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Locarno, Switzerland

Carlin E.S.,Istituto Ricerche Solari Locarno
Proceedings of the International Astronomical Union | Year: 2015

Is it physically feasible to perform the chromospheric diagnosis using spatial maps of scattering polarization at the solar disk center? To investigate it we synthesized polarization maps (in 8542 Å) resulting from MHD solar models and NLTE radiative transfer calculations that consider Hanle effect and vertical macroscopic motions. After explaining the physical context of forward scattering and presenting our results, we arrive at the definition of Hanle polarity inversion lines. We show how such features can give support for a clearer chromospheric diagnosis in which the magnetic and dynamic effects in the scattering polarization could be disentangled. Copyright © 2015 International Astronomical Union.

Carlin E.S.,Istituto Ricerche Solari Locarno | Ramos A.A.,Institute of Astrophysics of Canarias | Ramos A.A.,University of La Laguna
Astrophysical Journal | Year: 2015

This paper presents a synthetic tomography of the quiet solar chromosphere formed by spatial maps of scattering polarization. It has been calculated for the Ca II 8498, 8542, and 3934 Å lines by solving the non-LTE radiative transfer problem of the second kind in a three-dimensional atmosphere model obtained from realistic magneto-hydrodynamical simulations. Our investigation focuses on the linear polarization signals induced by kinematics, radiation field anisotropy, and the Hanle effect in forward-scattering geometry. Thus, instead of considering slit profiles at the limb as normally done in the study of the second solar spectrum, we synthesize and analyze spatial maps of polarization at the disk center. This allows us to understand the spatial signatures of dynamics and magnetic field in the linear polarization in order to discriminate them observationally. Our results suggest some ideas for chromospheric diagnosis that will be developed throughout a series of papers. In particular, Hanle polarity inversion lines and dynamic Hanle diagrams are two concepts introduced in the present work. We find that chromospheric dynamics and magnetic field topology create spatial polarization fingerprints that trace the dynamic situation of the plasma and the magnetic field. This allows us to reconstruct the magnetic field intensity in the middle chromosphere using Stokes V along grooves of null linear polarization. We finally address the problems of diagnosing Hanle saturation and kinematic amplification of scattering signals using Hanle diagrams. © 2015. The American Astronomical Society. All rights reserved..

Ishikawa R.,Japan National Astronomical Observatory | Asensio Ramos A.,Institute of Astrophysics of Canarias | Belluzzi L.,Istituto Ricerche Solari Locarno | Belluzzi L.,Kiepenheuer Institute for Solar Physics | And 5 more authors.
Astrophysical Journal | Year: 2014

Magnetic field measurements in the upper chromosphere and above, where the gas-to-magnetic pressure ratio β is lower than unity, are essential for understanding the thermal structure and dynamical activity of the solar atmosphere. Recent developments in the theory and numerical modeling of polarization in spectral lines have suggested that information on the magnetic field of the chromosphere-corona transition region could be obtained by measuring the linear polarization of the solar disk radiation at the core of the hydrogen Lyα line at 121.6 nm, which is produced by scattering processes and the Hanle effect. The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) sounding rocket experiment aims to measure the intensity (Stokes I) and the linear polarization profiles (Q/I and U/I) of the hydrogen Lyα line. In this paper, we clarify the information that the Hanle effect can provide by applying a Stokes inversion technique based on a database search. The database contains all theoretical Q/I and U/I profiles calculated in a one-dimensional semi-empirical model of the solar atmosphere for all possible values of the strength, inclination, and azimuth of the magnetic field vector, though this atmospheric region is highly inhomogeneous and dynamic. We focus on understanding the sensitivity of the inversion results to the noise and spectral resolution of the synthetic observations as well as the ambiguities and limitation inherent to the Hanle effect when only the hydrogen Lyα is used. We conclude that spectropolarimetric observations with CLASP can indeed be a suitable diagnostic tool for probing the magnetism of the transition region, especially when complemented with information on the magnetic field azimuth that can be obtained from other instruments. © 2014. The American Astronomical Society. All rights reserved..

Belluzzi L.,Istituto Ricerche Solari Locarno | Belluzzi L.,Kiepenheuer Institute for Solar Physics | Landi Degl'Innocenti E.,University of Florence | Bueno J.T.,Institute of Astrophysics of Canarias | Bueno J.T.,University of La Laguna
Astrophysical Journal | Year: 2015

A correct modeling of the scattering polarization profiles observed in some spectral lines of diagnostic interest, the sodium doublet being one of the most important examples, requires taking hyperfine structure (HFS) and quantum interference between different J-levels into account. An atomic model suitable for taking these physical ingredients into account is the so-called multiterm atom with HFS. In this work, we introduce and study the transfer and relaxation rates due to isotropic inelastic collisions with electrons, which enter the statistical equilibrium equations (SEE) for the atomic density matrix of this atomic model. Under the hypothesis that the electron-atom interaction is described by a dipolar operator, we provide useful relations between the rates describing the transfer and relaxation of quantum interference between different levels (whose numerical values are in most cases unknown) and the usual rates for the atomic level populations, for which experimental data and/or approximate theoretical expressions are generally available. For the particular case of a two-term atom with HFS, we present an analytical solution of the SEE for the spherical statistical tensors of the upper term, including both radiative and collisional processes, and we derive the expression of the emission coefficient in the four Stokes parameters. Finally, an illustrative application to the Na i D1 and D2 lines is presented. © 2015. The American Astronomical Society. All rights reserved..

Kleint L.,ETH Zurich | Berdyugina S.V.,Kiepenheuer Institute for Solar Physics | Shapiro A.I.,World Radiation Center | Bianda M.,Istituto Ricerche Solari Locarno
Astronomy and Astrophysics | Year: 2010

Context. Small-scale, weak magnetic fields are ubiquitous in the quiet solar atmosphere. Yet their properties and temporal and spatial variations are not well known. Aims. We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle. Methods. Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5″ on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of ∼10-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141Å region including two C2 triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C2R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields. Results. We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3-8 G. Conclusions. Our results demonstrate the potential of long-term observations of small-scale magnetic fields, which may vary with the solar cycle in both mean strength and spatial distribution. This provides important constraints on the energy budget of the solar cycle. Extending this synoptic program to many spectral lines would provide a sample of heights in the solar atmosphere. © 2010 ESO.

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