which achieve similar goals.
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Title: Interaction of Magnetic Fields with a Vortex Tube at Solar
Subgranular Scale
Authors: Fischer, C. E.; Vigeesh, G.; Lindner, P.; Borrero, J. M.;
Calvo, F.; Steiner, O.
2020ApJ...903L..10F Altcode: 2020arXiv201005577F
Using high-resolution spectropolarimetric data recorded with the
Swedish 1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be the
observational signature of underlying tubes of vortical flow with
their axis oriented parallel to the solar surface. Associated with
these horizontal vortex tubes, we detect in some cases a significant
signal in linear polarization, located at the trailing dark edge of
the granular lane. The linear polarization appears at a later stage of
the granular lane development, and is flanked by patches of circular
polarization. Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field aligned
with the granular lane. We analyze snapshots of a magnetohydrodynamic
numerical simulation and find cases in which the horizontal vortex
tube of the granular lane redistributes and transports the magnetic
field to the solar surface causing a polarimetric signature similar to
what is observed. We thus witness a mechanism capable of transporting
magnetic flux to the solar surface within granules. This mechanism is
probably an important component of the small-scale dynamo supposedly
acting at the solar surface and generating the quiet-Sun magnetic field.
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Title: Evolution of Stokes V area asymmetry related to a quiet Sun
cancellation observed with GRIS/IFU
Authors: Kaithakkal, A. J.; Borrero, J. M.; Fischer, C. E.;
Dominguez-Tagle, C.; Collados, M.
2020A&A...634A.131K Altcode: 2020arXiv200105465K
A quiet Sun magnetic flux cancellation event at the disk center was
recorded using the Integral Field Unit (IFU) mounted on the GREGOR
Infrared Spectrograph (GRIS). The GRIS instrument sampled the event
in the photospheric Si I 10827 Å spectral line. The cancellation was
preceded by a significant rise in line core intensity and excitation
temperature, which is inferred from Stokes inversions under local
thermodynamic equilibrium (LTE). The opposite polarity features
seem to undergo reconnection above the photosphere. We also found
that the border pixels neighboring the polarity inversion line
of one of the polarities exhibit a systematic variation of area
asymmetry. Area asymmetry peaks right after the line core intensity
enhancement and gradually declines thereafter. Analyzing Stokes
profiles recorded from either side of the polarity inversion line
could therefore potentially provide additional information on the
reconnection process related to magnetic flux cancellation. Further
analysis without assuming LTE will be required to fully characterize
this event. <P />Movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936941/olm">https://www.aanda.org</A>
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Title: Combining magnetohydrostatic constraints with Stokes profiles
inversions. I. Role of boundary conditions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
2019A&A...632A.111B Altcode:
Context. Inversion codes for the polarized radiative transfer
equation, when applied to spectropolarimetric observations (i.e.,
Stokes vector) in spectral lines, can be used to infer the temperature
T, line-of-sight velocity v<SUB>los</SUB>, and magnetic field B as
a function of the continuum optical-depth τ<SUB>c</SUB>. However,
they do not directly provide the gas pressure P<SUB>g</SUB> or density
ρ. In order to obtain these latter parameters, inversion codes rely
instead on the assumption of hydrostatic equilibrium (HE) in addition
to the equation of state (EOS). Unfortunately, the assumption of HE is
rather unrealistic across magnetic field lines, causing estimations
of P<SUB>g</SUB> and ρ to be unreliable. This is because the role
of the Lorentz force, among other factors, is neglected. Unreliable
gas pressure and density also translate into an inaccurate conversion
from optical depth τ<SUB>c</SUB> to geometrical height z. <BR /> Aims:
We aim at improving the determination of the gas pressure and density
via the application of magnetohydrostatic (MHS) equilibrium instead of
HE. <BR /> Methods: We develop a method to solve the momentum equation
under MHS equilibrium (i.e., taking the Lorentz force into account)
in three dimensions. The method is based on the iterative solution of
a Poisson-like equation. Considering the gas pressure P<SUB>g</SUB>
and density ρ from three-dimensional magnetohydrodynamic (MHD)
simulations of sunspots as a benchmark, we compare the results from the
application of HE and MHS equilibrium using boundary conditions with
different degrees of realism. Employing boundary conditions that can
be applied to actual observations, we find that HE retrieves the gas
pressure and density with an error smaller than one order of magnitude
(compared to the MHD values) in only about 47% of the grid points in
the three-dimensional domain. Moreover, the inferred values are within
a factor of two of the MHD values in only about 23% of the domain. This
translates into an error of about 160 - 200 km in the determination of
the z - τ<SUB>c</SUB> conversion (i.e., Wilson depression). On the
other hand, the application of MHS equilibrium with similar boundary
conditions allows determination of P<SUB>g</SUB> and ρ with an error
smaller than an order of magnitude in 84% of the domain. The inferred
values are within a factor of two in more than 55% of the domain. In
this latter case, the z - τ<SUB>c</SUB> conversion is obtained with an
accuracy of 30 - 70 km. Inaccuracies are due in equal part to deviations
from MHS equilibrium and to inaccuracies in the boundary conditions. <BR
/> Results: Compared to HE, our new method, based on MHS equilibrium,
significantly improves the reliability in the determination of the
density, gas pressure, and conversion between geometrical height z and
continuum optical depth τ<SUB>c</SUB>. This method could be used in
conjunction with the inversion of the radiative transfer equation for
polarized light in order to determine the thermodynamic, kinematic,
and magnetic parameters of the solar atmosphere.
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Title: Combining magneto-hydrostatic constraints with Stokes profiles
inversions
Authors: Borrero, J. M.; Pastor Yabar, A.; Rempel, M.; Ruiz Cobo, B.
2019arXiv191014131B Altcode:
Inversion codes for the polarized radiative transfer equation can
be used to infer the temperature $T$, line-of-sight velocity $v_{\rm
los}$, and magnetic field $\rm{\bf B}$ as a function of the continuum
optical-depth $\tau_{\rm c}$. However, they do not directly provide
the gas pressure $P_{\rm g}$ or density $\rho$. In order to obtain
these latter parameters, inversion codes rely instead on the assumption
of hydrostatic equilibrium (HE) in addition to the equation of state
(EOS). Unfortunately, the assumption of HE is rather unrealistic across
magnetic field lines. This is because the role of the Lorentz force,
among other factors, is neglected. This translates into an inaccurate
conversion from optical depth $\tau_{\rm c}$ to geometrical height
$z$. We aim at improving this conversion via the application of
magneto-hydrostatic (MHS) equilibrium instead of HE. We develop a
method to solve the momentum equation under MHS equilibrium (i.e.,
taking the Lorentz force into account) in three dimensions. The method
is based on the solution of a Poisson-like equation. Considering the
gas pressure $P_{\rm g}$ and density $\rho$ from three-dimensional
magneto-hydrodynamic (MHD) simulations of sunspots as a benchmark, we
compare the results from the application of HE and MHS equilibrium. We
find that HE retrieves the gas pressure and density within an order
of magnitude of the MHD values in only about 47 \% of the domain. This
translates into an error of about $160-200$ km in the determination of
the $z-\tau_{\rm c}$ conversion. On the other hand, the application of
MHS equilibrium allows determination of $P_{\rm g}$ and $\rho$ within
an order of magnitude in 84 \% of the domain. In this latter case, the
$z-\tau_{\rm c}$ conversion is obtained with an accuracy of $30-70$ km.
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Title: FIRTEZ-dz. A forward and inverse solver of the polarized
radiative transfer equation under Zeeman regime in geometrical scale
Authors: Pastor Yabar, A.; Borrero, J. M.; Ruiz Cobo, B.
2019A&A...629A..24P Altcode: 2019arXiv190808075P
We present a numerical code that solves the forward and inverse problem
of the polarized radiative transfer equation in geometrical scale under
the Zeeman regime. The code is fully parallelized, making it able to
easily handle large observational and simulated datasets. We checked
the reliability of the forward and inverse modules through different
examples. In particular, we show that even when properly inferring
various physical parameters (temperature, magnetic field components,
and line-of-sight velocity) in optical depth, their reliability in
height-scale depends on the accuracy with which the gas-pressure or
density are known. The code is made publicly available as a tool
to solve the radiative transfer equation and perform the inverse
solution treating each pixel independently. An important feature of
this code, that will be exploited in the future, is that working
in geometrical-scale allows for the direct calculation of spatial
derivatives, which are usually required in order to estimate the gas
pressure and/or density via the momentum equation in a three-dimensional
volume, in particular the three-dimensional Lorenz force.
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Title: Observations of solar small-scale magnetic flux-sheet emergence
Authors: Fischer, C. E.; Borrero, J. M.; Bello González, N.;
Kaithakkal, A. J.
2019A&A...622L..12F Altcode: 2019arXiv190105870F
<BR /> Aims: Two types of flux emergence were recently discovered
in numerical simulations: magnetic loops and magnetic sheet
emergence. While magnetic loop emergence has been documented well in
recent years using high-resolution full Stokes data from ground-based
telescopes as well as satellites, magnetic sheet emergence is
still an understudied process. We report here on the first clear
observational evidence of a magnetic sheet emergence and characterise
its development. <BR /> Methods: Full Stokes spectra from the Hinode
spectropolarimeter were inverted with the Stokes Inversion based on
Response functions (SIR) code to obtain solar atmospheric parameters
such as temperature, line-of-sight velocities, and full magnetic
field vector information. <BR /> Results: We analyse a magnetic
flux emergence event observed in the quiet-Sun internetwork. After a
large-scale appearance of linear polarisation, a magnetic sheet with
horizontal magnetic flux density of up to 194 Mx cm<SUP>-2</SUP>
hovers in the low photosphere spanning a region of 2-3 arcsec. The
magnetic field azimuth obtained through Stokes inversions clearly
shows an organised structure of transversal magnetic flux density
emerging. The granule below the magnetic flux sheet tears the
structure apart leaving the emerged flux to form several magnetic
loops at the edges of the granule. <BR /> Conclusions: A large amount
of flux with strong horizontal magnetic fields surfaces through
the interplay of buried magnetic flux and convective motions. The
magnetic flux emerges within 10 minutes and we find a longitudinal
magnetic flux at the foot points of the order of ∼10<SUP>18</SUP>
Mx. This is one to two orders of magnitude larger than what has been
reported for small-scale magnetic loops. The convective flows feed
the newly emerged flux into the pre-existing magnetic population on
a granular scale. <P />Movie attached to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834628/olm">https://www.aanda.org</A>
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Title: Three-lobed near-infrared Stokes V profiles in the quiet Sun
Authors: Kiess, Christoph; Borrero, Juan Manuel; Schmidt, Wolgang
2018A&A...616A.109K Altcode: 2018arXiv180309668K
Context. The 1.5-m GREGOR solar telescope can resolve structures as
small as 0.4” at near-infrared wavelengths on the Sun. At this spatial
resolution the polarized solar spectrum shows complex patterns, such as
large horizontal and/or vertical variations of the physical parameters
in the solar photosphere. <BR /> Aims: We investigate a region of the
quiet solar photosphere exhibiting three-lobed Stokes V profiles in
the Fe I spectral line at 15 648 Å. The data were acquired with the
GRIS spectropolarimeter attached to the GREGOR telescope. We aim at
investigating the thermal, kinematic and magnetic properties of the
atmosphere responsible for these measured complex signals. <BR />
Methods: The SIR inversion code is employed to retrieve the physical
parameters of the lower solar photosphere from the observed polarization
signals. We follow two different approaches. On the one hand, we
consider that the multi-lobe circular polarization signals are only
produced by the line-of-sight variation of the physical parameters. We
therefore invert the data assuming a single atmospheric component
that occupies the entire resolution element in the horizontal plane
and where the physical parameters vary with optical depth τ (i.e.,
line-of-sight). On the other hand, we consider that the multi-lobe
circular polarization signals are produced not by the optical depth
variations of the physical parameters but instead by their horizontal
variations. Here we invert the data assuming that the resolution element
is occupied by two different atmospheric components where the kinematic
and magnetic properties are constant along the line-of-sight. <BR
/> Results: Both approaches reveal some common features about the
topology responsible for the observed three-lobed Stokes V signals:
both a strong (>1000 Gauss) and a very weak (<10 Gauss) magnetic
field with opposite polarities and harboring flows directed in opposite
directions must co-exist (either vertically or horizontally interlaced)
within the resolution element. Conclusions.
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Title: Solar Magnetoconvection and Small-Scale Dynamo
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
2018smf..book..275B Altcode:
No abstract at ADS
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Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent
Developments in Observation and Simulation
Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K.
2017SSRv..210..275B Altcode: 2015SSRv..tmp..113B; 2015arXiv151104214B
A number of observational and theoretical aspects of solar
magnetoconvection are considered in this review. We discuss recent
developments in our understanding of the small-scale structure of
the magnetic field on the solar surface and its interaction with
convective flows, which is at the centre of current research. Topics
range from plage areas in active regions over the magnetic network
shaped by supergranulation to the ubiquituous `turbulent' internetwork
fields. On the theoretical side, we focus upon magnetic field generation
by small-scale dynamo action.
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Title: Penumbral thermal structure below the visible surface
Authors: Borrero, J. M.; Franz, M.; Schlichenmaier, R.; Collados,
M.; Asensio Ramos, A.
2017A&A...601L...8B Altcode: 2017arXiv170502832B
Context. The thermal structure of the penumbra below its visible
surface (I.e., τ<SUB>5</SUB> ≥ 1) has important implications for
our present understanding of sunspots and their penumbrae: their
brightness and energy transport, mode conversion of magneto-acoustic
waves, sunspot seismology, and so forth. <BR /> Aims: We aim at
determining the thermal stratification in the layers immediately
beneath the visible surface of the penumbra: τ<SUB>5</SUB> ∈ [1,3]
(≈70-80 km below the visible continuum-forming layer) <BR /> Methods:
We analyzed spectropolarimetric data (I.e., Stokes profiles) in three Fe
I lines located at 1565 nm observed with the GRIS instrument attached
to the 1.5-m solar telescope GREGOR. The data are corrected for the
smearing effects of wide-angle scattered light and then subjected
to an inversion code for the radiative transfer equation in order
to retrieve, among others, the temperature as a function of optical
depth T(τ<SUB>5</SUB>). <BR /> Results: We find that the temperature
gradient below the visible surface of the penumbra is smaller than
in the quiet Sun. This implies that in the region τ<SUB>5</SUB> ≥
1 the penumbral temperature diverges from that of the quiet Sun. The
same result is obtained when focusing only on the thermal structure
below the surface of bright penumbral filaments. <BR /> Conclusions:
We interpret these results as evidence of a thick penumbra, whereby
the magnetopause is not located near its visible surface. In addition,
we find that the temperature gradient in bright penumbral filaments is
lower than in granules. This can be explained in terms of the limited
expansion of a hot upflow inside a penumbral filament relative to
a granular upflow, as magnetic pressure and tension forces from the
surrounding penumbral magnetic field hinder an expansion like this.
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Title: Are Internetwork Magnetic Fields in the Solar Photosphere
Horizontal or Vertical?
Authors: Lites, B. W.; Rempel, M.; Borrero, J. M.; Danilovic, S.
2017ApJ...835...14L Altcode:
Using many observations obtained during 2007 with the
Spectro-Polarimeter of the Hinode Solar Optical Telescope, we explore
the angular distribution of magnetic fields in the quiet internetwork
regions of the solar photosphere. Our work follows from the insight of
Stenflo, who examined only linear polarization signals in photospheric
lines, thereby avoiding complications of the analysis arising from the
differing responses to linear and circular polarization. We identify
and isolate regions of a strong polarization signal that occupy only
a few percent of the observed quiet Sun area yet contribute most to
the net linear polarization signal. The center-to-limb variation of
the orientation of linear polarization in these strong signal regions
indicates that the associated magnetic fields have a dominant vertical
orientation. In contrast, the great majority of the solar disk is
occupied by much weaker linear polarization signals. The orientation of
the linear polarization in these regions demonstrates that the field
orientation is dominantly horizontal throughout the photosphere. We
also apply our analysis to Stokes profiles synthesized from the
numerical MHD simulations of Rempel as viewed at various oblique
angles. The analysis of the synthetic data closely follows that of
the observations, lending confidence to using the simulations as a
guide for understanding the physical origins of the center-to-limb
variation of linear polarization in the quiet Sun area.
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Title: Deep probing of the photospheric sunspot penumbra: no evidence
of field-free gaps
Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.;
Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.;
Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe,
O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.;
Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.;
Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H.
2016A&A...596A...2B Altcode: 2016arXiv160708165B
Context. Some models for the topology of the magnetic field in
sunspot penumbrae predict regions free of magnetic fields or with
only dynamically weak fields in the deep photosphere. <BR /> Aims:
We aim to confirm or refute the existence of weak-field regions in
the deepest photospheric layers of the penumbra. <BR /> Methods:
We investigated the magnetic field at log τ<SUB>5</SUB> = 0 is
by inverting spectropolarimetric data of two different sunspots
located very close to disk center with a spatial resolution of
approximately 0.4-0.45”. The data have been recorded using the GRIS
instrument attached to the 1.5-m solar telescope GREGOR at the El
Teide observatory. The data include three Fe I lines around 1565 nm,
whose sensitivity to the magnetic field peaks half a pressure scale
height deeper than the sensitivity of the widely used Fe I spectral
line pair at 630 nm. Before the inversion, the data were corrected
for the effects of scattered light using a deconvolution method with
several point spread functions. <BR /> Results: At log τ<SUB>5</SUB>
= 0 we find no evidence of regions with dynamically weak (B<
500 Gauss) magnetic fields in sunspot penumbrae. This result is much
more reliable than previous investigations made on Fe I lines at 630
nm. Moreover, the result is independent of the number of nodes employed
in the inversion, is independent of the point spread function used to
deconvolve the data, and does not depend on the amount of stray light
(I.e., wide-angle scattered light) considered.
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Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
2016A&A...596A...4F Altcode: 2016arXiv160800513F
Context. A significant part of the penumbral magnetic field returns
below the surface in the very deep photosphere. For lines in the
visible, a large portion of this return field can only be detected
indirectly by studying its imprints on strongly asymmetric and
three-lobed Stokes V profiles. Infrared lines probe a narrow layer
in the very deep photosphere, providing the possibility of directly
measuring the orientation of magnetic fields close to the solar
surface. <BR /> Aims: We study the topology of the penumbral magnetic
field in the lower photosphere, focusing on regions where it returns
below the surface. <BR /> Methods: We analyzed 71 spectropolarimetric
datasets from Hinode and from the GREGOR infrared spectrograph. We
inferred the quality and polarimetric accuracy of the infrared data
after applying several reduction steps. Techniques of spectral
inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral
area covered by reversed-polarity and three-lobed Stokes V profiles for
sunspots at disk center. We determined the amount of reversed-polarity
and three-lobed Stokes V profiles in visible and infrared data of
sunspots at various heliocentric angles. From the results, we computed
center-to-limb variation curves, which were interpreted in the context
of existing penumbral models. <BR /> Results: Observations in visible
and near-infrared spectral lines yield a significant difference in the
penumbral area covered by magnetic fields of opposite polarity. In
the infrared, the number of reversed-polarity Stokes V profiles is
smaller by a factor of two than in the visible. For three-lobed Stokes
V profiles the numbers differ by up to an order of magnitude.
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Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
2016A&A...596A...6L Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector. <BR /> Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account. <BR /> Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM). <BR /> Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields. <BR /> Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.
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Title: Kinematics and Magnetic Properties of a Light Bridge in a
Decaying Sunspot
Authors: Falco, M.; Borrero, J. M.; Guglielmino, S. L.; Romano, P.;
Zuccarello, F.; Criscuoli, S.; Cristaldi, A.; Ermolli, I.; Jafarzadeh,
S.; Rouppe van der Voort, L.
2016SoPh..291.1939F Altcode: 2016arXiv160607229F; 2016SoPh..tmp..107F
We present the results obtained by analysing high spatial and spectral
resolution data of the solar photosphere acquired by the CRisp Imaging
SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011 of a
large sunspot with a light bridge (LB) observed in NOAA AR 11263. These
data are complemented by simultaneous Hinode Spectropolarimeter (SP)
observation in the Fe I 630.15 nm and 630.25 nm lines. The continuum
intensity map shows a discontinuity in the radial distribution of the
penumbral filaments in correspondence with the LB, which shows a dark
lane (≈0.3<SUP>″</SUP> wide and ≈8.0<SUP>″</SUP> long) along its
main axis. The available data were inverted with the Stokes Inversion
based on Response functions (SIR) code and physical parameters maps were
obtained. The line-of-sight (LOS) velocity of the plasma along the LB
derived from the Doppler effect shows motions towards and away from the
observer up to 0.6 kms−<SUP>1</SUP> that are lower in value than the
LOS velocities observed in the neighbouring penumbral filaments. The
noteworthy result is that we find motions towards the observer of up to
0.6 kms−<SUP>1</SUP> in the dark lane where the LB is located between
two umbral cores, while the LOS velocity motion towards the observer
is strongly reduced where the LB is located between an umbral core
at one side and penumbral filaments on the other side. Statistically,
the LOS velocities correspond to upflows or downflows, and comparing
these results with Hinode/SP data, we conclude that the surrounding
magnetic field configuration (whether more or less inclined) could have
a role in maintaining the conditions for the process of plasma pile-up
along the dark lane. The results obtained from our study support and
confirm outcomes of recent magneto-hydrodynamic simulations showing
upflows along the main axis of an LB.
---------------------------------------------------------
Title: Markov Properties of the Magnetic Field in the Quiet Solar
Photosphere
Authors: Gorobets, A. Y.; Borrero, J. M.; Berdyugina, S.
2016ApJ...825L..18G Altcode: 2016arXiv160500074G
The observed magnetic field on the solar surface is characterized by a
very complex spatial and temporal behavior. Although feature-tracking
algorithms have allowed us to deepen our understanding of this behavior,
subjectivity plays an important role in the identification and tracking
of such features. In this paper, we study the temporal stochasticity of
the magnetic field on the solar surface without relying on either the
concept of magnetic feature or on the subjective assumptions about their
identification and interaction. The analysis is applied to observations
of the magnetic field of the quiet solar photosphere carried out with
the Imaging Magnetograph eXperiment (IMaX) instrument on board the
stratospheric balloon, Sunrise. We show that the joint probability
distribution functions of the longitudinal ({B}<SUB>\parallel </SUB>)
and transverse ({B}<SUB>\perp </SUB>) components of the magnetic field,
as well as of the magnetic pressure ({B}<SUP>2</SUP>={B}<SUB>\perp
</SUB><SUP>2</SUP>+{B}<SUB>\parallel </SUB><SUP>2</SUP>), verify the
necessary and sufficient condition for the Markov chains. Therefore,
we establish that the magnetic field as seen by IMaX with a resolution
of 0.″15-0.″18 and 33 s cadence, which can be considered as a
memoryless temporal fluctuating quantity.
---------------------------------------------------------
Title: Magneto-static Modeling of the Mixed Plasma Beta Solar
Atmosphere Based on Sunrise/IMaX Data
Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki,
S. K.; Martínez Pillet, V.; Borrero, J. M.
2015ApJ...815...10W Altcode: 2015arXiv151105568W
Our aim is to model the three-dimensional magnetic field structure
of the upper solar atmosphere, including regions of non-negligible
plasma beta. We use high-resolution photospheric magnetic field
measurements from SUNRISE/IMaX as the boundary condition for a
magneto-static magnetic field model. The high resolution of IMaX
allows us to resolve the interface region between the photosphere
and corona, but modeling this region is challenging for the following
reasons. While the coronal magnetic field is thought to be force-free
(the Lorentz force vanishes), this is not the case in the mixed plasma
β environment in the photosphere and lower chromosphere. In our model,
pressure gradients and gravity forces are self-consistently taken into
account and compensate for the non-vanishing Lorentz force. Above a
certain height (about 2 Mm) the non-magnetic forces become very weak
and consequently the magnetic field becomes almost force-free. Here,
we apply a linear approach where the electric current density consists
of a superposition of a field-line parallel current and a current
perpendicular to the Sun's gravity field. We illustrate the prospects
and limitations of this approach and give an outlook for an extension
toward a nonlinear model.
---------------------------------------------------------
Title: Variation in sunspot properties between 1999 and 2014
Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt,
W.; Collados, M.
2015A&A...578A..43R Altcode:
<BR /> Aims: We study the variation in the magnetic field strength,
area, and continuum intensity of umbrae in solar cycles 23 and 24. <BR
/> Methods: We analyzed a sample of 374 sunspots observed from 1999
until 2014 with the Tenerife Infrared Polarimeter at the German
Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter
at the Dunn Solar Telescope. The sample of field strength, area,
and intensities was used to trace any long-term or cyclic trend of
umbral properties in the last 15 years. <BR /> Results: Sunspots
are systematically weaker, that is, have a weaker field strength and
stronger continuum intensity, toward the end of cycle 23 than they
had at the maximum of cycle 23. The linear trend reverses with the
onset of cycle 24. We find that the field strength decreases in the
declining phase of cycle 23 by about 112 (± 16) G yr<SUP>-1</SUP>,
while it increases in the rising phase of cycle 24 by about 138 (±
72) G yr<SUP>-1</SUP>. The umbral intensity shows the opposite trend:
the intensity increases with a rate of 0.7 (± 0.3)% of I<SUB>c</SUB>
yr<SUP>-1</SUP> toward the end of cycle 23 and decreases with a rate
of 3.8 (± 1.5)% of I<SUB>c</SUB> yr<SUP>-1</SUP> toward the maximum of
cycle 24. The distribution of the umbral maximum field strength in cycle
24 is similar to that of cycle 23, but is slightly shifted toward lower
values by about 80 G, corresponding to a possible long-term gradient in
umbral field strength of about 7 ± 4 G yr<SUP>-1</SUP>. If instead of
the maximum umbral field we consider the average value over the entire
umbra, the distribution shifts by about 44 Gauss. <BR /> Conclusions:
The umbral brightness decreases in the rising stage of a solar cycle,
but increases from maximum toward the end of the cycle. Our results
do not indicate a drastic change of the solar cycle toward a grand
minimum in the near future.
---------------------------------------------------------
Title: Comparison of inversion codes for polarized line formation
in MHD simulations. I. Milne-Eddington codes
Authors: Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M.
2014A&A...572A..54B Altcode: 2014arXiv1409.3376B
Milne-Eddington (M-E) inversion codes for the radiative transfer
equation are the most widely used tools to infer the magnetic field
from observations of the polarization signals in photospheric and
chromospheric spectral lines. Unfortunately, a comprehensive comparison
between the different M-E codes available to the solar physics
community is still missing, and so is a physical interpretation of their
inferences. In this contribution we offer a comparison between three
of those codes (VFISV, ASP/HAO, and HeLIx<SUP>+</SUP>). These codes are
used to invert synthetic Stokes profiles that were previously obtained
from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD)
simulations. The results of the inversion are compared with each other
and with those from the MHD simulations. In the first case, the M-E
codes retrieve values for the magnetic field strength, inclination and
line-of-sight velocity that agree with each other within σ<SUB>B</SUB>
≤ 35 (Gauss), σ<SUB>γ</SUB> ≤ 1.2°, and σ<SUB>v</SUB> ≤
10 m s<SUP>-1</SUP>, respectively. Additionally, M-E inversion codes
agree with the numerical simulations, when compared at a fixed optical
depth, within σ<SUB>B</SUB> ≤ 130 (Gauss), σ<SUB>γ</SUB> ≤ 5°,
and σ<SUB>v</SUB> ≤ 320 m s<SUP>-1</SUP>. Finally, we show that
employing generalized response functions to determine the height at
which M-E codes measure physical parameters is more meaningful than
comparing at a fixed geometrical height or optical depth. In this case
the differences between M-E inferences and the 3D MHD simulations
decrease to σ<SUB>B</SUB> ≤ 90 (Gauss), σ<SUB>γ</SUB> ≤ 3°,
and σ<SUB>v</SUB> ≤ 90 m s<SUP>-1</SUP>.
---------------------------------------------------------
Title: High speed magnetized flows in the quiet Sun
Authors: Quintero Noda, C.; Borrero, J. M.; Orozco Suárez, D.;
Ruiz Cobo, B.
2014A&A...569A..73Q Altcode: 2014arXiv1407.7477Q
Context. We analyzed spectropolarimetric data recorded with
Hinode/SP in quiet-Sun regions located at the disk center. We found
single-lobed Stokes V profiles showing highly blue- and red-shifted
signals. Oftentimes both types of events appear to be related to
each other. <BR /> Aims: We aim to set constraints on the nature and
physical causes of these highly Doppler-shifted signals, as well as to
study their spatial distribution, spectropolarimetric properties, size,
and rate of occurrence. Also, we plan to retrieve the variation of the
physical parameters with optical depth through the photosphere. <BR />
Methods: We have examined the spatial and polarimetric properties of
these events using a variety of data from the Hinode spacecraft. We
have also inferred the atmospheric stratification of the physical
parameters by means of the inversion of the observed Stokes profiles
employing the Stokes Inversion based on Response functions (SIR)
code. Finally, we analyzed their evolution using a time series from
the same instrument. <BR /> Results: Blue-shifted events tend to appear
over bright regions at the edge of granules, while red-shifted events
are seen predominantly over dark regions on intergranular lanes. Large
linear polarization signals can be seen in the region that connects
them. The magnetic structure inferred from the time series revealed that
the structure corresponds to a Ω-loop, with one footpoint always over
the edge of a granule and the other inside an intergranular lane. The
physical parameters obtained from the inversions of the observed
Stokes profiles in both events show an increase with respect to the
Harvard-Smithonian reference atmosphere in the temperature at log
τ<SUB>500</SUB> ∈ (-1, -3) and a strong magnetic field, B ≥ 1 kG,
at the bottom of the atmosphere that quickly decreases upward until
vanishing at log τ<SUB>500</SUB> ≈ -2. In the blue-shifted events,
the LOS velocities change from upflows at the bottom to downflows at
the top of the atmosphere. Red-shifted events display the opposite
velocity stratification. The change of sign in LOS velocity happens at
the same optical depth in which the magnetic field becomes zero. <BR
/> Conclusions: The physical mechanism that best explains the inferred
magnetic field configuration and flow motions is a siphon flow along an
arched magnetic flux tube. Further investigation is required, however,
as the expected features of a siphon flow cannot be unequivocally
identified.
---------------------------------------------------------
Title: Temporal relation between quiet-Sun transverse fields and
the strong flows detected by IMaX/SUNRISE
Authors: Quintero Noda, C.; Martínez Pillet, V.; Borrero, J. M.;
Solanki, S. K.
2013A&A...558A..30Q Altcode: 2013arXiv1309.0627Q
Context. Localized strongly Doppler-shifted Stokes V signals were
detected by IMaX/SUNRISE. These signals are related to newly emerged
magnetic loops that are observed as linear polarization features. <BR
/> Aims: We aim to set constraints on the physical nature and causes
of these highly Doppler-shifted signals. In particular, the temporal
relation between the appearance of transverse fields and the strong
Doppler shifts is analyzed in some detail. <BR /> Methods: We calculated
the time difference between the appearance of the strong flows and the
linear polarization. We also obtained the distances from the center
of various features to the nearest neutral lines and whether they
overlap or not. These distances were compared with those obtained from
randomly distributed points on observed magnetograms. Various cases of
strong flows are described in some detail. <BR /> Results: The linear
polarization signals precede the appearance of the strong flows by on
average 84 ± 11 s. The strongly Doppler-shifted signals are closer
(0.″19) to magnetic neutral lines than randomly distributed points
(0.″5). Eighty percent of the strongly Doppler-shifted signals are
close to a neutral line that is located between the emerging field and
pre-existing fields. That the remaining 20% do not show a close-by
pre-existing field could be explained by a lack of sensitivity or
an unfavorable geometry of the pre-existing field, for instance, a
canopy-like structure. <BR /> Conclusions: Transverse fields occurred
before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop
that first gives rise to the linear polarization signals, interacts
with pre-existing fields (generating new neutral line configurations),
and produces the observed strong flows. This explanation is indicative
of frequent small-scale reconnection events in the quiet Sun.
---------------------------------------------------------
Title: Is Magnetic Reconnection the Cause of Supersonic Upflows in
Granular Cells?
Authors: Borrero, J. M.; Martínez Pillet, V.; Schmidt, W.; Quintero
Noda, C.; Bonet, J. A.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.
2013ApJ...768...69B Altcode: 2013arXiv1303.2557B
In a previous work, we reported on the discovery of supersonic magnetic
upflows on granular cells in data from the SUNRISE/IMaX instrument. In
the present work, we investigate the physical origin of these events
employing data from the same instrument but with higher spectral
sampling. By means of the inversion of Stokes profiles we are able
to recover the physical parameters (temperature, magnetic field,
line-of-sight velocity, etc.) present in the solar photosphere at the
time of these events. The inversion is performed in a Monte-Carlo-like
fashion, that is, repeating it many times with different initializations
and retaining only the best result. We find that many of the events are
characterized by a reversal in the polarity of the magnetic field along
the vertical direction in the photosphere, accompanied by an enhancement
in the temperature and by supersonic line-of-sight velocities. In
about half of the studied events, large blueshifted and redshifted
line-of-sight velocities coexist above/below each other. These features
can be explained in terms of magnetic reconnection, where the energy
stored in the magnetic field is released in the form of kinetic
and thermal energy when magnetic field lines of opposite polarities
coalesce. However, the agreement with magnetic reconnection is not
perfect and, therefore, other possible physical mechanisms might also
play a role.
---------------------------------------------------------
Title: Evolution of the Fine Structure of Magnetic Fields in the
Quiet Sun: Observations from Sunrise/IMaX and Extrapolations
Authors: Wiegelmann, Thomas; Solanki, Sami; Borrero, Juan; Peter,
Hardi; Sunrise Team
2013EGUGA..15.5251W Altcode:
Observations with the balloon-borne Sunrise/ Imaging Magnetograph
eXperiment (IMaX) provide high spatial resolution (roughly 100 km at
disk center) measurements of the magnetic field in the photosphere of
the quiet Sun. To investigate the magnetic structure of the chromosphere
and corona, we extrapolate these photospheric measurements into
the upper solar atmosphere and analyse a 22-minute long time series
with a cadence of 33 seconds. Using the extrapolated magnetic-field
lines as tracer, we investigate temporal evolution of the magnetic
connectivity in the quiet Sun's atmosphere. The majority of magnetic
loops are asymmetric in the sense that the photospheric field strength
at the loop footpoints is very different. We find that the magnetic
connectivity of the loops changes rapidly with a typical connection
recycling time of about 3 ± 1 minutes in the upper solar atmosphere and
12 ± 4 minutes in the photosphere. This is considerably shorter than
previously found. Nonetheless, our estimate of the energy released by
the associated magnetic-reconnection processes is not likely to be the
sole source for heating the chromosphere and corona in the quiet Sun.
---------------------------------------------------------
Title: Evolution of the Fine Structure of Magnetic Fields in the
Quiet Sun: Observations from Sunrise/IMaX and Extrapolations
Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Peter,
H.; Barthol, P.; Gandorfer, A.; Martínez Pillet, V.; Schmidt, W.;
Knölker, M.
2013SoPh..283..253W Altcode:
Observations with the balloon-borne Sunrise/Imaging Magnetograph
eXperiment (IMaX) provide high spatial resolution (roughly 100 km at
disk center) measurements of the magnetic field in the photosphere of
the quiet Sun. To investigate the magnetic structure of the chromosphere
and corona, we extrapolate these photospheric measurements into
the upper solar atmosphere and analyze a 22-minute long time series
with a cadence of 33 seconds. Using the extrapolated magnetic-field
lines as tracer, we investigate temporal evolution of the magnetic
connectivity in the quiet Sun's atmosphere. The majority of magnetic
loops are asymmetric in the sense that the photospheric field strength
at the loop foot points is very different. We find that the magnetic
connectivity of the loops changes rapidly with a typical connection
recycling time of about 3±1 minutes in the upper solar atmosphere and
12±4 minutes in the photosphere. This is considerably shorter than
previously found. Nonetheless, our estimate of the energy released by
the associated magnetic-reconnection processes is not likely to be the
sole source for heating the chromosphere and corona in the quiet Sun.
---------------------------------------------------------
Title: Inferring the magnetic field vector in the quiet Sun. III. Disk
variation of the Stokes profiles and isotropism of the magnetic field
Authors: Borrero, J. M.; Kobel, P.
2013A&A...550A..98B Altcode: 2012arXiv1212.0788B
Recent investigations of the magnetic field vector properties in the
solar internetwork have provided diverging results. While some works
found that the internetwork is mostly pervaded by horizontal magnetic
fields, other works argued in favor of an isotropic distribution of
the magnetic field vector. Motivated by these seemingly contradictory
results and by the fact that most of these works have employed
spectropolarimetric data at disk center only, we have revisited
this problem employing high-quality data (noise level σ ≈ 3 ×
10<SUP>-4</SUP> in units of the quiet-Sun intensity) at different
latitudes recorded with the Hinode/SP instrument. Instead of applying
traditional inversion codes of the radiative transfer equation to
retrieve the magnetic field vector at each spatial point on the solar
surface and studying the resulting distribution of the magnetic field
vector, we surmised a theoretical distribution function of the magnetic
field vector and used it to obtain the theoretical histograms of the
Stokes profiles. These histograms were then compared to the observed
ones. Any mismatch between them was ascribed to the theoretical
distribution of the magnetic field vector, which was subsequently
modified to produce a better fit to the observed histograms. With
this method we find that Stokes profiles with signals above 2 ×
10<SUP>-3</SUP> (in units of the continuum intensity) cannot be
explained by an isotropic distribution of the magnetic field vector. We
also find that the differences between the histograms of the Stokes
profiles observed at different latitudes cannot be explained in terms of
line-of-sight effects. However, they can be explained by a distribution
of the magnetic field vector that inherently varies with latitude. We
note that these results are based on a series of assumptions that,
although briefly discussed in this paper, need to be considered in
more detail in the future.
---------------------------------------------------------
Title: Inferring the magnetic field vector in the quiet
Sun. II. Interpreting results from the inversion of Stokes profiles
Authors: Borrero, J. M.; Kobel, P.
2012A&A...547A..89B Altcode: 2012arXiv1209.4830B
In a previous paper, we argued that the inversion of Stokes profiles
applied to spectropolarimetric observations of the solar internetwork
yield unrealistically large values of the inclination of the magnetic
field vector (γ). This is because photon noise in Stokes Q and U are
interpreted by the inversion code as valid signals, that leads to an
overestimation of the transverse component B<SUB>⊥</SUB>, thus the
inclination γ. However, our study was based on the analysis of linear
polarization signals that featured only uncorrelated noise. In this
paper, we develop this idea further and study this effect in Stokes Q
and U profiles that also show correlated noise. In addition, we extend
our study to the three components of the magnetic field vector, as well
as the magnetic filling factor α. With this, we confirm the tendency
to overestimate γ when inverting linear polarization profiles that,
although non-zero, are still below the noise level. We also establish
that the overestimation occurs mainly for magnetic fields that are
nearly vertical γ ≲ 20°. This indicates that a reliable inference
of the inclination of the magnetic field vector cannot be achieved
by analyzing only Stokes I and V. In addition, when inverting Stokes
Q and U profiles below the noise, the inversion code retrieves a
randomly uniform distribution of the azimuth of the magnetic field
vector φ. To avoid these problems, we propose only inverting Stokes
profiles for which the linear polarization signals are sufficiently
above the noise level. However, this approach is also biased because,
in spite of allowing for a very accurate retrieval of the magnetic
field vector from the selected Stokes profiles, it selects only profiles
arising from highly inclined magnetic fields.
---------------------------------------------------------
Title: The continuum intensity as a function of magnetic
field. II. Local magnetic flux and convective flows
Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M.
2012A&A...542A..96K Altcode: 2014arXiv1402.3474K
Context. To deepen our understanding of the role of small-scale magnetic
fields in active regions (ARs) and in the quiet Sun (QS) on the solar
irradiance, it is fundamental to investigate the physical processes
underlying their continuum brightness. Previous results showed that
magnetic elements in the QS reach larger continuum intensities than
in ARs at disk center, but left this difference unexplained. <BR />
Aims: We use Hinode/SP disk center data to study the influence of
the local amount of magnetic flux on the vigour of the convective
flows and the continuum intensity contrasts. <BR /> Methods: The
apparent (i.e. averaged over a pixel) longitudinal field strength
and line-of-sight (LOS) plasma velocity were retrieved by means
of Milne-Eddington inversions (VFISV code). We analyzed a series
of boxes taken over AR plages and the QS, to determine how the
continuum intensity contrast of magnetic elements, the amplitude of
the vertical flows and the box-averaged contrast were affected by the
mean longitudinal field strength in the box (which scales with the total
unsigned flux in the box). <BR /> Results: Both the continuum brightness
of the magnetic elements and the dispersion of the LOS velocities
anti-correlate with the mean longitudinal field strength. This can
be attributed to the "magnetic patches" (here defined as areas where
the longitudinal field strength is above 100 G) carrying most of the
flux in the boxes. There the velocity amplitude and the spatial scale
of convection are reduced. Due to this hampered convective transport,
these patches appear darker than their surroundings. Consequently, the
average brightness of a box decreases as the the patches occupy a larger
fraction of it and the amount of embedded flux thereby increases. <BR />
Conclusions: Our results suggest that as the magnetic flux increases
locally (e.g. from weak network to strong plage), the heating of the
magnetic elements is reduced by the intermediate of a more suppressed
convective energy transport within the larger and stronger magnetic
patches. This, together with the known presence of larger magnetic
features, could explain the previously found lower contrasts of the
brightest magnetic elements in ARs compared to the QS. The inhibition
of convection also affects the average continuum brightness of a
photospheric region, so that at disk center, an area of photosphere in
strong network or plage appears darker than a purely quiet one. This
is qualitatively consistent with the predictions of 3D MHD simulations.
---------------------------------------------------------
Title: First Results from the SUNRISE Mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller,
T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González,
M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta,
J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González,
N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt,
W.; Steiner, O.; Title, A. M.
2012ASPC..455..143S Altcode:
The SUNRISE balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that reveal the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the SUNRISE data, which include a number of discoveries.
---------------------------------------------------------
Title: Supersonic Magnetic Flows in the Quiet Sun Observed with
SUNRISE/IMaX
Authors: Borrero, J. M.; Pillet, V. M.; Schlichenmaier, R.; Schmidt,
W.; Berkefeld, T.; Solanki, S. K.; Bonet, J. A.; Iniesta, J. C. d. T.;
Domingo, V.; Barthol, P.; Gandorfer, A.
2012ASPC..455..155B Altcode: 2012arXiv1202.4354B
In this contribution we describe some recent observations of high-speed
magnetized flows in the quiet Sun granulation. These observations
were carried out with the Imaging Magnetograph eXperiment (IMaX)
onboard the stratospheric balloon SUNRISE, and possess an unprecedented
spatial resolution and temporal cadence. These flows were identified as
highly shifted circular polarization (Stokes V) signals. We estimate
the LOS velocity responsible for these shifts to be larger than 6 km
s<SUP>-1</SUP>, and therefore we refer to them as supersonic magnetic
flows. The average lifetime of the detected events is 81.3 s and
they occupy an average area of about 23 000 km<SUP>2</SUP>. Most of
the events occur within granular cells and correspond therefore to
upflows. However some others occur in intergranular lanes or bear no
clear relation to the convective velocity pattern. We analyze a number
of representative examples and discuss them in terms of magnetic loops,
reconnection events, and convective collapse.
---------------------------------------------------------
Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
2012ASPC..455..337L Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
---------------------------------------------------------
Title: Polarization Calibration of the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO)
Authors: Schou, J.; Borrero, J. M.; Norton, A. A.; Tomczyk, S.;
Elmore, D.; Card, G. L.
2012SoPh..275..327S Altcode:
As part of the overall ground-based calibration of the Helioseismic
and Magnetic Imager (HMI) instrument an extensive set of polarimetric
calibrations were performed. This paper describes the polarimetric
design of the instrument, the test setup, the polarimetric model,
the tests performed, and some results. It is demonstrated that HMI
achieves an accuracy of 1% or better on the crosstalks between Q,
U, and V and that our model can reproduce the intensities in our
calibration sequences to about 0.4%. The amount of depolarization
is negligible when the instrument is operated as intended which,
combined with the flexibility of the polarimeter design, means that
the polarimetric efficiency is excellent.
---------------------------------------------------------
Title: Evolution of the fine structure of magnetic fields in the
quiet Sun: Combining Sunrise observations and modelling
Authors: Wiegelmann, T.; Solanki, S.; Borrero, J.; Martinez Pillet,
V.; Sunrise Team
2011AGUFMSH41B..06W Altcode:
Observations with the balloon borne SUNRISE/IMAX instrument provide
us with unprecedented high spatial resolution (pixel size 40 km)
measurements of the magnetic field in the photosphere of the quiet
Sun. To investigate the magnetic structure of the chromosphere and
corona we extrapolate these photospheric measurements into the upper
solar atmosphere and analyse a timeseries with a cadence of 33s. We find
that the majority of closed loops which reach into the chromosphere
or corona have one foot point in strong photospheric magnetic field
regions (B>300 G). Most loops are asymmetric and the weaker foot
point is often located in the internetwork. We find that the magnetic
connectivity of the loops changes rapidly with a typical recycling
time of about 2 min in the upper solar atmosphere and 14 min in the
photosphere. We discuss, to which extend the observed topological
changes can be interpreted as evidence for magnetic reconnection and
the relevance of these processes for coronal heating.
---------------------------------------------------------
Title: HMI vector magnetic field products: the long-awaited release
has come! Now what?
Authors: Centeno, R.; Barnes, G.; Borrero, J.; Couvidat, S. P.;
Hayashi, K.; Hoeksema, J. T.; Leka, K. D.; Liu, Y.; Schou, J.; Schuck,
P. W.; Sun, X.; Tomczyk, S.
2011AGUFMSH31A1985C Altcode:
HMI vector magnetic field test products will be released, alongside
with the corresponding documentation, soon after the submission of this
abstract. These data represent a stage of the project at which the HMI
vector team has a large degree of confidence in the results. However,
longer-term research topics on how to improve certain aspects of the
data pipeline in general -and the spectral line inversion code in
particular- are being pursued as we get valuable input from the user
community. I will give a brief summary of the characteristics of the
released inversion data products and an update of where we stand now.
---------------------------------------------------------
Title: VFISV: Very Fast Inversion of the Stokes Vector for the
Helioseismic and Magnetic Imager
Authors: Borrero, J. M.; Tomczyk, S.; Kubo, M.; Socas-Navarro, H.;
Schou, J.; Couvidat, S.; Bogart, R.
2011SoPh..273..267B Altcode: 2009arXiv0901.2702B
In this paper we describe in detail the implementation and main
properties of a new inversion code for the polarized radiative transfer
equation (VFISV: Very Fast Inversion of the Stokes Vector). VFISV will
routinely analyze pipeline data from the Helioseismic and Magnetic
Imager (HMI) on-board of the Solar Dynamics Observatory (SDO). It
will provide full-disk maps (4096×4096 pixels) of the magnetic field
vector on the Solar Photosphere every ten minutes. For this reason
VFISV is optimized to achieve an inversion speed that will allow
it to invert sixteen million pixels every ten minutes with a modest
number (approx. 50) of CPUs. Here we focus on describing a number of
important details, simplifications and tweaks that have allowed us to
significantly speed up the inversion process. We also give details on
tests performed with data from the spectropolarimeter on-board of the
Hinode spacecraft.
---------------------------------------------------------
Title: Magnetic Structure of Sunspots
Authors: Borrero, Juan M.; Ichimoto, Kiyoshi
2011LRSP....8....4B Altcode: 2011arXiv1109.4412B
In this review we give an overview about the current state-of-knowledge
of the magnetic field in sunspots from an observational point of
view. We start by offering a brief description of tools that are most
commonly employed to infer the magnetic field in the solar atmosphere
with emphasis in the photosphere of sunspots. We then address separately
the global and local magnetic structure of sunspots, focusing on the
implications of the current observations for the different sunspots
models, energy transport mechanisms, extrapolations of the magnetic
field towards the corona, and other issues.
---------------------------------------------------------
Title: The Sun at high resolution: first results from the Sunrise
mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
2011IAUS..273..226S Altcode:
The Sunrise balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system and further infrastructure. The first
science flight of Sunrise yielded high-quality data that reveal the
structure, dynamics and evolution of solar convection, oscillations
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the Sunrise data, which include a number of discoveries.
---------------------------------------------------------
Title: The continuum intensity as a function of magnetic
field. I. Active region and quiet Sun magnetic elements
Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M.
2011A&A...531A.112K Altcode: 2011arXiv1105.1958K
Context. Small-scale magnetic fields are major contributors to
the solar irradiance variations. Hence, the continuum intensity
contrast of magnetic elements in the quiet Sun (QS) network and in
active region (AR) plage is an essential quantity that needs to be
measured reliably. <BR /> Aims: By using Hinode/SP disk center data
at a constant, high spatial resolution, we aim at updating results of
earlier ground-based studies of contrast vs. magnetogram signal, and to
look for systematic differences between AR plages and QS network. <BR
/> Methods: The field strength, filling factor and inclination of the
field was retrieved by means of a Milne-Eddington inversion (VFISV
code). As in earlier studies, we then performed a pixel-by-pixel study
of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel)
longitudinal magnetic field over large fields of view in ARs and in the
QS. <BR /> Results: The continuum contrast of magnetic elements reaches
larger values in the QS (on average 3.7%) than in ARs (on average
1.3%). This could not be attributed to any systematic difference in
the chosen contrast references, so that it mainly reflects an intrinsic
brightness difference. The larger contrasts in the QS are in agreement
with earlier, lower resolution results, although our values are larger
due to our better spatial resolution. At Hinode's spatial resolution,
moreover, the relationship between contrast and apparent longitudinal
field strength exhibits a peak at around 700 G in both the QS and ARs,
whereas earlier lower resolution studies only found a peak in the QS and
a monotonic decrease in ARs. We attribute this discrepancy both to our
careful removal of the pores and their close surroundings affected by
the telescope diffraction, as well as to the enhanced spatial resolution
and very low scattered light of the Hinode Solar Optical Telescope. We
verified that the magnetic elements producing the peak in the contrast
curve are rather vertical in the AR and in the QS, so that the larger
contrasts in the QS cannot be explained by larger inclinations, as had
been proposed earlier. The opposite polarities in ARs do not exhibit
any noticeable difference in inclination either, although they reach
different contrasts when the amount of flux is significantly unbalanced
between the polarities. <BR /> Conclusions: According to our inversions,
the magnetic elements producing the peak of the contrast curves have
similar properties (field strength, inclination, filling factor) in ARs
and in the QS, so that the larger brightness of magnetic elements in the
QS remains unexplained. Indirect evidence suggests that the contrast
difference is not primarily due to any difference in average size of
the magnetic elements. A possible explanation lies in the different
efficiencies of convective energy transport in the QS and in ARs,
which will be the topic of a second paper.
---------------------------------------------------------
Title: The Continuum Contrast of Magnetic Elements as a Function of
Magnetic Field (Disk Center): Early Studies and Hinode/SP Results
Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M.
2011ASPC..437..297K Altcode:
To deepen our understanding of the role of small-scale magnetic
fields on the solar irradiance, it is essential to quantify the
continuum contrast of magnetic elements in the quiet Sun (QS)
network and in active region (AR) plage. By using Hinode/SP disk
center data at constant spatial resolution, we aimed at updating
results of earlier ground-based studies of contrast vs. magnetogram
signal, and to look for systematic differences between AR plages
and QS network. We performed a pixel-per-pixel study of continuum
contrast vs. longitudinal flux density over large fields of view in
AR and in QS (as in earlier studies). Even at Hinode's resolution,
the contrast of magnetic elements reaches larger values in the QS than
in ARs. We show that this difference cannot be explained by different
inclinations of magnetic elements in ARs and QS. We compared our
contrast vs. magnetic flux density with earlier studies and attributed
the differences both to our proper removal of the pores and their
surrounding diffraction-spread radiation, as well as to our enhanced
spatial resolution and quasi-absence of scattered light. At Hinode's
resolution, the contrast of magnetic elements peaks on average at
similar magnetic flux densities in ARs and in the QS, which indicates
that the brightest flux tubes have similar sizes in ARs and QS.
---------------------------------------------------------
Title: HMI: First Results
Authors: Centeno, R.; Tomczyk, S.; Borrero, J. M.; Couvidat,
S. Hayashi, K.; Hoeksema, T.; Liu, Y.; Schou, J.
2011ASPC..437..147C Altcode: 2010arXiv1012.3796C
The Helioseismic and Magnetic Imager (HMI) has just started producing
data that will help determine what the sources and mechanisms of
variability in the Sun's interior are. The instrument measures the
Doppler shift and the polarization of the Fe I 6173 Å line, on the
entire solar disk at a relatively-high cadence, in order to study
the oscillations and the evolution of the full vector magnetic field
of the solar Photosphere. After the data are properly calibrated,
they are given to a Milne-Eddington inversion code (VFISV, Borrero et
al. 2010) whose purpose is to infer certain aspects of the physical
conditions in the Sun's Photosphere, such as the full 3-D topology
of the magnetic field and the line-of-sight velocity at the solar
surface. We will briefly describe the characteristics of the inversion
code, its advantages and limitations -both in the context of the model
atmosphere and the actual nature of the data-, and other aspects of its
performance on such a remarkable data load. Also, a cross-comparison
with near-simultaneous maps from the Spectro-Polarimeter (SP) onboard
Hinode will be made.
---------------------------------------------------------
Title: Inferring the magnetic field vector in the quiet Sun. I. Photon
noise and selection criteria
Authors: Borrero, J. M.; Kobel, P.
2011A&A...527A..29B Altcode: 2010arXiv1011.4380B
In the past, spectropolarimetric data from Hinode/SP were employed
to infer the distribution of the magnetic field vector in the quiet
Sun. While some authors found predominantly horizontal magnetic fields,
others favor an isotropic distribution. We investigate whether it is
actually possible to accurately retrieve the magnetic field vector
in regions with very low polarization signals (e.g. internetwork),
employing the FeI line pair at 6300 Å. We first perform inversions
of the Stokes vector observed with Hinode/SP in the quiet Sun at
disk center in order to confirm the distributions retrieved by other
authors. We then carry out several Monte-Carlo simulations with
synthetic data, with which we show that the observed distribution of
the magnetic field vector can be explained in terms of purely vertical
(γ = 0°) and weak fields (bar{B<20} G), which are misinterpreted
by the analysis technique (Stokes inversion code) as being horizontal
(γ ≈ 90°) and stronger (bar{B ≈ 100} G), owing to the effect of
the photon noise. This challenges the correctness of previous results,
which presented the distributions for the magnetic field vector
peaking at γ = 90° and bar{B=100} G. We propose that an accurate
determination of the magnetic field vector can be achieved by decreasing
the photon noise to a point where most of the observed profiles posses
Stokes Q or U profiles that are above the noise level. Unfortunately,
for noise levels as low as 2.8 × 10<SUP>-4</SUP>, only 30 % of the
observed region with Hinode/SP have sufficiently strong Q or U signals,
implying that the magnetic field vector remains unknown in the rest
of the internetwork.
---------------------------------------------------------
Title: Supersonic Magnetic Upflows in Granular Cells Observed with
SUNRISE/IMAX
Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.;
Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.;
Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M.
2010ApJ...723L.144B Altcode: 2010arXiv1009.1227B
Using the IMaX instrument on board the SUNRISE stratospheric balloon
telescope, we have detected extremely shifted polarization signals
around the Fe I 5250.217 Å spectral line within granules in the solar
photosphere. We interpret the velocities associated with these events
as corresponding to supersonic and magnetic upflows. In addition, they
are also related to the appearance of opposite polarities and highly
inclined magnetic fields. This suggests that they are produced by the
reconnection of emerging magnetic loops through granular upflows. The
events occupy an average area of 0.046 arcsec<SUP>2</SUP> and last for
about 80 s, with larger events having longer lifetimes. These supersonic
events occur at a rate of 1.3 × 10<SUP>-5</SUP> occurrences per second
per arcsec<SUP>2</SUP>.
---------------------------------------------------------
Title: Magnetic Loops in the Quiet Sun
Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Martínez
Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Bonet, J. A.;
Barthol, P.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.185W Altcode: 2010arXiv1009.4715W
We investigate the fine structure of magnetic fields in the atmosphere
of the quiet Sun. We use photospheric magnetic field measurements from
SUNRISE/IMaX with unprecedented spatial resolution to extrapolate
the photospheric magnetic field into higher layers of the solar
atmosphere with the help of potential and force-free extrapolation
techniques. We find that most magnetic loops that reach into the
chromosphere or higher have one footpoint in relatively strong magnetic
field regions in the photosphere. Ninety-one percent of the magnetic
energy in the mid-chromosphere (at a height of 1 Mm) is in field
lines, whose stronger footpoint has a strength of more than 300 G,
i.e., above the equipartition field strength with convection. The
loops reaching into the chromosphere and corona are also found to be
asymmetric in the sense that the weaker footpoint has a strength B <
300 G and is located in the internetwork (IN). Such loops are expected
to be strongly dynamic and have short lifetimes, as dictated by the
properties of the IN fields.
---------------------------------------------------------
Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with
the SUNRISE/IMAX Instrument
Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez
Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero,
J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.;
Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M.
2010ApJ...723L.164L Altcode: 2010arXiv1009.0996L
Until today, the small size of magnetic elements in quiet-Sun areas has
required the application of indirect methods, such as the line-ratio
technique or multi-component inversions, to infer their physical
properties. A consistent match to the observed Stokes profiles could
only be obtained by introducing a magnetic filling factor that specifies
the fraction of the observed pixel filled with magnetic field. Here,
we investigate the properties of a small magnetic patch in the quiet
Sun observed with the IMaX magnetograph on board the balloon-borne
telescope SUNRISE with unprecedented spatial resolution and low
instrumental stray light. We apply an inversion technique based on
the numerical solution of the radiative transfer equation to retrieve
the temperature stratification and the field strength in the magnetic
patch. The observations can be well reproduced with a one-component,
fully magnetized atmosphere with a field strength exceeding 1 kG and
a significantly enhanced temperature in the mid to upper photosphere
with respect to its surroundings, consistent with semi-empirical flux
tube models for plage regions. We therefore conclude that, within the
framework of a simple atmospheric model, the IMaX measurements resolve
the observed quiet-Sun flux tube.
---------------------------------------------------------
Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as
Measured from SUNRISE
Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler,
M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.;
Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez
Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.154H Altcode:
We present high-resolution images of the Sun in the near-ultraviolet
spectral range between 214 nm and 397 nm as obtained from the first
science flight of the 1 m SUNRISE balloon-borne solar telescope. The
quiet-Sun rms intensity contrasts found in this wavelength range are
among the highest values ever obtained for quiet-Sun solar surface
structures—up to 32.8% at a wavelength of 214 nm. We compare the
rms contrasts obtained from the observational data with theoretical
intensity contrasts obtained from numerical magnetohydrodynamic
simulations. For 388 nm and 312 nm the observations agree well with
the numerical simulations whereas at shorter wavelengths discrepancies
between observed and simulated contrasts remain.
---------------------------------------------------------
Title: Quiet-Sun intensity contrasts in the near ultraviolet
Authors: Hirzberger, Johann; Feller, Alex; Riethmüller, Tino L.;
Schüssler, Manfred; Borrero, Juan M.; Afram, Nadine; Unruh, Yvonne C.;
Berdyugina, Svetlana V.; Gandorfer, Achim; Solanki, Sami K.; Barthol,
Peter; Bonet, Jose A.; Martínez Pillet, Valentin; Berkefeld, Thomas;
Knölker, Michael; Schmidt, Wolfgang; Title, Alan M.
2010arXiv1009.1050H Altcode:
We present high-resolution images of the Sun in the near ultraviolet
spectral range between 214 nm and 397 nm as obtained from the first
science flight of the 1-m Sunrise balloon-borne solar telescope. The
quiet-Sun rms intensity contrasts found in this wavelength range
are among the highest values ever obtained for quiet-Sun solar
surface structures - up to 32.8% at a wavelength of 214 nm. We
compare with theoretical intensity contrasts obtained from numerical
magneto-hydrodynamic simulations. For 388 nm and 312 nm the observations
agree well with the numerical simulations whereas at shorter wavelengths
discrepancies between observed and simulated contrasts remain.
---------------------------------------------------------
Title: Spectropolarimetric analysis of 3D MHD sunspot simulations
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
2010AN....331..567B Altcode:
We have employed 3D non-grey MHD simulations of sunspots to compute
theoretical Stokes profiles and compare the levels of circular and
linear polarization in the simulations with those observed in a real
sunspot. We find that the spatial distribution and average values
of these quantities agree very well with the observations, although
the polarization levels in the simulations are slightly larger. This
can be explained by a slightly larger magnetic field strength or a
larger temperature gradient in the simulated penumbra as compared to
the observations.
---------------------------------------------------------
Title: Convective Motions and Net Circular Polarization in Sunspot
Penumbrae
Authors: Borrero, J. M.; Solanki, S. K.
2010ApJ...709..349B Altcode: 2009arXiv0911.2570B
We have employed a penumbral model, which includes the Evershed flow
and convective motions inside penumbral filaments, to reproduce the
azimuthal variation of the net circular polarization (NCP) in sunspot
penumbrae at different heliocentric angles for two different spectral
lines. The theoretical NCP fits the observations as satisfactorily
as penumbral models based on flux tubes. The reason for this is that
the effect of convective motions on the NCP is very small compared
to the effect of the Evershed flow. In addition, the NCP generated by
convective upflows cancels out the NCP generated by the downflows. We
have also found that, in order to fit the observed NCP, the strength
of the magnetic field inside penumbral filaments must be very close
to 1000 G. In particular, field-free or weak-field filaments fail to
reproduce both the correct sign of the NCP and its dependence on the
azimuthal and heliocentric angles.
---------------------------------------------------------
Title: UV intensity distributions of the quiet Sun observed with
Sunrise
Authors: Hirzberger, Johann; Feller, A.; Riethmueller, T.; Borrero,
J. M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knoelker, M.; Martínez Pillet, V.; Schmidt, W.; Solanki, S.; Title, A.
2010cosp...38.1735H Altcode: 2010cosp.meet.1735H
High resolution solar images in the near UV have been obtained with
the Solar UV Filtergraph (SUFI) onboard the Sunrise balloon borne
observatory, amongst others in wavelength regions not accessible
from the ground. We present intensity distributions of the quiet
Sun at different heliocentric angles, from disk center to the solar
limb. These results, obtained in spectral windows at 214 nm, 313 nm
(OH band), 388 nm (CN band) and 396.7 nm (CaIIH), represent an important
validation of numerical models of the solar photosphere and are, thus,
fundamental ingredients for our understanding of the thermal processes
in the solar surface region.
---------------------------------------------------------
Title: Models and observations of sunspot penumbrae
Authors: Borrero, Juan Manuel
2009ScChG..52.1670B Altcode: 2008arXiv0810.0080B
The mysteries of sunspot penumbrae have been under an intense
scrutiny for the past 10 years. During this time, some models
have been proposed and refuted, while the surviving ones had to be
modified, adapted and evolved to explain the ever-increasing array of
observational constraints. In this contribution I will review two of
the present models, emphasizing their contributions to this field,
but also pinpointing some of their inadequacies to explain a number
of recent observations at very high spatial resolution (0.32″). To
help explaining these new observations I propose some modifications to
each of those models. These modifications bring those two seemingly
opposite models closer together into a general picture that agrees
well with recent 3D magneto-hydrodynamic simulations.
---------------------------------------------------------
Title: Dipolar Evolution in a Coronal Hole Region
Authors: Yang, Shuhong; Zhang, Jun; Borrero, Juan Manuel
2009ApJ...703.1012Y Altcode: 2009arXiv0908.0578Y
Using observations from the Solar and Heliospheric Observatory, the
Solar Terrestrial Relations Observatory, and Hinode, we investigate
magnetic field evolution in an equatorial coronal hole region. Two
dipoles emerge one by one. The negative element of the first dipole
disappears due to the interaction with the positive element of the
second dipole. During this process, a jet and a plasma eruption are
observed. The opposite polarities of the second dipole separate at
first, and then cancel with each other, which is first reported in
a coronal hole. With the reduction of unsigned magnetic flux of the
second dipole from 9.8 × 10<SUP>20</SUP> Mx to 3.0 × 10<SUP>20</SUP>
Mx in two days, 171 Å brightness decreases by 75% and coronal loops
shrink obviously. At the cancellation sites, the transverse fields are
strong and point directly from the positive elements to the negative
ones, meanwhile Doppler redshifts with an average velocity of 0.9 km
s<SUP>-1</SUP> are observed, comparable to the horizontal velocity (1.0
km s<SUP>-1</SUP>) derived from the canceling island motion. Several
days later, the northeastern part of the coronal hole, where the dipoles
are located, appears as a quiet region. These observations support the
idea that the interaction between the two dipoles is caused by flux
reconnection, while the cancellation between the opposite polarities
of the second dipole is due to the submergence of original loops. These
results will help us to understand coronal hole evolution.
---------------------------------------------------------
Title: Are There Field-Free Gaps near τ = 1 in Sunspot Penumbrae?
Authors: Borrero, J. M.; Solanki, S. K.
2008ApJ...687..668B Altcode: 2008arXiv0806.4454B
The vertical stratification of the magnetic field strength in sunspot
penumbrae is investigated by means of spectropolarimetric observations
at high spatial resolution from the Hinode spacecraft. Assuming that
the magnetic field changes linearly with optical depth we find that, in
those regions where the magnetic field is more inclined and the Evershed
flow is strongest (penumbral intraspines), the magnetic field can either
increase or decrease with depth. Allowing more degrees of freedom to the
magnetic field stratification reveals that the magnetic field initially
decreases from log τ<SUB>5</SUB> = - 3 until log τ<SUB>5</SUB>simeq -
1.0, but increases again below that. The presence of strong magnetic
fields near the continuum is at odds with the existence of regions
void of magnetic fields at, or right below, the τ<SUB>5</SUB> = 1
level in the penumbra. However, they are compatible with the presence
of a horizontal flux-tube-like field embedded in a magnetic atmosphere.
---------------------------------------------------------
Title: Solar spectropolarimetry at high spatial resolution: Quiet-Sun
magnetic fields
Authors: Bellot Rubio, L. R.; Borrero, J. M.
2008ESPM...12..2.4B Altcode:
Spectropolarimetry permits quantitative inferences of solar magnetic
fields to be carried out. It is in fact the only means at our disposal
to characterize the physical properties of small magnetic structures
from low spatial-resolution observations. The accuracy of the results
derived from spectropolarimetric measurements has improved dramatically
with the advent of simultaneous observations in different spectral
regions and high angular resolution measurements. The main advantage
brought about by high spatial resolution is that there is less need
to model complex scenarios involving different fields and/or flows
coexisting in the pixel. Currently it is possible to achieve resolutions
of 0.3 arcsec from space (with the Hinode satellite) and 0.15 arcsec
from the ground (with the 1-m Swedish Solar Telescope). In this talk
we will show examples of high spatial resolution spectropolarimetric
observations and how they are challenging our understanding of quiet
Sun magnetic fields and sunspot penumbrae. In addition, we will
describe previously unknown issues that affect diffraction-limited
observations. Proper interpretations of the measurements require these
effects to be taken into account.
---------------------------------------------------------
Title: Evidence of magnetic field wrapping around penumbral filaments
Authors: Borrero, J. M.; Lites, B. W.; Solanki, S. K.
2008A&A...481L..13B Altcode: 2007arXiv0712.2548B
We employ high-spatial resolution spectropolarimetric observations
from the Solar Optical Telescope on-board the Hinode spacecraft to
investigate the fine structure of the penumbral magnetic fields. The
Stokes vector of two neutral iron lines at 630 nm is inverted at every
spatial pixel to retrieve the depth-dependence of the magnetic field
vector, line-of-sight velocity and thermodynamic parameters. We show
that the azimuthal angle of the magnetic field vector has opposite sign
on both sides above the penumbral filaments. This is consistent with
the wrapping of an inclined field around the horizontal filaments. The
wrapping effect is stronger for filaments with larger horizontal
extensions. In addition, we find that the external magnetic field can
penetrate into the intraspines, leading to non-radial magnetic fields
inside them. These findings shed some light on the controversial
small-scale structure of the sunspot penumbra.
---------------------------------------------------------
Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and
6302 Å
Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.;
Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez
González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez
Almeida, J.
2008ApJ...674..596S Altcode:
The reliability of quiet-Sun magnetic field diagnostics based on the
Fe I lines at 6302 Å has been questioned by recent work. Here we
present the results of a thorough study of high-resolution multiline
observations taken with the new spectropolarimeter SPINOR, comprising
the 5250 and 6302 Å spectral domains. The observations were analyzed
using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Å lines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magnetohydrodynamic constrains. When this is done, the 5250 Å lines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point toward a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic
field strengths. Instead, we advocate for a combined analysis of both
spectral ranges, which yields a better determination of the quiet-Sun
magnetic properties. Finally, we propose the use of two other Fe I
lines (at 4122 and 9000 Å) with identical line opacities that seem
to work much better than the others.
---------------------------------------------------------
Title: Temporal evolution of the Evershed flow in
sunspots. II. Physical properties and nature of Evershed clouds
Authors: Cabrera Solana, D.; Bellot Rubio, L. R.; Borrero, J. M.;
Del Toro Iniesta, J. C.
2008A&A...477..273C Altcode: 2007arXiv0709.1601C
Context: Evershed clouds (ECs) represent the most conspicuous
variation of the Evershed flow in sunspot penumbrae. <BR />Aims:
We determine the physical properties of ECs from high spatial and
temporal resolution spectropolarimetric measurements. This information
is used to investigate the nature of the EC phenomenon. <BR />Methods:
The Stokes profiles of four visible and three infrared spectral lines
are subject to inversions based on simple one-component models as well
as more sophisticated realizations of penumbral flux tubes embedded
in a static ambient field (uncombed models). <BR />Results: According
to the one-component inversions, the EC phenomenon can be understood
as a perturbation of the magnetic and dynamic configuration of the
penumbral filaments along which the ECs move. The uncombed inversions,
on the other hand, suggest that ECs are the result of enhancements
in the visibility of penumbral flux tubes. We conjecture that
these enhancements are caused by a perturbation of the thermodynamic
properties of the tubes, rather than by changes in the vector magnetic
field. This mechanism is investigated performing numerical experiments
of thick penumbral tubes in mechanical equilibrium with a background
field. <BR />Conclusions: While the one-component inversions confirm
many of the properties indicated by a simple line parameter analysis
(Paper I of this series), we give more credit to the results of the
uncombed inversions because they take into account, at least in an
approximate manner, the fine structure of the penumbra. <P />Appendix
A is only available in electronic form at http://www.aanda.org
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Title: On the Role of Magnetic Fields in Abundance Determinations
Authors: Borrero, J. M.
2008ApJ...673..470B Altcode: 2007arXiv0709.3809B
Although there is considerable evidence supporting an ubiquitous
magnetic field in solar/stellar photospheres, its impact in the
determination of abundances has never been quantified. In this work
we investigate whether the magnetic field plays a measurable role for
this kind of study. To that end, we carry out simulations of spectral
line formation in the presence of a magnetic field and then use those
profiles to derive the abundance of several atomic species (Fe, Si, C,
and O), neglecting the magnetic field. In this way, we find that the
derived iron abundance can be significantly biased, with systematic
errors of up to 0.1 dex. In the case of silicon, carbon, and oxygen,
their role is very marginal (errors smaller than 0.02 dex). We also
find that the effect of the magnetic field strongly depends on its
inclination with respect to the observer. We show that fields that
are aligned with the observer lead to an underestimation of the real
abundance, whereas more inclined ones overestimate it. In the case
of a mixture of fields with different inclinations, these effects are
likely to partly cancel each other out, making the role of the magnetic
field even less important. Finally, we derive a simple model that can
be used to determine the suitability of a spectral line when we wish
to avoid the bias introduced by the neglect of the magnetic field.
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Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å
Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados,
M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González,
M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J.
2007arXiv0710.1099S Altcode:
The reliability of quiet Sun magnetic field diagnostics based on the
\ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We
present here the results of a thorough study of high-resolution
multi-line observations taken with the new spectro-polarimeter SPINOR,
comprising the 5250 and 6302 Åspectral domains. The observations were
analyzed using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Ålines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magneto-hydrodynamical constrains. When this is done, the 5250 Ålines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point towards a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic field
strengths. Instead, we advocate for a combined analysis of both spectral
ranges, which yields a better determination of the quiet Sun magnetic
properties. Finally, we propose the use of two other \ion{Fe}{1} lines
(at 4122 and 9000 Å) with identical line opacities that seem to work
much better than the others.
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Title: Flux Tubes as the Origin of Net Circular Polarization in
Sunspot Penumbrae
Authors: Borrero, J. M.; Bellot Rubio, L. R.; Müller, D. A. N.
2007ApJ...666L.133B Altcode: 2007arXiv0707.4145B
We employ a three-dimensional magnetohydrostatic model of a
horizontal flux tube, embedded in a magnetic surrounding atmosphere,
to successfully reproduce the azimuthal and center-to-limb variations
of the net circular polarization observed in sunspot penumbrae. This
success is partly due to the realistic modeling of the interaction
between the flux tube and the surrounding magnetic field.
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Title: The structure of sunspot penumbrae. IV. MHS equilibrium for
penumbral flux tubes and the origin of dark core penumbral filaments
and penumbral grains
Authors: Borrero, J. M.
2007A&A...471..967B Altcode: 2007arXiv0704.3219B
Aims:We study the magnetohydrostatic equilibrium of magnetic flux
tubes with circular cross sections embedded in a magnetic surrounding
atmosphere. <BR />Methods: We solve the static momentum equation
in 2.5D to obtain the thermodynamics that are consistent with a
prescribed velocity and magnetic fields. <BR />Results: We show that
force balance is roughly satisfied if the flux tube's magnetic field
is aligned with its axis. Equilibrium is guaranteed if this magnetic
field possesses a transverse component. Several forms of this transverse
field are investigated. The resulting magnetic field configurations are
critically reviewed in terms of the results from spectropolarimetric
observations. The thermodynamic structure that allows the flux tube
to be in mechanical equilibrium is also calculated. We show that the
inferred pressure, density, and temperature stratification reproduce
intensity features similar to dark core penumbral filaments and
penumbral grains.
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Title: Modified p-modes in penumbral filaments?
Authors: Bloomfield, D. S.; Solanki, S. K.; Lagg, A.; Borrero, J. M.;
Cally, P. S.
2007A&A...469.1155B Altcode: 2007arXiv0705.0481B
Aims:The primary objective of this study is to search for and identify
wave modes within a sunspot penumbra. <BR />Methods: Infrared
spectropolarimetric time series data are inverted using a model
comprising two atmospheric components in each spatial pixel. Fourier
phase difference analysis is performed on the line-of-sight velocities
retrieved from both components to determine time delays between the
velocity signals. In addition, the vertical separation between the
signals in the two components is calculated from the Stokes velocity
response functions. <BR />Results: The inversion yields two atmospheric
components, one permeated by a nearly horizontal magnetic field, the
other with a less-inclined magnetic field. Time delays between the
oscillations in the two components in the frequency range 2.5-4.5 mHz
are combined with speeds of atmospheric wave modes to determine wave
travel distances. These are compared to expected path lengths obtained
from response functions of the observed spectral lines in the different
atmospheric components. Fast-mode (i.e., modified p-mode) waves exhibit
the best agreement with the observations when propagating toward the
sunspot at an angle ~50° to the vertical.
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Title: Magnetic Field Vector Retrieval With the Helioseismic and
Magnetic Imager
Authors: Borrero, J. M.; Tomczyk, S.; Norton, A.; Darnell, T.; Schou,
J.; Scherrer, P.; Bush, R.; Liu, Y.
2007SoPh..240..177B Altcode: 2006astro.ph.11565B
We investigate the accuracy to which we can retrieve the solar
photospheric magnetic field vector using the Helioseismic and Magnetic
Imager (HMI) that will fly onboard of the Solar Dynamics Observatory by
inverting simulated HMI profiles. The simulated profiles realistically
take into account the effects of the photon noise, limited spectral
resolution, instrumental polarization modulation, solar p modes, and
temporal averaging. The accuracy of the determination of the magnetic
field vector is studied by considering the different operational modes
of the instrument.
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Title: Modified p-modes in penumbral filaments
Authors: Bloomfield, D. S.; Lagg, A.; Solanki, S. K.; Borrero, J. M.
2007msfa.conf..241B Altcode:
A time series analysis was performed on velocity signals in a sunspot
penumbra to search for possible wave modes. The spectropolarimetric
photospheric data obtained by the Tenerife Infrared Polarimeter were
inverted using the SPINOR code. An atmospheric model comprising two
magnetic components and one stray-light component gave an optimal fit
to the data. Fourier phase difference analysis between line-of-sight
velocities of both magnetic components provided time delays between
the two atmospheres. These delays were combined with the speeds of
atmospheric wave modes and compared to height separations derived from
velocity response functions to determine the wave mode.
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Title: Magnetic Field Vector Retrieval with HMI
Authors: Borrero, J. M.; Tomczyk, S.; Norton, A. A.; Darnell, T.;
Schou, J.; Scherrer, P.; Bush, R. I.; Lui, Y.
2006ASPC..358..144B Altcode:
The Helioseismic and Magnetic Imager (HMI), on board the Solar Dynamics
Observatory (SDO), will begin data acquisition in 2008. It will provide
the first full-disk, high temporal cadence observations of the full
Stokes vector with a 0.5 arcsec pixel size. This will allow for a
continuous monitoring of the Solar magnetic-field vector. HMI data will
advance our understanding of the small- and large-scale magnetic field
evolution, its relation to the solar and global dynamic processes,
coronal field extrapolations, flux emergence, magnetic helicity, and
the nature of the polar magnetic fields. We summarize HMI's expected
operation modes, focusing on the polarization cross-talk induced
by the solar oscillations, and how this affects the magnetic-field
vector determination.
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Title: The Uncombed Penumbra
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
2006ASPC..358...19B Altcode: 2006astro.ph..2130B
The uncombed penumbral model explains the structure of the sunspot
penumbra in terms of thick magnetic fibrils embedded in a surrounding,
magnetic atmosphere. This model has been successfully applied to explain
the polarization signals emerging from the sunspot penumbra. Thick
penumbral fibrils face some physical problems, however. In this
contribution we will offer possible solutions to these shortcomings.
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Title: On the fine structure of sunspot penumbrae. III. The vertical
extension of penumbral filaments
Authors: Borrero, J. M.; Solanki, S. K.; Lagg, A.; Socas-Navarro,
H.; Lites, B.
2006A&A...450..383B Altcode: 2005astro.ph.10586B
In this paper we study the fine structure of the penumbra as inferred
from the uncombed model (flux tube embedded in a magnetic surrounding)
when applied to penumbral spectropolarimetric data from the neutral
iron lines at 6300 Å. The inversion infers very similar radial
dependences in the physical quantities (LOS velocity, magnetic field
strength etc.) as those obtained from the inversion of the Fe I 1.56
μm lines. In addition, the large Stokes V area asymmetry exhibited
by the visible lines helps to constrain the size of the penumbral
flux tubes. As we demonstrate here, the uncombed model is able to
reproduce the area asymmetry with striking accuracy, returning flux
tubes as thick as 100-300 kilometers in the vertical direction, in
good agreement with previous investigations.
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Title: The uncombed penumbra
Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K.
2006astro.ph..2129B Altcode:
The uncombed penumbral model explains the structure of the sunspot
penumbra in terms of thick magnetic fibrils embedded in a magnetic
surrounding atmosphere. This model has been successfully applied
to explain the polarization signals emerging from the sunspot
penumbra. Thick penumbral fibrils face some physical problems,
however. In this contribution we will offer possible solutions to
these shortcomings.
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Title: On the fine structure of sunspot penumbrae. II. The nature
of the Evershed flow
Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Collados, M.
2005A&A...436..333B Altcode: 2005astro.ph..3677B
We investigate the fine structure of the sunspot penumbra by means of
a model that allows for a flux tube in horizontal pressure balance
with the magnetic background atmosphere in which it is embedded. We
apply this model to spectropolarimetric observations of two neutral
iron lines at 1.56 μm and invert several radial cuts in the penumbra
of the same sunspot at two different heliocentric angles. In the inner
part of the penumbra we find hot flux tubes that are somewhat inclined
to the horizontal. They become gradually more horizontal and cooler
with increasing radial distance. This is accompanied by an increase
in the velocity of the plasma and a decrease of the gas pressure
difference between flux tube and the background component. At large
radial distances the flow speed exceeds the critical speed and evidence
is found for the formation of a shock front. These results are in good
agreement with simulations of the penumbral fine structure and provide
strong support for the siphon flow as the physical mechanism driving
the Evershed flow.
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Title: Thermal-magnetic relation in a sunspot and a map of its
Wilson depression
Authors: Mathew, S. K.; Solanki, S. K.; Lagg, A.; Collados, M.;
Borrero, J. M.; Berdyugina, S.
2004A&A...422..693M Altcode:
We present relations between thermal and magnetic quantities in a
simple, isolated sunspot, as deduced from the inversion of 1.56 μm
spectropolarimetric data. We used a combination of two infrared Fe I
lines at 15 648.5 Å and 15 652.8 Å/ in the inversions. Due to the
high Zeeman sensitivity of these lines, we can study this relationship
in the entire sunspot. The relevant parameters were derived both as a
function of location within the sunspot and of height in the atmosphere
using an inversion technique based on response functions. In this paper
we relate the magnetic vector with temperature. We find a non-linear
relationship between the various components of the magnetic vector and
temperature, which confirm the results from earlier investigations. We
also computed the Wilson depression and the plasma β for the observed
sunspot and compare our results with earlier findings.
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Title: On the fine structure of sunspot penumbrae. I. A quantitative
comparison of two semiempirical models with implications for the
Evershed effect
Authors: Borrero, J. M.; Solanki, S. K.; Bellot Rubio, L. R.; Lagg,
A.; Mathew, S. K.
2004A&A...422.1093B Altcode:
Sunspot penumbrae exhibit prominent fine structure. Different
interpretations of spectropolarimetric observations suggest different,
sometimes contradictory, properties of this fine structure. In this
paper we show that the results of inversions of penumbral infrared
profiles based on one-component models with gradients of the atmospheric
parameters and two-component models without gradients are compatible
with each other. Our analysis reconciles the results of previous
investigations and provides further support for the picture that sunspot
penumbrae are composed of penumbral flux tubes embedded in a magnetic
background. The magnetic field in the tubes is more horizontal and
weaker than that of the background atmosphere. While the tubes carry
most of the Evershed flow, the background is essentially at rest. We
notice also that the magnetic field strength in the flux tubes drops
much more slowly with radial distance than the background field. This
finding is discussed as a possible driver for the Evershed flow.
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Title: The fine structure of the sunspot penumbra
Authors: Borrero, Juan Manuel
2004PhDT.......307B Altcode:
No abstract at ADS
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Title: Three dimensional structure of a regular sunspot from the
inversion of IR Stokes profiles
Authors: Mathew, S. K.; Lagg, A.; Solanki, S. K.; Collados, M.;
Borrero, J. M.; Berdyugina, S.; Krupp, N.; Woch, J.; Frutiger, C.
2003A&A...410..695M Altcode:
The magnetic, thermal and velocity structure of a regular sunspot,
observed close to solar disk center is presented. Spectropolarimetric
data obtained with the Tenerife Infrared Polarimeter (TIP) in two
infrared FeI lines at 15 648.5 Å and 15 652.8 Å are inverted
employing a technique based on response functions to retrieve the
atmospheric stratification at every point in the sunspot. In order
to improve the results for the umbra, profiles of Zeeman split OH
lines blending the FeI 15 652.8 Å are also consistently fit. Thus
we obtain maps of temperature, line-of-sight velocity, magnetic
field strength, inclination, and azimuth, as a function of both
location within the sunspot and height in the atmosphere. We present
these maps for an optical depth range between log tau<SUB>5</SUB> =
0 and log tau<SUB>5</SUB> = -1.5, where these lines provide accurate
results. We find decreasing magnetic field strength with increasing
height all over the sunspot, with a particularly large vertical field
gradient of ~ -4 G km<SUP>-1</SUP> in the umbra. We also observe the
so called “spine” structures in the penumbra, i.e. extended radial
features with a stronger and more vertical magnetic field than the
surroundings. Also we found that the magnetic field zenith angle
increases with height. From the velocity map it is clear that the
Evershed flow avoids the spines and mostly concentrates in the more
inclined intervening field. The field inclination at a few locations
in the outer penumbra in lower layers goes beyond 90<SUP>o</SUP>. These
locations coincide with the strongest flows in the velocity map.
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Title: Infrared Polarimetry at the MPAe: The Solar Atmosphere from
the Photosphere to the Upper Chromosphere
Authors: Lagg, Andreas; Woch, Joachim; Solanki, Sami K.; Mathew,
Shibu; Borrero, Juan M.; Krupp, N.; Raouafi, N. E.
2003ANS...324...29L Altcode: 2003ANS...324..D04L
No abstract at ADS
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Title: Accurate atomic parameters for near-infrared spectral lines
Authors: Borrero, J. M.; Bellot Rubio, L. R.; Barklem, P. S.; del
Toro Iniesta, J. C.
2003A&A...404..749B Altcode:
A realistic two-component model of the quiet solar photosphere is
used to fit the intensity spectrum of the Sun in the wavelength
range 0.98-1.57 mu m. Our approach differs from earlier attempts in
many respects: proper account of convective inhomogeneities is made,
accurate collisional broadening parameters from quantum mechanical
computations are used, and the effects of possible blends in the local
continuum are corrected empirically. This allows us to derive oscillator
strengths and central wavelengths for virtually any unblended line of
the solar spectrum. The accuracy of the inferred atomic parameters,
about 0.06 dex for oscillator strengths and 5 mÅ at 1 mu m for central
wavelengths, is similar to that of the best laboratory measurements. We
apply our method to 83 near-infrared lines belonging to 6 different
atomic species. The availability of accurate oscillator strengths and
central wavelengths for lines of different species is essential for
the interpretation of high resolution spectroscopic observations. The
method is especially useful in the infrared, a wavelength domain where
laboratory measurements are scarce.
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Title: Two-Component Modeling of Convective Motions in the Solar
Photosphere and Determination of Atomic Parameters
Authors: Borrero, J. M.; Bellot Rubio, L. R.
2003IAUS..210P..C9B Altcode:
No abstract at ADS
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Title: Accurate Atomic Parameters from the Solar Spectrum
Authors: Bellot Rubio, Luis Ramon; Borrero, Juan Manuel; Barklem,
Paul; del Toro Iniesta, Jose Carlos
2003IAUJD..20E..16B Altcode:
A realistic two-component model of the quiet solar photosphere is used
to fit the full shape of the intensity profiles of unblended lines in
the solar spectrum. Our approach differs from previous attempts in many
respects: proper account of granulation inhomogeneities is made accurate
collisional broadening parameters from quantum mechanical computations
are used and possible absorptions in the local continuum due to blends
are corrected empirically. This allows us to derive oscillator strengths
and central wavelengths for any clean line with an accuracy comparable
with that of the best laboratory measurements. The availability of
very precise atomic parameters for lines of different species is
essential for the interpretation of high resolution spectroscopic
observations. Abundance determinations and investigations of granular
motions in stellar atmospheres are among the applications that would
benefit from such accurate atomic data. As an example we determine the
oscillator strengths and central wavelengths of 100 unblended lines
in the near-infrared (0.99-1.56 microns) a wavelength domain where
laboratory measurements are particularly scarce.
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Title: Modeling the Fine Structure of a Sunspot Penumbra through
the Inversion of Stokes Profiles
Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Frutiger, C.;
Collados, M.; Bellot Rubio, L. R.
2003ASPC..286..235B Altcode: 2003ctmf.conf..235B
No abstract at ADS
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Title: Iron abundance in the solar photosphere. Application of a
two-component model atmosphere
Authors: Bellot Rubio, L. R.; Borrero, J. M.
2002A&A...391..331B Altcode:
A realistic two-component model of the quiet Sun is used to determine
the solar abundance of iron from the inversion of a number of Fe I and
Fe II spectral lines for which accurate atomic parameters (oscillator
strengths, central wavelengths, and collisional broadening cross
sections) exist. From 33 Fe I lines we infer an abundance of A_Fe =
7.43 +/- 0.06, whereas we estimate A_Fe = 7.45 +/- 0.08 from 10 Fe
II lines. These values are in excellent agreement with the results of
analyses based on realistic 3D hydrodynamical simulations of the solar
granulation, and imply a low photospheric iron abundance. We investigate
the effects of convective motions and granular temperatures and conclude
that both are important for reliable abundance determinations. For Fe
I lines, the effects of convective motions can be simulated by using
a microturbulent velocity of about 1 km s<SUP>-1</SUP>, whereas it
is possible to account for temperature inhomogeneities by adopting an
average temperature stratification which is cooler than the Holweger
& Müller model in the upper layers.
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Title: A two-component model of the solar photosphere from the
inversion of spectral lines
Authors: Borrero, J. M.; Bellot Rubio, L. R.
2002A&A...385.1056B Altcode:
A two-component model of the solar photosphere is obtained from the
inversion of the intensity profiles of 22 Fe I spectral lines for which
very accurate atomic data (oscillator strengths, central wavelengths,
and collisional broadening parameters) exist. The model is meant to
describe the effects of convective motions in the solar photosphere. It
has been subject to various tests to confront its predictions with
observations of the solar spectrum. The model is able to reproduce the
observed line shifts and equivalent widths of about 800 spectral lines
of iron and other species. It is also capable of matching the observed
center-to-limb variation of the continuum intensity with unprecedented
accuracy. This allows us to determine line-transition parameters from
the fitting of the solar spectrum. Exploratory calculations demonstrate
that the model can be used to derive transition probabilities and
central wavelengths of Fe I and Fe II lines, as well as other elements,
within the uncertainties of the best laboratory measurements.