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  • 1. Asensio Ramos, A.
    et al.
    Díaz Baso, Carlos José
    Stockholm University, Faculty of Science, Department of Astronomy.
    Stokes inversion based on convolutional neural networks2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A102Article in journal (Refereed)
    Abstract [en]

    Context. Spectropolarimetric inversions are routinely used in the field of solar physics for the extraction of physical information from observations. The application to two-dimensional fields of view often requires the use of supercomputers with parallelized inversion codes. Even in this case, the computing time spent on the process is still very large.

    Aims. Our aim is to develop a new inversion code based on the application of convolutional neural networks that can quickly provide a three-dimensional cube of thermodynamical and magnetic properties from the interpreation of two-dimensional maps of Stokes profiles.

    Methods. We trained two different architectures of fully convolutional neural networks. To this end, we used the synthetic Stokes profiles obtained from two snapshots of three-dimensional magneto-hydrodynamic numerical simulations of different structures of the solar atmosphere.

    Results. We provide an extensive analysis of the new inversion technique, showing that it infers the thermodynamical and magnetic properties with a precision comparable to that of standard inversion techniques. However, it provides several key improvements: our method is around one million times faster, it returns a three-dimensional view of the physical properties of the region of interest in geometrical height, it provides quantities that cannot be obtained otherwise (pressure and Wilson depression) and the inferred properties are decontaminated from the blurring effect of instrumental point spread functions for free. The code, models, and data are all open source and available for free, to allow both evaluation and training.

  • 2.
    Diaz Baso, Carlos José
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Danilovic, Sanja
    Stockholm University, Faculty of Science, Department of Astronomy.
    Solar image denoising with convolutional neural networks2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 629, article id A99Article in journal (Refereed)
    Abstract [en]

    The topology and dynamics of the solar chromosphere are greatly affected by the presence of magnetic fields. The magnetic field can be inferred by analyzing polarimetric observations of spectral lines. Polarimetric signals induced by chromospheric magnetic fields are, however, particularly weak, and in most cases very close to the detection limit of current instrumentation. Because of this, there are only few observational studies that have successfully reconstructed the three components of the magnetic field vector in the chromosphere. Traditionally, the signal-to-noise ratio of observations has been improved by performing time-averages or spatial averages, but in both cases, some information is lost. More advanced techniques, like principal-component analysis, have also been employed to take advantage of the sparsity of the observations in the spectral direction. In the present study, we use the spatial coherence of the observations to reduce the noise using deep-learning techniques. We designed a neural network that is capable of recovering weak signals under a complex noise corruption (including instrumental artifacts and non-linear post-processing). The training of the network is carried out without a priori knowledge of the clean signals, or an explicit statistical characterization of the noise or other corruption. We only use the same observations as our generative model. The performance of this method is demonstrated on both synthetic experiments and real data. We show examples of the improvement in typical signals obtained in current telescopes such as the Swedish 1 m Solar Telescope. The presented method can recover weak signals equally well no matter what spectral line or spectral sampling is used. It is especially suitable for cases when the wavelength sampling is scarce.

  • 3.
    Diaz Baso, Carlos José
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Laguna, Spain.
    Martinez Gonzalez, M. J.
    Asensio Ramos, A.
    Spectropolarimetric analysis of an active region filament I. Magnetic and dynamical properties from single component inversions2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 625, article id A128Article in journal (Refereed)
    Abstract [en]

    Aims. The determination of the magnetic filed vector in solar filaments is made possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10 830 angstrom. We study the vector magnetic field of an active region filament (NOAA 12087). Methods. Spectropolarimetric data of this active region was acquired with the GRIS instrument at the GREGOR telescope and studied simultaneously in the chromosphere with the He I 10 830 angstrom multiplet and in the photosphere Si I 10 827 angstrom line. As has been done in previous studies, only a single-component model was used to infer the magnetic properties of the filament. The results are put into a solar context with the help of the Solar Dynamic Observatory images. Results. Some results clearly point out that a more complex inversion had to be performed. First, the Stokes V map of He I does not show a clear signature of the presence of the filament. Second, the local azimuth map follows the same pattern as Stokes V; it appears that polarity of Stokes V is conditioning the inference to very different magnetic fields even with similar linear polarization signals. This indication suggests that the Stokes V could be dominated from below by the magnetic field coming from the active region, and not from the filament itself. This evidence, and others, will be analyzed in depth and a more complex inversion will be attempted in the second part of this series.

  • 4.
    Diaz Baso, Carlos José
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Laguna, Spain.
    Martinez Gonzalez, M. J.
    Asensio Ramos, A.
    Spectropolarimetric analysis of an active region filament. II. Evidence of the limitations of a single-component model2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 625, article id A129Article in journal (Refereed)
    Abstract [en]

    Aims. Our aim is to demonstrate the limitations of using a single-component model to study the magnetic field of an active region filament. To do this, we analyzed the polarimetric signals of the He I 10830 angstrom multiplet, which were acquired with the Infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain). Methods. After a first analysis of the general properties of the filament using HAZEL under the assumption of a single-component model atmosphere, in this second part we focus our attention on the observed Stokes profiles and the signatures that cannot be explained with this model. Results. We have found an optically thick filament whose blue and red components have the same sign in the linear polarization as an indication of radiative transfer effects. Moreover, the circular polarization signals inside the filament show strong magnetic field gradients. We also show that even a filament with such high absorption still shows signatures of the circular polarization that is generated by the magnetic field below the filament. The reason is that the absorption of the spectral line decays very quickly toward the wings, just where the circular polarization has a larger amplitude. In order to separate the two contributions, we explore the possibility of a two-component model, but the Inference becomes impossible to overcome because very many solutions are compatible with the observations.

  • 5.
    Díaz Baso, Carlos José
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Spain.
    Martinez González, M. J.
    Asensio Ramos, A.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Diagnostic potential of the Ca II 8542 Ångstrom line for solar filaments2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A178Article in journal (Refereed)
    Abstract [en]

    Aims. In this study we explore the diagnostic potential of the chromospheric Ca II line at 8542 angstrom for studying the magnetic and dynamic properties of solar filaments. We have acquired high spatial resolution spectropolarimetric observations in the Ca II 8542 angstrom line using the CRISP instrument at the Swedish 1 m Solar Telescope. Methods. We used the NICOLE inversion code to infer physical properties from observations of a solar filament. We discuss the validity of the results due to the assumption of hydrostatic equilibrium. We have used observations from other telescopes such as CHROTEL and SDO, in order to study large scale dynamics and the long term evolution of the filament. Results. We show that the Ca II 8542 angstrom line encodes information of the temperature, line-of-sight velocity and magnetic field vector from the region where the filament is located. The current noise levels only allow us to estimate an upper limit of 260 G for the total magnetic field of the filament. Our study also reveals that if we consider information from the aforementioned spectral line alone, the geometric height, the temperature and the density could be degenerated parameters outside the hydrostatic equilibrium approach.

  • 6.
    Yadav, Rahul
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Díaz Baso, Carlos José
    Stockholm University, Faculty of Science, Department of Astronomy.
    Prasad, Avijeet
    Libbrecht, Tine
    Stockholm University, Faculty of Science, Department of Astronomy.
    Robustini, Carolina
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, Andrés
    Three-dimensional magnetic field structure of a flux-emerging region in the solar atmosphere2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 632, article id A112Article in journal (Refereed)
    Abstract [en]

    We analyze high-resolution spectropolarimetric observations of a flux-emerging region (FER) in order to understand its magnetic and kinematic structure. Our spectropolarimetric observations in the HeI 10830 angstrom spectral region of a FER were recorded with GRIS at the 1.5 m aperture GREGOR telescope. A Milne-Eddington-based inversion code was employed to extract the photospheric information of the SiI spectral line, whereas the HeI triplet line was analyzed with the Hazel inversion code, which takes into account the joint action of the Hanle and the Zeeman effects. The spectropolarimetric analysis of the SiI line reveals a complex magnetic structure near the vicinity of the FER, where a weak (350-600 G) and horizontal magnetic field was observed. In contrast to the photosphere, the analysis of the HeI triplet presents a smooth variation of the magnetic field vector (ranging from 100 to 400 G) and velocities across the FER. Moreover, we find supersonic downflows of similar to 40 km s(-1) appearing near the foot points of loops connecting two pores of opposite polarity, whereas strong upflows of 22 km s(-1) appear near the apex of the loops. At the location of supersonic downflows in the chromosphere, we observed downflows of 3 km s(-1) in the photosphere. Furthermore, nonforce-free field extrapolations were performed separately at two layers in order to understand the magnetic field topology of the FER. We determine, using extrapolations from the photosphere and the observed chromospheric magnetic field, that the average formation height of the HeI triplet line is similar to 2 Mm from the solar surface. The reconstructed loops using photospheric extrapolations along an arch filament system have a maximum height of similar to 10.5 Mm from the solar surface with a foot-point separation of similar to 19 Mm, whereas the loops reconstructed using chromospheric extrapolations reach around similar to 8.4 Mm above the solar surface with a foot-point separation of similar to 16 Mm at the chromospheric height. The magnetic topology in the FER suggests the presence of small-scale loops beneath the large loops. Under suitable conditions, due to magnetic reconnection, these loops can trigger various heating events in the vicinity of the FER.

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