arXiv subjects

J. Trujillo Bueno

Publications and source records attributed to J. Trujillo Bueno.

At least 73 records · Page 4Linked to original sources

arXiv2007

Towards the Modelling of the Second Solar Spectrum

This paper addresses the modelling issue of the linearly-polarized solar limb spectrum, which is due to scattering processes and it offers a rich diagnostic potential for exploring solar magnetic fields via the Hanle effect. However, this so-called second solar spectrum is full of mysterious spectral features, which cannot be understood with simplified polarization transfer theories, thus suggesting that the underlying scattering physics is more complex than previously thought. In this paper we argue that the physical understanding of the second solar spectrum requires the consideration of scattering processes in multilevel atomic models, taking fully into account the transfer of atomic polarization among all the levels involved. The consideration of lower-level atomic polarization leads to non-linear and non-local coupled sets of equations, even for the two-level model atom case considered in this paper. The unknowns of the problem are the irreducible tensor components of the atomic density matrix whose self-consistent values have first to be obtained to be able to calculate the emergent Stokes profiles. To solve numerically this non-LTE problem of the second kind we present some iterative methods that are very suitable for developing a general multilevel scattering polarization code. We demonstrate that there exists metastable-level atomic polarization in the solar chromosphere, which suggests that the solution to some recently-formulated "paradoxes" is to be found by carefully revising our current ideas about the chromospheric magnetic field.

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arXiv2007

Three-dimensional Radiative Transfer with Multilevel Atoms

The efficient numerical solution of Non-LTE multilevel transfer problems requires the combination of highly convergent iterative schemes with fast and accurate formal solution methods of the radiative transfer (RT) equation. This contribution begins presenting a method for the formal solution of the RT equation in three-dimensional (3D) media with horizontal periodic boundary conditions. This formal solver is suitable for both, unpolarized and polarized 3D radiative transfer and it can be easily combined with the iterative schemes for solving non-LTE multilevel transfer problems that we have developed over the last few years. We demonstrate this by showing some schematic 3D multilevel calculations that illustrate the physical effects of horizontal radiative transfer. These Non-LTE calculations have been carried out with our code MUGA 3D, a 3D multilevel Non-LTE code based on the Gauss-Seidel iterative scheme that Trujillo Bueno and Fabiani Bendicho (1995) developed for RT applications.

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arXiv2007

The Hanle Effect in 1D, 2D and 3D

This paper addresses the problem of scattering line polarization and the Hanle effect in one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) media for the case of a two-level model atom without lower-level polarization and assuming complete frequency redistribution. The theoretical framework chosen for its formulation is the QED theory of Landi Degl'Innocenti (1983), which specifies the excitation state of the atoms in terms of the irreducible tensor components of the atomic density matrix. The self-consistent values of these density-matrix elements is to be determined by solving jointly the kinetic and radiative transfer equations for the Stokes parameters. We show how to achieve this by generalizing to Non-LTE polarization transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho (1995). These methods essentially maintain the simplicity of the Lambda-iteration method, but their convergence rate is extremely high. Finally, some 1D and 2D model calculations are presented that illustrate the effect of horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance line polarization signals.

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arXiv2007

Spicule emission profiles observed in \ion{He}{i} 10830 Å

Off-the-limb observations with high spatial and spectral resolution will help us understand the physical properties of spicules in the solar chromosphere Spectropolarimetric observations of spicules in the \ion{He}{i} 10830 Å multiplet were obtained with the Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope at the Observatorio del Teide (Tenerife, Spain). The analysis shows the variation of the off-limb emission profiles as a function of the distance to the visible solar limb. The ratio between the intensities of the blue and the red components of this triplet $({\cal R}=I_{\rm blue}/I_{\rm red})$ is an observational signature of the optical thickness along the light path, which is related to the intensity of the coronal irradiation. We present observations of the intensity profiles of spicules above a quiet Sun region. The observable ${\cal R}$ as a function of the distance to the visible limb is also given. We have compared our observational results to the intensity ratio obtained from detailed radiative transfer calculations in semi-empirical models of the solar atmosphere assuming spherical geometry. The agreement is purely qualitative. We argue that future models of the solar chromosphere and transition region should account for the observational constraints presented here.

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arXiv2007

Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?

The physical interpretation of scattering line polarization offers a novel diagnostic window for exploring the thermal and magnetic structure of the quiet regions of the solar atmosphere. Here we evaluate the impact of isotropic collisions with neutral hydrogen atoms on the scattering polarization signals of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the IR triplet of Ca II, with emphasis on the collisional transfer rates between nearby J-levels. To this end, we calculate the linear polarization produced by scattering processes considering realistic multilevel models and solving the statistical equilibrium equations for the multipolar components of the atomic density matrix. We confirm that the lower levels of the 13 lines of multiplet 42 of Ti I are completely depolarized by elastic collisions. We find that upper-level collisional depolarization turns out to have an unnoticeable impact on the emergent linear polarization amplitudes, except for the ${λ4536$ line for which it is possible to notice a rather small depolarization caused by the collisional transfer rates. Concerning the Ca II lines, we show that the collisional rates play no role on the polarization of the upper level of the K line, while they have a rather small depolarizing effect on the atomic polarization of the metastable lower levels of the Ca II IR triplet.

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arXiv2007

A User-Friendly Code to Diagnose Chromospheric Plasmas

The physical interpretation of spectropolarimetric observations of lines of neutral helium, such as those of the 10830 A multiplet, represents an excellent opportunity for investigating the magnetism of plasma structures in the solar chromosphere. Here we present a powerful forward modeling and inversion code that permits either to calculate the emergent intensity and polarization for any given magnetic field vector or to infer the dynamical and magnetic properties from the observed Stokes profiles. This diagnostic tool is based on the quantum theory of spectral line polarization, which self-consistently accounts for the Hanle and Zeeman effects in the most general case of the incomplete Paschen-Back effect regime. We also take into account radiative transfer effects. An efficient numerical scheme based on global optimization methods has been applied. Our Stokes inversion code permits a fast and reliable determination of the global minimum.

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arXiv2006

The Hanle Effect in Atomic and Molecular Lines: A New Look at the Sun's Hidden Magnetism

This paper reviews some of the most recent advances in the application of the Hanle effect to solar physics, and how these developments are allowing us to explore the magnetism of the photospheric regions that look ``empty'' in solar magnetograms--that is, the Sun's ``hidden'' magnetism. In particular, we show how a joint analysis of the Hanle effect in atomic and molecular lines indicates that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere, carried mainly by tangled fields at sub-resolution scales with strengths between the equipartition field values and 1 kG.

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arXiv2006

A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere

We report on spectropolarimetric observations of a near-IR line of Mn I located at 15262.702 A whose intensity and polarization profiles are very sensitive to the presence of hyperfine structure. A theoretical investigation of the magnetic sensitivity of this line to the magnetic field uncovers several interesting properties. The most important one is that the presence of strong Paschen-Back perturbations due to the hyperfine structure produces an intensity line profile whose shape changes according to the absolute value of the magnetic field strength. A line ratio technique is developed from the intrinsic variations of the line profile. This line ratio technique is applied to spectropolarimetric observations of the quiet solar photosphere in order to explore the probability distribution function of the magnetic field strength. Particular attention is given to the quietest area of the observed field of view, which was encircled by an enhanced network region. A detailed theoretical investigation shows that the inferred distribution yields information on the average magnetic field strength and the spatial scale at which the magnetic field is organized. A first estimation gives ~250 G for the mean field strength and a tentative value of ~0.45" for the spatial scale at which the observed magnetic field is horizontally organized.

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arXiv2006

Influence of atomic polarization and horizontal illumination on the Stokes profiles of the He I 10830 multiplet

The polarization observed in the spectral lines of the He I 10830 multiplet carries valuable information on the dynamical and magnetic properties of plasma structures in the solar chromosphere and corona, such as spicules, prominences, filaments, emerging magnetic flux regions, etc. Here we investigate the influence of atomic level polarization on the emergent Stokes profiles for a broad range of magnetic field strengths, in both 90 degree and forward scattering geometry. We show that, contrary to a widespread belief, the selective emission and absorption processes caused by the presence of atomic level polarization may have an important influence on the emergent linear polarization, even for magnetic field strengths as large as 1000 G. Consequently, the modeling of the Stokes Q and U profiles should not be done by taking only into account the contribution of the transverse Zeeman effect within the framework of the Paschen-Back effect theory, unless the magnetic field intensity of the observed plasma structure is sensibly larger than 1000 G. We point out also that in low-lying optically thick plasma structures, such as those of active region filaments, the (horizontal) radiation field generated by the structure itself may substantially reduce the positive contribution to the anisotropy factor caused by the (vertical) radiation field coming from the underlying solar photosphere, so that the amount of atomic level polarization may turn out to be negligible. Only under such circumstances may the emergent linear polarization of the He I 10830 multiplet in such regions of the solar atmosphere be dominated by the contribution caused by the transverse Zeeman effect.

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arXiv2005

Very Efficient Methods for Multilevel Radiative Transfer in Atomic and Molecular Lines

The development of fast numerical methods for multilevel radiative transfer (RT) applications often leads to important breakthroughs in astrophysics, because they allow the investigation of problems that could not be properly tackled using the methods previously available. Probably, the most familiar example is the so-called Multilevel Accelerated $Λ$-Iteration (MALI) technique of Rybicki & Hummer for the case of a local approximate operator, which is based on Jacobi iteration. However, there are superior operator-splitting methods, based on Gauss-Seidel (GS) and Successive Overrelaxation (SOR) iteration, which provide a dramatic increase in the speed with which non-LTE multilevel transfer problems can be solved in one, two and three-dimensional geometries. Such RT methods, which were introduced by Trujillo Bueno & Fabiani Bendicho ten years ago, are the main subject of the first part of this paper. We show in some detail how they can be applied for solving multilevel RT problems in spherical geometry, for both atomic and molecular line transitions. The second part of the article addresses the issue of the calculation of the molecular number densities when the approximation of instantaneous chemical equilibrium turns out to be inadequate, which happens to be the case whenever the dynamical time scales of the astrophysical plasma under consideration are much shorter than the time needed by the molecules to form.

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arXiv2005

A Qualitative Interpretation of the Second Solar Spectrum of Ce ll

This is a theoretical investigation on the formation of the linearly polarized line spectrum of ionized cerium in the sun. We calculate the scattering line polarization pattern emergent from a plane-parallel layer of Ce {\sc ii} atoms illuminated from below by the photospheric radiation field, taking into account the differential pumping induced in the various magnetic sublevels by the anisotropic radiation field. We find that the line polarization pattern calculated with this simple model is in good qualitative agreement with reported observations. Interestingly, the agreement improves when some amount of atomic level depolarization is considered. We find that the best fit to the observations corresponds to the situation where the ground and metastable levels are depolarized to about one fifth of the corresponding value obtained in the absence of any depolarizing mechanism. One possibility to have this situation is that the depolarizing rate value of elastic collisions is exactly $D=10^6{\rm s}^{-1}$, which is rather unlikely. Therefore, we interpret that fact as} due to the presence of a turbulent magnetic field in the limit of saturated Hanle effect for the lower-levels. For this turbulent magnetic field we obtain a lower limit of 0.8 gauss and an upper limit of 200-300 gauss.

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arXiv2005

Evidence for Fine Structure in the Chromospheric Umbral Oscillation

Novel spectro-polarimetric observations of the \ion{He}{1} multiplet are used to explore the dynamics of the chromospheric oscillation above sunspot umbrae. The results presented here provide strong evidence in support of the two-component model proposed by Socas-Navarro and co-authors. According to this model, the waves propagate only inside channels of sub-arcsecond width (the ``active'' component), whereas the rest of the umbra remains nearly at rest (the ``quiet'' component). Although the observations support the fundamental elements of that model, there is one particular aspect that is not compatible with our data. We find that, contrary to the scenario as originally proposed, the active component remains through the entire oscillation cycle and harbors both the upflowing and the downflowing phase of the oscillation.

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arXiv2005

Evidence for collisional depolarization in the MgH lines of the second solar spectrum

Analysis of the Hanle effect in solar molecular lines allows us to obtain empirical information on hidden, mixed-polarity magnetic fields at subresolution scales in the (granular) upflowing regions of the `quiet' solar photosphere. Here we report that collisions seem to be very efficient in depolarizing the rotational levels of MgH lines. This has the interesting consequence that in the upflowing regions of the quiet solar photosphere the strength of the hidden magnetic field cannot be sensibly larger than 10 G, assuming the simplest case of a single valued microturbulent field that fills the entire upflowing photospheric volume. Alternatively, an equally good theoretical fit to the observed scattering polarization amplitudes can be achieved by assuming that the rate of depolarizing collisions is an order of magnitude smaller than in the previous collisionally dominated case, but then the required strength of the hidden field in the upflowing regions turns out to be unrealistically high. These constraints reinforce our previously obtained conclusion that there is a vast amount of hidden magnetic energy and unsigned magnetic flux localized in the (intergranular) downflowing regions of the quiet solar photosphere.

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arXiv2005

Theory and Modeling of the Zeeman and Paschen-Back effects in Molecular Lines

This paper describes a very general approach to the calculation of the Zeeman splitting effect produced by an external magnetic field on the rotational levels of diatomic molecules. The method is valid for arbitrary values of the total electronic spin and of the magnetic field strength -that is, it holds for molecular electronic states of any multiplicity and for both the Zeeman and incomplete Paschen-Back regimes. It is based on an efficient numerical diagonalization of the effective Zeeman Hamiltonian, which can incorporate easily all the contributions one may eventually be interested in, such as the hyperfine interaction of the external magnetic field with the spin motions of the nuclei. The reliability of the method is demonstrated by comparing our results with previous ones obtained via formulae valid only for doublet states. We also present results for molecular transitions arising between non-doublet electronic states, illustrating that their Zeeman patterns show signatures produced by the Paschen-Back effect.

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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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arXiv2005

Observation and modeling of anomalous CN polarization profiles produced by the molecular Paschen-Back effect in sunspots

We report novel spectropolarimetric observations of sunspots carried out with the Tenerife Infrared Polarimeter (TIP) in a near-IR spectral region around 15410 A, which is known to contain two groups of prominent OH lines that show circular polarization signals of opposite polarity. Surrounding these well-known OH lines, we have discovered the presence of CN lines of the $Δv=1$ band which show anomalous polarization profiles. Although the Stokes V signals of the OH lines are antisymmetric and with a sizable amplitude, the CN lines show almost negligible circular polarization. On the contrary, the linear polarization signals turn out to be much stronger in the CN lines than in the OH lines. Interestingly, these CN lines present striking antisymmetric linear polarization profiles, which we are able to explain and model via the Paschen-Back effect theory for diatomic molecules. The presence of such peculiar CN lines in the same spectral region of the OH lines may be useful to improve our empirical knowledge of solar magnetic fields via the simultaneous observation and modeling of the transverse and longitudinal Zeeman effects in two different molecular species.

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arXiv2005

Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization

The theory of the generation and transfer of polarized radiation, mainly developed for interpreting solar spectropolarimetric observations, allows to reconsider, in a more rigorous and elegant way, a physical mechanism that has been suggested some years ago to interpret the high degree of polarization often observed in astronomical masers. This mechanism, for which the name of 'dichroic maser' is proposed, can operate when a low density molecular cloud is illuminated by an anisotropic source of radiation (like for instance a nearby star). Here we investigate completely unsaturated masers and show that selective stimulated emission processes are capable of producing highly polarized maser radiation in a non-magnetic environment. The polarization of the maser radiation is linear and is directed tangentially to a ring equidistant to the central star. We show that the Hanle effect due to the presence of a magnetic field can produce a rotation (from the tangential direction) of the polarization by more that 45 degrees for some selected combinations of the strength, inclination and azimuth of the magnetic field vector. However, these very same conditions produce a drastic inhibition of the maser effect. The rotations of about 90 degrees observed in SiO masers in the evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al (2000) may then be explainedby a local modification of the anisotropy of the radiation field, being transformed from mainly radial to mainly tangential.

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arXiv2005

The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism

An attractive diagnostic tool for investigating the magnetism of the solar chromosphere is the observation and theoretical modeling of the Hanle and Zeeman effects in spicules, as shown in this letter for the first time. Here we report on spectropolarimetric observations of solar chromospheric spicules in the He I 10830 Åmultiplet and on their theoretical modeling accounting for radiative transfer effects. We find that the magnetic field in the observed (quiet Sun) spicular material at a height of about 2000 km above the visible solar surface has a strength of the order of 10 G and is inclined by approximately $35^{\circ}$ with respect to the local vertical direction. Our empirical finding based on full Stokes-vector spectropolarimetry should be taken into account in future magnetohydrodynamical simulations of spicules.

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