arXiv subjects

H. Socas-Navarro

Publications and source records attributed to H. Socas-Navarro.

52 records · Page 3Linked to original sources

arXiv2005

Spectro-polarimetric observations and non-LTE Modeling of Ellerman bombs

Ellerman bombs are bright emission features observed in the wings of H$α$, usually in the vicinity of magnetic concentrations. Here we show that they can also be detected in the Ca II infrared triplet lines, which are easier to interpret and therefore allow for more detailed diagnostics. We present full Stokes observations of the 849.8 and 854.2 nm lines acquired with the new spectro-polarimeter SPINOR. The data shows no significant linear polarization at the level of 3x10$^{-4}$. The circular polarization profiles exhibit measureable signals with a very intricate pattern of peaks. A non-LTE analysis of the spectral profiles emerging from these features reveal the presence of strong downflows (~10 km/s) in a hot layer between the upper photosphere and the lower chromosphere.

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arXiv2005

Solar Site Survey for the Advanced Technology Solar Telecope. I. Analysis of the Seeing Data

The site survey for the Advanced Technology Solar Telescope concluded recently after more than two years of data gathering and analysis. Six locations, including lake, island and continental sites, were thoroughly probed for image quality and sky brightness. The present paper describes the analysis methodology employed to determine the height stratification of the atmospheric turbulence. This information is crucial because day-time seeing is often very different between the actual telescope aperture (~30 m) and the ground. Two independent inversion codes have been developed to analyze simultaneously data from a scintillometer array and a solar differential image monitor. We show here the results of applying them to a sample subset of data from May 2003, which was used for testing. Both codes retrieve a similar seeing stratification through the height range of interest. A quantitative comparison between our analysis procedure and actual in situ measurements confirms the validity of the inversions. The sample data presented in this paper reveal a qualitatively different behavior for the lake sites (dominated by high-altitude seeing) and the rest (dominated by near-ground turbulence).

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arXiv2005

The Three-Dimensional Structure of a Sunspot Magnetic Field

Here we report on observations of the three-dimensional structure of a sunspot magnetic field from the photosphere to the chromosphere, obtained with the new visible/infrared spectro-polarimeter SPINOR. The observations, interpreted with a non-LTE modeling technique, reveal a surprisingly complex topology with areas of opposite-sign torsion, suggesting that flux-ropes of opposite helicities may coexist together in the same spot.

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arXiv2005

Are Electric Currents Heating the Magnetic Chromosphere?

This paper presents an analysis of three-dimensional vector currents and temperatures observed in a sunspot from the photosphere to the chromosphere, spanning a range of heights of approximately 1500 km. With this unique dataset, based on novel spectro-polarimetric observations of the 850 nm spectral region, it is possible to conduct for the first time an empirical study of the relation between currents and chromospheric heating. It is shown that, while resistive current dissipation contributes to heat the sunspot chromosphere, it is not the dominant factor. The heating effect of current dissipation is more important in the penumbra of the sunspot, but even there it is still a relatively modest contribution.

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arXiv2005

An Artificial Neural Network Approach to the Solution of Molecular Chemical Equilibrium

A novel approach is presented for the solution of instantaneous chemical equilibrium problems. The chemical equilibrium can be considered, due to its intrinsically local character, as a mapping of the three-dimensional parameter space spanned by the temperature, hydrogen density and electron density into many one-dimensional spaces representing the number density of each species. We take advantage of the ability of artificial neural networks to approximate non-linear functions and construct neural networks for the fast and efficient solution of the chemical equilibrium problem in typical stellar atmosphere physical conditions. The neural network approach has the advantage of providing an analytic function, which can be rapidly evaluated. The networks are trained with a learning set (that covers the entire parameter space) until a relative error below 1% is reached. It has been verified that the networks are not overtrained by using an additional verification set. The networks are then applied to a snapshot of realistic three-dimensional convection simulations of the solar atmosphere showing good generalization properties.

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arXiv2005

Polynomial Approximants for the Calculation of Polarization Profiles in the \ion{He}{1} 10830 ÅMultiplet

The \ion{He}{1} multiplet at 10830 Åis formed in the incomplete Paschen-Back regime for typical conditions found in solar and stellar atmospheres. The positions and strengths of the various components that form the Zeeman structure of this multiplet in the Paschen-Back regime are approximated here by polynomials. The fitting errors are smaller than $\sim10^{-2}$ mÅin the component positions and $\sim10^{-3}$ in the relative strengths. The approximant polynomials allow for a very fast implementation of the incomplete Paschen-Back regime in numerical codes for the synthesis and inversion of polarization profiles in this important multiplet.

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arXiv2004

Motivation and Initial Results from SPINOR in the Near Infrared

SPINOR is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic operation over a very broad range of wavelengths, from $\sim$400 up to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc). SPINOR is scheduled for commissioning by the end of 2005. In this paper we report on the current status of the project and present actual observations of active regions in the Ca II infrared triplet and the He I multiplet at 1083 nm.

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arXiv2004

Observational Evidence for Small-Scale Mixture of Weak and Strong Fields in the Quiet Sun

Three different maps of the quiet Sun, observed with the Advanced Stokes Polarimeter (ASP) and the Diffraction-Limited Stokes Polarimeter (DLSP), show evidence of strong ($\simeq$1700 G) and weak ($<$500 G) fields coexisting within the resolution element both at network and internetwork locations. The angular resolution of the observations is of 1\arcsec (ASP) and 0.\arcsec6 (DLSP), respectively. Even at the higher DLSP resolution, a significant fraction of the network magnetic patches harbor a mixture of strong and weak fields. Internetwork elements that exhibit kG fields when analyzed with a single-component atmosphere are also shown to harbor considerable amounts of weak fields. Only those patches for which a single-component analysis yields weak fields do not show this mixture of field strengths. Finally, there is a larger fractional area of weak fields in the convective upflows than in the downflows.

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arXiv2004

Feature Extraction Techniques for the Analysis of Spectral Polarization Profiles

This paper introduces a novel feature extraction technique for the analysis of spectral line Stokes profiles. The procedure is based on the use of an auto-associative artificial neural network containing non-linear hidden layers. The neural network extracts a small subset of parameters from the profiles (features), from which it is then able to reconstruct the original profile. This new approach is compared to two other procedures that have been proposed in previous works, namely principal component analysis and Hermitian function expansions. Depending on the target application, each one of these three techniques has some advantages and disadvantages, which are discussed here.

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arXiv2004

Strategies for Spectral Profile Inversion using Artificial Neural Networks

This paper explores three different strategies for the inversion of spectral lines (and their Stokes profiles) using artificial neural networks. It is shown that a straightforward approach in which the network is trained with synthetic spectra from a simplified model leads to considerable errors in the inversion of real observations. This problem can be overcome in at least two different ways that are studied here in detail. The first method makes use of an additional pre-processing auto-associative neural network to project the observed profile into the theoretical model subspace. The second method considers a suitable regularization of the neural network used for the inversion. These new techniques are shown to be robust and reliable when applied to the inversion of both synthetic and observed data, with errors typically below $\sim$100 G.

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arXiv2004

Shocks in the Quiet Solar Photosphere: A Rather Common Occurrence

We present observations of the quiet solar photosphere in the \ion{Fe}{1} lines at 6302 Åwhere at least four different spatial locations exhibit upwards-directed supersonic flows. These upflows can only be detected in the circular polarization profiles as a double-peaked structure in the blue lobe of both \ion{Fe}{1} lines. We have detected cases of either magnetic polarity in the data. The polarization signals associated with the upflows are very weak, which is probably why they had not been seen before in this type of observations. We propose that the observed flows are the signature of aborted convective collapse, similar to the case reported by \citeN{BRRHC+01}. Our data indicates that this phenomenon occurs frequently in the quiet Sun, which means that many magnetic elements (although the fraction is still unknown) are destroyed even before they are formed completely. The spectral signatures of supersonic upflows reported here are probably present in most spectro-polarimetric observations of sufficient signal-to-noise and spatial resolution.

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arXiv2004

Physical Properties of Spicules from Simultaneous Spectro-Polarimetric Observations of He I and Ca II Lines

We present full Stokes observations from SPINOR in the \ion{Ca}{2} infrared triplet and the \ion{He}{1} multiplet at 1083 nm from which some properties of spicules have been derived. There are important advantages in multi-line observations, particularly from different elements. We find that the orientation of the plane of polarization is very different for the Ca and He lines, which provides the first direct model-independent evidence for magnetic fields in spicules. Our data shows that the Ca and He lines have almost identical widths. Since the Ca atom is 10 times heavier than He, we are able to conclude that most of the broadening is non-thermal ($\simeq$16 km s$^{-1}$) and to set an upper limit of 13 kK to the spicular temperatures. The bisectors of the lines span a velocity range of over 15 km s$^{-1}$ for the He line and 30 km s$^{-1}$ for the Ca ones. The vertical gradient of line-of-sight velocities is also very different for both elements. We obtain 2.8 km s$^{-1}$ Mm$^{-1}$ from He versus 6.4 km s$^{-1}$ Mm$^{-1}$ from Ca. These properties, and others from similar observations, should be taken into account in future physical models of spicules.

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arXiv2004

Polarimetric Calibration of Large-Aperture Telescopes I: The Beam-Expansion Method

This paper describes a concept for the high-accuracy absolute calibration of the instrumental polarization introduced by the primary mirror of a large-aperture telescope. This procedure requires a small aperture with polarization calibration optics (e.g., mounted on the dome) followed by a lens that opens the beam to illuminate the entire surface of the mirror. The Jones matrix corresponding to this calibration setup (with a diverging incident beam) is related to that of the normal observing setup (with a collimated incident beam) by an approximate correction term. Numerical models of parabolic on-axis and off-axis mirrors with surface imperfections are used to explore its accuracy.

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arXiv2004

Polarimetric Calibration of Large-Aperture Telescopes II: The sub-aperture method

A new method for absolute polarimetric calibration of large telescopes is presented. The proposed method is highly accurate and is based on the calibration of a small sub-aperture, which is then extended to the full system by means of actual observations of an astronomical source. The calibration procedure is described in detail along with numerical simulations that explore its robustness and accuracy. The advantages and disadvantages of this technique with respect to other possible alternatives are discussed.

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arXiv2004

Signatures of Incomplete Paschen-Back Splitting in the Polarization Profiles of the He I 10830 multiplet

We investigate the formation of polarization profiles induced by a magnetic field in the He I multiplet at 1083,0 nm . Our analysis considers the Zeeman splitting in the incomplete Paschen-Back regime. The effects turn out to be important and produce measurable signatures on the profiles, even for fields significantly weaker than the level-crossing field ($\sim$400 G). When compared to profiles calculated with the usual linear Zeeman effect, the incomplete Paschen-Back profiles exhibit the following conspicuous differences: a) a non-Doppler blueshift of the Stokes V zero-crossing wavelength of the blue component; b) area and peak asymmetries, even in the absence of velocity and magnetic gradients; c) a $\sim$25% reduction in the amplitude of the red component. These features do not vanish in the weak field limit. The spectral signatures that we analyze in this paper may be found in previous observations published in the literature.

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arXiv2001

Magnetic properties of photospheric regions having very low magnetic flux

The magnetic properties of the quiet Sun are investigated using a novel inversion code, FATIMA, based on the Principal Component Analysis of the observed Stokes profiles. The stability and relatively low noise sensitivity of this inversion procedure allows for the systematic inversion of large data sets with very weak polarization signal. Its application to quiet Sun observations of network and internetwork regions reveals that a significant fraction of the quiet Sun contains kilogauss fields (usually with very small filling factors) and confirms that the pixels with weak polarization account for most of the magnetic flux. Mixed polarities in the resolution element are also found to occur more likely as the polarization weakens.

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