General
Magnetic fields pervade all astrophysical plasmas and govern most of the variability in the Universe at intermediate time scales. They are present in stars across the whole Hertzsprung-Russell diagram, in galaxies, and even perhaps in the intergalactic medium. Polarized light provides the most reliable source of information at our disposal for the remote sensing of astrophysical magnetic fields, including those on the Sun. In particular, the diagnostics of solar and stellar magnetic fields requires the measurement and physical interpretation of polarization signatures in spectral lines, which are induced by various physical mechanisms taking place at the atomic level. In addition to the familiar Zeeman effect, polarization can also be generated by various other physical processes, such as atomic level polarization induced by anisotropic pumping mechanisms, quantum interference between fine-structured or hyperfine-structured energy levels, the Hanle effect, etc. Interestingly, the polarization produced by such mechanisms is sensitive to the physical conditions of the astrophysical plasma under consideration and, in particular, to the presence of magnetic fields in a parameter domain that goes from field intensities as small as 1 micro-G to many thousands of Gauss.
The main objective of this project is to explore in depth the physics and origin of polarized radiation in astrophysical plasmas as well as its diagnostic use for understanding cosmical magnetic fields, with emphasis on the magnetism of the extended solar atmosphere. Our investigations deal with:
-the theoretical understanding of relevant polarization physics, which requires new insights into the quantum theory of polarized light scattering in the presence of magnetic and electric fields.
-the development of plasma diagnostic tools for the investigation of astrophysical magnetic fields, with emphasis on the magnetism of the extended solar atmosphere, circumstellar envelopes and planetary nebulae.
-spectropolarimetric observations and their physical interpretation.
-radiative transfer in three-dimensional models of stellar atmospheres, resulting from magneto-hydrodynamical simulations.
-atomic and molecular spectroscopy and spectro-polarimetry, with applications in several fields of astrophysics.
This research project is formed by a group of scientists convinced of the importance of complementing theoretical and observational investigations in order to face some of the present challenges of 21st century Astrophysics.
Members
Results
- We applied deep learning techniques to the analysis of observations. Using convolutional neural networks, we developed techniques for the deconvolution of observational data. These techniques were also used to accelerate the deconvolution process of ground-based observations, achieving a cadence of around a hundred images processed per second.
- We developed an inference technique based on bayesian statistics in order to interpret the observations provided by the CLASP international experiment. By parametrizing a state-of-the-art magneto-hydrodynamical model of the solar atmosphere, we found that the geometrical complexity of the transition region must be much more complex than the one provided by the model.
- We solved the problem of polarized radiation transfer in magneto-convection simulations that account for small-scale dynamo action for the Sr I line at 460.7 nm. We found that the model with most of the convection zone magnetized close to the equipartition and a surface mean field strength of 170G is compatible with the available observations.
- We studied the magnetic sensitivity of the Ca I line at 422.7nm. The linear polarization at the core is sensitive to the Hanle effect, while the linear polarization in the wings is sensitive to the magneto-optical effects, as a consequence of the newly found effect resulting from the joint action of partial redistribution and the Zeeman effects.
- We studied the formation of the H-alpha, Mg II h-k, and Ca II H-K and 845.2nm in a model atmosphere of a flaring bipolar active region, solving the radiation transfer problem taking into account partial redistribution in full 3D geometry and out of local thermodynamical equilibrium. We succeeded in reproducing common observational features of such flaring regions.
Scientific activity
Related publications
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Non-LTE Determination of the Silicon Abundance Using a Three-dimensional Hydrodynamical Model of the Solar Photosphere
Confrontations of spectroscopic observations with local thermodynamic equilibrium (LTE) spectral syntheses in a three-dimensional (3D) hydrodynamical model of the solar photosphere led to a downward revision of the photospheric and meteoritic silicon abundances. Here we derive the photospheric silicon abundance taking into account non-LTE (NLTE)
Shchukina, N. et al.Advertised on:
82012 -
Determining the Magnetization of the Quiet Sun Photosphere from the Hanle Effect and Surface Dynamo Simulations
The bulk of the quiet solar photosphere is thought to be significantly magnetized, due to the ubiquitous presence of a tangled magnetic field at subresolution scales with an average strength langBrang ~ 100 G. This conclusion was reached through detailed three-dimensional (3D) radiative transfer modeling of the Hanle effect in the Sr I 4607 Å line
Shchukina, Nataliya et al.Advertised on:
42011 -
The Scattering Polarization of the Lyα Lines of H I and He II Taking into Account Partial Frequency Redistribution and J-state Interference Effects
Recent theoretical investigations have pointed out that the cores of the Lyα lines of H I and He II should show measurable scattering polarization signals when observing the solar disk, and that the magnetic sensitivity, through the Hanle effect, of such linear polarization signals is suitable for exploring the magnetism of the solar transition
Belluzzi, L. et al.Advertised on:
82012 -
The Polarization of the Solar Mg II h and k Lines
Although the h and k lines of Mg II are expected to be of great interest for probing the upper solar chromosphere, relatively little is known about their polarization properties which encode the information on the magnetic field. Here we report the first results of an investigation whose main goal is to understand the physical mechanisms that
Belluzzi, L. et al.Advertised on:
52012 -
The Physical Origin and the Diagnostic Potential of the Scattering Polarization in the Li I Resonance Doublet at 6708 Å
High-sensitivity measurements of the linearly polarized solar limb spectrum produced by scattering processes in quiet regions of the solar atmosphere showed that the Q/I profile of the lithium doublet at 6708 Å has an amplitude ~10-4 and a curious three-peak structure, qualitatively similar to that found and confirmed by many observers in the Na I
Belluzzi, Luca et al.Advertised on:
112009 -
The Lyα Lines of H I and He II: A Differential Hanle Effect for Exploring the Magnetism of the Solar Transition Region
The Lyα line of He II at 304 Å is one of the spectral lines of choice for EUV channels of narrowband imagers on board space telescopes, which provide spectacular intensity images of the outer solar atmosphere. Since the magnetic field information is encoded in the polarization of the spectral line radiation, it is important to investigate whether
Trujillo-Bueno, J. et al.Advertised on:
22012 -
The Impact of Quantum Interference between Different J-levels on Scattering Polarization in Spectral Lines
The spectral line polarization produced by optically pumped atoms contains a wealth of information on the thermal and magnetic structure of a variety of astrophysical plasmas, including that of the solar atmosphere. A correct decoding of such information from the observed Stokes profiles requires a clear understanding of the effects that
Belluzzi, L. et al.Advertised on:
122011 -
Spectropolarimetric forward modelling of the lines of the Lyman-series using a self-consistent, global, solar coronal model
Context. The presence and importance of the coronal magnetic field is illustrated by a wide range of phenomena, such as the abnormally high temperatures of the coronal plasma, the existence of a slow and fast solar wind, the triggering of explosive events such as flares and CMEs. Aims: We investigate the possibility of using the Hanle effect to
Khan, A. et al.Advertised on:
52011 -
The Magnetic Sensitivity of the Stokes I Profile of the 15260 Å Line of Mn I
The near-IR line of Mn I at 15262.7 Å has recently been proposed as a new tool for diagnosing the magnetic field strength and magnetic energy density associated with unresolved magnetic structures, due to the enhanced sensitivity of the Stokes I profile. In this paper, the behavior of the line, focusing on the properties of the Stokes I profile, is
Asensio Ramos, A.Advertised on:
12009 -
PCA detection and denoising of Zeeman signatures in polarised stellar spectra
Aims: Our main objective is to develop a denoising strategy to increase the signal to noise ratio of individual spectral lines of stellar spectropolarimetric observations. Methods: We use a multivariate statistics technique called Principal Component Analysis. The cross-product matrix of the observations is diagonalized to obtain the eigenvectors
Martínez González, M. J. et al.Advertised on:
82008 -
Near-IR internetwork spectro-polarimetry at different heliocentric angles
Aims:The analysis of near infrared spectropolarimetric data at the internetwork in different regions on the solar surface could offer constraints that reject current modeling of these quiet areas. Methods: We present spectro-polarimetric observations of very quiet regions for different values of the heliocentric angle for the Fe i lines at 1.56 μm
Martínez González, M. J. et al.Advertised on:
22008 -
Multiline Zeeman signatures through line addition
Context: To obtain a significant Zeeman signature in the polarised spectra of a magnetic star, we usually “add” the contributions of numerous spectral lines; the ultimate goal is to recover the spectropolarimetric prints of the magnetic field in these line additions. Aims: Here we want to clarify the meaning of these techniques of line addition; in
Semel, M. et al.Advertised on:
92009 -
Multiline Spectropolarimetry of the Quiet Sun at 5250 and 6302 Å
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
Socas-Navarro, H. et al.Advertised on:
22008 -
Markov properties of solar granulation
Aims: We estimate the minimum length on which solar granulation can be considered to be a Markovian process. Methods: We measure the variation in the bright difference between two pixels in images of the solar granulation for different distances between the pixels. This scale-dependent data is empirically analyzed to find the minimum scale on which
Asensio Ramos, A.Advertised on:
12009 -
Evidence for Quasi-Isotropic Magnetic Fields from Hinode Quiet-Sun Observations
Some recent investigations of spectropolarimetric observations of the Zeeman effect in the Fe I lines at 630 nm carried out with the Hinode solar space telescope have concluded that the strength of the magnetic field vector in the internetwork regions of the quiet Sun is in the hG regime and that its inclination is predominantly horizontal. We
Asensio Ramos, A.Advertised on:
82009 -
Error propagation in polarimetric demodulation
The polarization analysis of light is typically carried out using modulation schemes. The light of an unknown polarization state is passed through a set of known modulation optics, and a detector is used to measure the total intensity passing the system. The modulation optics is modified several times, and, with the aid of several such measurements
Asensio Ramos, A. et al.Advertised on:
52008 -
Bayesclumpy: Bayesian Inference with Clumpy Dusty Torus Models
Our aim is to present a fast and general Bayesian inference framework based on the synergy between machine learning techniques and standard sampling methods and apply it to infer the physical properties of clumpy dusty torus using infrared photometric high spatial resolution observations of active galactic nuclei. We make use of the Metropolis
Asensio Ramos, A. et al.Advertised on:
52009 -
Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects
A big challenge in solar and stellar physics in the coming years will be to decipher the magnetism of the solar outer atmosphere (chromosphere and corona) along with its dynamic coupling with the magnetic fields of the underlying photosphere. To this end, it is important to develop rigorous diagnostic tools for the physical interpretation of
Asensio Ramos, A. et al.Advertised on:
82008 -
Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX
We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500 G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma
Martínez-González, M. J. et al.Advertised on:
42011 -
Torus and Active Galactic Nucleus Properties of Nearby Seyfert Galaxies: Results from Fitting Infrared Spectral Energy Distributions and Spectroscopy
We used the CLUMPY torus models and a Bayesian approach to fit the infrared spectral energy distributions and ground-based high angular resolution mid-infrared spectroscopy of 13 nearby Seyfert galaxies. This allowed us to put tight constraints on torus model parameters such as the viewing angle i, the radial thickness of the torus Y, the angular
Alonso-Herrero, Almudena et al.Advertised on:
82011
