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ABSTRACT: We study the presence of reversed polarity magnetic flux in sunspot penumbra.
We applied a new regularized method to deconvolve spectropolarimetric data
observed with the spectropolarimeter SP onboard Hinode. The new regularization
is based on a principal component decomposition of the Stokes profiles. The
resulting Stokes profiles were inverted to infer the magnetic field vector
using SIR. We find, for the first time, reversed polarity fields at the border
of many bright penumbral filaments in the whole penumbra.
11/2012;
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ABSTRACT: A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun
at disk center, observed at the German Vacuum Tower Telescope (Observatorio del
Teide), was used to study the global and spatial variations of different line
parameters. In order to determine the vertical structure of the photosphere two
lines with well separated formation heights have been considered. The data have
been filtered of p-modes to isolate the pure convective phenomenon. From our
studies of global correlation coefficients and coherence and phase shift
analyzes between the several line parameters, the following results can be
reported. The convective velocity pattern preserves structures larger than 1.0"
up to the highest layers of the photosphere (~ 435 km). However, at these
layers, in the intensity pattern only structures larger than 2.0" are still
connected with those at the continuum level although showing inverted
brightness contrast. This confirms an inversion of temperature that we have
found at a height of ~140 km. A possible evidence of gravity waves superimposed
to the convective motions is derived from the phase shift analysis. We
interpret the behavior of the full width at half maximum and the equivalent
width as a function of the distance to the granular borders, as a consequence
of enhanced turbulence and/or strong velocity gradients in the intergranular
lanes.
05/2012;
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ABSTRACT: From the inversion of a time series of high resolution slit spectrograms
obtained from the quiet sun, the spatial and temporal distribution of the
thermodynamical quantities and the vertical flow velocity is derived as a
function of logarithmic optical depth and geometrical height. Spatial coherence
and phase shift analyzes between temperature and vertical velocity depict the
height variation of these physical quantities for structures of different size.
An average granular cell model is presented, showing the granule-intergranular
lane stratification of temperature, vertical velocity, gas pressure and density
as a function of logarithmic optical depth and geometrical height. Studies of a
specific small and a specific large granular cell complement these results. A
strong decay of the temperature fluctuations with increasing height together
with a less efficient penetration of smaller cells is revealed. The T -T
coherence at all granular scales is broken already at log tau =-1 or z~170 km.
At the layers beyond, an inversion of the temperature contrast is revealed.
Vertical velocities are in phase throughout the photosphere and penetrate into
the highest layers under study.
05/2012;
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ABSTRACT: The aim of this work is the determination of the twist, writhe, and self-magnetic helicity of penumbral filaments located in an inner sunspot penumbra. For this purpose, we inverted data taken with the spectropolarimeter on board Hinode with the SIR (Stokes Inversion based on Response function) code. For the construction of a three-dimensional geometrical model we applied a genetic algorithm minimizing the divergence of and the net magnetohydrodynamic force, consequently a force-free solution would be reached if possible. We estimated two proxies to the magnetic helicity frequently used in literature: the force-free parameter α z and the current helicity term . We show that both proxies are only qualitative indicators of the local twist as the magnetic field in the area under study significantly departs from a force-free configuration. The local twist shows significant values only at the borders of bright penumbral filaments with opposite signs on each side. These locations are precisely correlated to large electric currents. The average twist (and writhe) of penumbral structures is very small. The spines (dark filaments in the background) show a nearly zero writhe. The writhe per unit length of the intraspines diminishes with increasing length of the tube axes. Thus, the axes of tubes related to intraspines are less wrung when the tubes are more horizontal. As the writhe of the spines is very small, we can conclude that the writhe reaches only significant values when the tube includes the border of an intraspine.
The Astrophysical Journal 01/2012; 745(2):141. · 6.02 Impact Factor
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ABSTRACT: We determine the entire electrical current density vector in a geometrical 3D
volume of the inner penumbra of a sunspot from an inversion of
spectropolarimetric data obtained with Hinode/SP. Significant currents are seen
to wrap around the hotter, more elevated regions with lower and more horizontal
magnetic field that harbor strong upflows and radial outflows (the
intraspines). The horizontal component of the current density vector is 3-4
times larger than the vertical; nearly all previous studies only obtain the
vertical component and thus strongly underestimate the current density. The
current density and the magnetic field vectors form an angle of about 20
degrees. The plasma beta at the 0 km level is larger than 1 in the intraspines
and is one order of magnitude lower in the background component of the penumbra
(spines). At the 200 km level, the plasma beta is below 0.3 nearly everywhere.
The plasma beta surface as well as the surface optical depth unity are very
corrugated. At the borders of intraspines and inside, the magnetic field is not
force-free at deeper layers and nearly force free at the top layers. The
magnetic field of the spines is close to being potential everywhere. The
dissipated ohmic energy is five orders of magnitudes smaller than the solar
energy flux and thus negligible for the energy balance of the penumbra.
08/2010;
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ABSTRACT: Inversions of spectropolarimetric observations of penumbral filaments deliver
the stratification of different physical quantities in an optical depth scale.
However, without establishing a geometrical height scale their
three-dimensional geometrical structure can not be derived. This is crucial in
understanding the correct spatial variation of physical properties in the
penumbral atmosphere and to provide insights into the mechanism capable of
explaining the observed penumbral brightness. The aim of this work is to
determine a global geometrical height scale in the penumbra by minimizing the
divergence of the magnetic field vector and the deviations from static
equilibrium as imposed by a force balance equation that includes pressure
gradients, gravity and the Lorentz force. Optical depth models are derived from
the SIR inversion of spectropolarimetric data of an active region observed with
SOT on-board the Hinode satellite. We use a genetic algorithm to determine the
boundary condition for the inference of geometrical heights. The retrieved
geometrical height scale permits the evaluation of the Wilson depression at
each pixel and the correlation of physical quantities at each height. Our
results fit into the uncombed penumbral scenario, i.e., a penumbra composed of
flux tubes with channelled mass flow and with a weaker and more horizontal
magnetic field as compared with the background field. The ascending material is
hotter and denser than their surroundings. We do not find evidence of
overturning convection or field free regions in the inner penumbral area
analyzed. The penumbral brightness can be explained by the energy transfer of
the ascending mass carried by the Evershed flow, if the physical quantities
below z=-75km are extrapolated from the results of the inversion.
07/2010;
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ABSTRACT: In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the atmospheric conditions. We introduce the fixed departure coefficients (FDC) approximation in order to compute these response functions in a fast and straightforward manner. The accuracy of this approximation is checked comparing FDC response functions with those obtained from full NLTE computations. An NLTE inversion code based on these response functions has been developed and extensively tested. Reference synthetic profiles, similar to those expected from real observations, are given as input to the inversion algorithm and the recovered models are shown to be compatible with the reference models within the error bars. Our NLTE inversion code thus provides a new tool for the investigation of the chromospheres of the Sun and other stars.
The Astrophysical Journal 01/2009; 507(1):470. · 6.02 Impact Factor
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ABSTRACT: We present results of the inversion of spatially averaged Stokes I and V profiles emerging from plage regions near disk center. The observations analyzed in this work were recorded with the Advanced Stokes Polarimeter. All atmospheric quantities determining the radiative transfer in the thin flux-tube approximation are inferred self-consistently with the help of the inversion code described by Bellot Rubio et al. With regard to thermodynamics, the retrieved model atmospheres are found to behave as expected on theoretical grounds. For the first time, velocities inside and outside the tubes have been derived empirically. The magnetic atmospheres resulting from the inversion are characterized by the absence of significant motions in high layers but show strong velocity gradients in deeper layers. These gradients turn out to be essential for reproducing the whole shape of the observed profiles and, in particular, the asymmetries and the extended red tail of Stokes V. Our scenario predicts that the Stokes V zero-crossing wavelengths of Fe I and Fe II lines are redshifted by small but nonnegligible amounts, which is indeed confirmed by observations made with the Fourier Transform Spectrometer. According to recent numerical simulations, the internal downflows derived from the inversion could be produced by the strong shear that takes place in the intermediate layer between the magnetized interior and the ambient medium. Another possible origin is magnetic flux undergoing convective collapse within the resolution element.
The Astrophysical Journal 12/2008; 535(1):489. · 6.02 Impact Factor
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ABSTRACT: We describe a new LTE inversion code for the analysis of Stokes profiles emerging from unresolved magnetic elements. It has been specifically designed to obtain the thermal, dynamic, and magnetic properties of these structures in a self-consistent manner by fitting the whole shape of the observed spectra. The inversion code is based on a previous scheme by Ruiz Cobo & del Toro Iniesta and implements the thin flux-tube model as a reasonable description of reality. All physical parameters considered relevant for the problem (including velocity fields) are retrieved by means of a Marquardt nonlinear least-squares algorithm. We present the results of extensive tests aimed at characterizing the behavior of the code so as to understand its limitations for the analysis of real observations. The code is found to produce accurate results even with only two spectral lines and noisy Stokes I and V profiles. A detailed error treatment, in which the covariances between parameters are explicitly included, is also carried out in order to investigate the uniqueness and reliability of the inferred model atmospheres.
The Astrophysical Journal 12/2008; 535(1):475. · 6.02 Impact Factor
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ABSTRACT: 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.
The Astrophysical Journal 12/2008; 674(1):596. · 6.02 Impact Factor
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ABSTRACT: We report on the detection of magnetic field strength and velocity oscillations in the photosphere of a sunspot umbra. Our analysis is based on the inversion of the full Stokes vector of three Fe I lines at 15650 Å, from which the stratification with optical depth of the different atmospheric parameters has been derived. This allows us to estimate the amplitude of the oscillations and the phase lag between the fluctuations in the line-of-sight velocity and field strength. Our results suggest that the inferred magnetic field oscillations are caused by opacity fluctuations that move upward and downward the region where the spectral lines are sensitive to magnetic fields.
The Astrophysical Journal 12/2008; 534(2):989. · 6.02 Impact Factor
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ABSTRACT: High-resolution observations of sunspots have revealed the existence of dark cores inside the bright filaments of the penumbra. Here we present the stationary solution of the heat transfer equation in a stratified penumbra consisting of nearly horizontal magnetic flux tubes embedded in a stronger and more vertical field. The tubes and the external medium are in horizontal mechanical equilibrium. This model produces bright filaments with dark cores as a consequence of the higher density of the plasma inside the flux tube, which shifts the surface of optical depth unity toward higher (cooler) layers. Our results suggest that the surplus brightness of the penumbra is a natural consequence of the Evershed flow, and that magnetic flux tubes about 250 km in diameter can explain the morphology of sunspot penumbra.
11/2008;
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ABSTRACT: We analyse spectropolarimetric observations of the penumbra of the NOAA AR
10953 at high spatial resolution (0.3"). The full Stokes profiles of the Fe I
lines at 630.1 nm and 630.2 nm have been obtained with the Solar Optical
Telescope (SOT) on board the Hinode satellite. The data have been inverted by
means of the SIR code, deriving the stratifications of temperature, line of
sight velocity, and the components of the magnetic field vector in optical
depth. In order to evaluate the gas pressure and to obtain an adequate
geometrical height scale, the motion equation has been integrated for each
pixel taking into account the terms of the Lorentz force. To establish the
boundary condition, a genetic algorithm has been applied. The final resulting
magnetic field has a divergence compatible with 0 inside its uncertainties.
First analyses of the correlation of the Wilson depression with velocity,
temperature, magnetic field strength, and field inclination strongly support
the uncombed penumbral model proposed by Solanki & Montavon (1993).
10/2008;
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ABSTRACT: Context: Observations at 0.1" have revealed the existence of dark cores in the bright filaments of sunspot penumbrae. Expectations are high that such dark-cored filaments are the basic building blocks of the penumbra, but their nature remains unknown. Aims: We investigate the origin of dark cores in penumbral filaments and the surplus brightness of the penumbra. To that end we use an uncombed penumbral model. Methods: The 2D stationary heat transfer equation is solved in a stratified atmosphere consisting of nearly horizontal magnetic flux tubes embedded in a stronger and more vertical field. The tubes carry an Evershed flow of hot plasma. Results: This model produces bright filaments with dark cores as a consequence of the higher density of the plasma inside the tubes, which shifts the surface of optical depth unity toward higher (cooler) layers. Our calculations suggest that the surplus brightness of the penumbra is a natural consequence of the Evershed flow, and that magnetic flux tubes about 250 km in diameter can explain the morphology of sunspot penumbrae.
07/2008;
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ABSTRACT: The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 \AA has 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 \AA 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 \AA 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 magneto-hydrodynamical constrains. When this is done, the 5250 \AA lines do not seem to provide any significant advantage over those at 6302 \AA . 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 \AA) with identical line opacities that seem to work much better than the others.
11/2007;
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ABSTRACT: We study the contradictory magnetic field strength distributions retrieved from independent analyses of spectropolarimetric observations in the near-infrared (1.56 micron) and in the visible (630 nm) at internetwork regions. In order to solve this apparent controversy, we present simultaneous and co-spatial 1.56 micron and 630 nm observations of an internetwork area. The properties of the circular and linear polarization signals, as well as the Stokes V area and amplitude asymmetries, are discussed. As a complement, inversion techniques are also used to infer the physical parameters of the solar atmosphere. As a first step, the infrared and visible observations are analysed separately to check their compatibility. Finally, the simultaneous inversion of the two data sets is performed. The magnetic flux densities retrieved from the individual analysis of the infrared and visible data sets are strongly correlated. The polarity of the Stokes V profiles is the same at co-spatial pixels in both wavelength ranges. This indicates that both 1.56 micron and 630 nm observations trace the same magnetic structures on the solar surface. The simultaneous inversion of the two pairs of lines reveals an internetwork full of sub-kG structures that fill only 2 % of the resolution element. A correlation is found between the magnetic field strength and the continuum intensity: equipartition fields (B ~500 G) tend to be located in dark intergranular lanes, whereas weaker field structures are found inside granules. The most probable unsigned magnetic flux density is 10 Mx/cm2. The net magnetic flux density in the whole field of view is nearly zero. This means that both polarities cancel out almost exactly in our observed internetwork area. Comment: Accepted for publication in A&A
11/2007;
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ABSTRACT: This paper reports on our new findings from the International Time
Program observations at the Canaries islands, Spain, in July 2005. We
have found small-scale photospheric events with extremely red-shifted
Stokes V signals in sunspot moat regions. A preliminary estimate of the
physical conditions for an observed Stokes V profile indicates the
presence of a downward motion with a supersonic speed in the order of 10
km/s. With the currently evaluated observational information, we
interprete the supersonic flows as downward motion from magnetic
reconnection occurring at the upper chromosphere or lower photosphere.
With coordinated observations of the Solar-B onboard telescopes, Stokes
measurements by the SOT spectro-polarimeter would give new information
for further understanding the nature of these events with strongly
red-shifted Stokes V, and for discussing the physical conditions
involving in possible magnetic reconnections in the lower solar
atmosphere.
09/2007; 369:113.
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ABSTRACT: The aim of this work is to study the structure of the magnetic field vector in the internetwork and search for the presence of small-scale loops. We invert 1.56 micron spectropolarimetric observations of internetwork regions at disc centre by applying the SIR code. This allows us to recover the atmospheric parameters that play a role in the formation of these spectral lines. We are mainly interested in the structure of the magnetic field vector. We find that many opposite polarity elements of the internetwork are connected by short (2-6''), low-lying (photospheric) loops. These loops connect at least the 10-20 % of the internetwork flux visible in our data. Also we have some evidence that points towards a dynamic scenario which can be produced by the emergence of internetwork magnetic flux. Comment: Accepted for publication in A&A letters
05/2007;
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ABSTRACT: We took simultaneous observations in Fe I 1.5 µ and Fe I 630 nm
of an internetwork region. We discuss the compatibility of the data in
the two spectral ranges, and present results for the magnetic field
distribution. We show that the larger contribution comes from hG field
strengths.
11/2006; 358:36.
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ABSTRACT: The purpose of this work is to analyze the reliability of the magnetic field strengths inferred from the 630 nm pair of Fe I lines at internetwork quiet Sun regions. Some numerical experiments have been performed that demonstrate the inability of these lines to recover the magnetic field strength in such low flux solar regions. It is shown how different model atmospheres, with magnetic field strengths ranging from few hundred Gauss to kiloGauss, give rise to Stokes profiles that can not be distinguished. The reasons for this degeneracy are discussed. Comment: Accepted for publication in A&A
05/2006;
