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  • 1. Asensio Ramos, A.
    et al.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sparse inversion of Stokes profiles I. Two-dimensional Milne-Eddington inversions2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 577, article id A140Article in journal (Refereed)
    Abstract [en]

    Context. Inversion codes are numerical tools used to infer physical properties from observations. Despite their success, the quality of current spectropolarimetric observations and those expected in the near future presents a challenge to current inversion codes. Aims. The pixel-by-pixel strategy of inverting spectropolarimetric data that we currently use needs to be surpassed and improved. The inverted physical parameters have to take into account the spatial correlation that is present in the data and that contains valuable physical information. Methods. We used the concept of sparsity or compressibility to develop a new generation of inversion codes for the Stokes parameters. The inversion code uses numerical optimization techniques based on the idea of proximal algorithms to impose sparsity. In so doing, we allow for the first time exploiting the spatial correlation on the maps of physical parameters. Sparsity also regularizes the solution by reducing the number of unknowns. Results. We compare the results of the new inversion code with pixel-by-pixel inversions to demonstrate the increased robustness of the solution. We also show how the method can easily compensate for the effect of the telescope point spread function, producing solutions with an enhanced contrast.

  • 2. Asensio Ramos, A.
    et al.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Martinez Gonzalez, M. J.
    Pastor Yabar, A.
    Inversion of Stokes profiles with systematic effects2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 590, article id A87Article in journal (Refereed)
    Abstract [en]

    Quantitative thermodynamical, dynamical and magnetic properties of the solar and stellar plasmas are obtained by interpreting their emergent non-polarized and polarized spectrum. This inference requires the selection of a set of spectral lines that are particularly sensitive to the physical conditions in the plasma and a suitable parametric model of the solar/stellar atmosphere. Nonlinear inversion codes are then used to fit the model to the observations. However, the presence of systematic effects, like nearby or blended spectral lines, telluric absorption, or incorrect correction of the continuum, among others, can strongly affect the results. We present an extension to current inversion codes that can deal with these effects in a transparent way. The resulting algorithm is very simple and can be applied to any existing inversion code with the addition of a few lines of code as an extra step in each iteration.

  • 3. Asensio Ramos, A.
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Martínez González, M. J.
    Socas-Navarro, H.
    Inference of the chromospheric magnetic field orientation in the Ca II 8542 angstrom line fibrils2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A133Article in journal (Refereed)
    Abstract [en]

    Context. Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes.

    Aims. Use statistical analysis to test whether the association between fibrils and magnetic field lines is justified.

    Methods. We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro- polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image.

    Results. We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of similar to 16 degrees, while this dispersion goes up to similar to 34 degrees in less magnetized regions.

  • 4. Asensio Ramos, A.
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pastor Yabar, A.
    Real-time, multiframe, blind deconvolution of solar images2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A73Article in journal (Refereed)
    Abstract [en]

    The quality of images of the Sun obtained from the ground are severely limited by the perturbing effect of the Earth's turbulent atmosphere. The post-facto correction of the images to compensate for the presence of the atmosphere require the combination of high-order adaptive optics techniques, fast measurements to freeze the turbulent atmosphere, and very time-consuming blind deconvolution algorithms. Under mild seeing conditions, blind deconvolution algorithms can produce images of astonishing quality. They can be very competitive with those obtained from space, with the huge advantage of the flexibility of the instrumentation thanks to the direct access to the telescope. In this contribution we make use of deep learning techniques to significantly accelerate the blind deconvolution process and produce corrected images at a peak rate of similar to 100 images per second. We present two different architectures that produce excellent image corrections with noise suppression while maintaining the photometric properties of the images. As a consequence, polarimetric signals can be obtained with standard polarimetric modulation without any significant artifact. With the expected improvements in computer hardware and algorithms, we anticipate that on-site real-time correction of solar images will be possible in the near future.

  • 5.
    Bjørgen, Johan P.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sukhorukov, Andrii V.
    Stockholm University, Faculty of Science, Department of Astronomy. National Academy of Sciences of Ukraine, Ukraine.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran B.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hansteen, Viggo H.
    Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, article id A62Article in journal (Refereed)
    Abstract [en]

    Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. Aims. We aim to model the Ca II H and K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere. Methods. We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare this data with SST/CHROMIS images. Results. The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H and K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere.

  • 6. Brown, Stephen A.
    et al.
    Fletcher, Lyndsay
    Kerr, Graham S.
    Labrosse, Nicolas
    Kowalski, Adam F.
    De La Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Modeling of the Hydrogen Lyman Lines in Solar Flares2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 862, no 1, article id 59Article in journal (Refereed)
    Abstract [en]

    The hydrogen Lyman lines (91.2 nm < lambda < 121.6 nm) are significant contributors to the radiative losses of the solar chromosphere, and they are enhanced during flares. We have shown previously that the Lyman lines observed by the Extreme Ultraviolet Variability instrument onboard the Solar Dynamics Observatory exhibit Doppler motions equivalent to speeds on the order of 30 km s(-1). However, contrary to expectations, both redshifts and blueshifts were present and no dominant flow direction was observed. To understand the formation of the Lyman lines, particularly their Doppler motions, we have used the radiative hydrodynamic code, RADYN, along with the radiative transfer code, RH, to simulate the evolution of the flaring chromosphere and the response of the Lyman lines during solar flares. We find that upflows in the simulated atmospheres lead to blueshifts in the line cores, which exhibit central reversals. We then model the effects of the instrument on the profiles, using the Extreme Ultraviolet Variability Experiment (EVE) instrument's properties. What may be interpreted as downflows (redshifted emission) in the lines, after they have been convolved with the instrumental line profile, may not necessarily correspond to actual downflows. Dynamic features in the atmosphere can introduce complex features in the line profiles that will not be detected by instruments with the spectral resolution of EVE, but which leave more of a signature at the resolution of the Spectral Investigation of the Coronal Environment instrument onboard the Solar Orbiter.

  • 7.
    Bühler, David
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Esteban Pozuelo, Sara
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran B.
    Stockholm University, Faculty of Science, Department of Astronomy.
    The Dark Side of Penumbral Microjets: Observations in H alpha2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 876, no 1, article id 47Article in journal (Refereed)
    Abstract [en]

    We present data of 10 penumbral microjets (PMJs) observed in a H alpha, Ca II 8542 angstrom, and Fe I 6302 angstrom line pair with the Swedish 1 m Solar Telescope (SST) with CRISP and Ca II K with SST/CHROMIS in active region NOAA 12599 on 2016 October 12 at mu = 0.68. All four Stokes parameters of the Ca II 8542 angstrom and Fe I 6302 angstrom lines were observed and a series of test pixels were inverted using the Stockholm inversion code. Our analysis revealed for the first time that PMJs are visible in H alpha, where they appear as dark features with average line-of-sight (LOS) upflows of 1.1 +/- 0.6 km s(-1), matching the LOS velocities from the inversions. Based on the H alpha observations we extend the previous average length and lifetime of PMJs to 2815 +/- 530 km and 163 +/- 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs of up to 17 km s(-1) tend to give increased velocities with distance traveled. Furthermore, two of our PMJs with significant Stokes V signal indicate that the PMJs possess an increased LOS magnetic field of up to 100 G compared to the local pre-/post- PMJ magnetic field, which propagates as quickly as the PMJs' POS velocities. Finally, we present evidence that PMJs display an on average 1 minute gradual precursory brightening that only manifests itself in the cores of the Ca II lines. We conclude that PMJs are not ordinary jets but likely are manifestations of heat fronts that propagate at the local Alfven velocity.

  • 8. Cristaldi, A.
    et al.
    Guglielmino, S. L.
    Zuccarello, F.
    Romano, P.
    Falco, M.
    van der Voort, L. Rouppe
    Rodriguez, Jaime de la Cruz
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ermolli, I.
    Criscuoli, S.
    DYNAMIC PROPERTIES ALONG THE NEUTRAL LINE OF A DELTA SPOT INFERRED FROM HIGH-RESOLUTION OBSERVATIONS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 789, no 2, p. 162-Article in journal (Refereed)
    Abstract [en]

    Delta (delta) spots are complex magnetic configurations of sunspots characterized by umbrae of opposite polarity sharing a common penumbra. In order to investigate the fine structure of the region separating the two magnetic polarities of a delta spot, we studied the morphology, the magnetic configuration, and the velocity field in such a region using observations of active region (AR) NOAA 11267 obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data shows upflows and downflows of similar to +/- 3 km s(-1) in proximity of the delta spot polarity inversion line (PIL), and horizontal motions along the PIL of the order of similar to 1 km s(-1). The results obtained from the SIR inversion of CRISP data also indicate that the transverse magnetic field in the brighter region separating the two opposite magnetic polarities of the delta spot is tilted about similar to 45 degrees with respect to the PIL. Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations confirm the presence of motions of similar to +/- 3 km s(-1) in proximity of the PIL, which were observed to last 15 hr. From the data analyzed, we conclude that the steady, persistent, and subsonic motions observed along the delta spot PIL can be interpreted as being due to Evershed flows occurring in the penumbral filaments that show a curved, wrapped configuration. The fluting of the penumbral filaments and their bending, continuously increased by the approaching motion of the negative umbra toward the positive one, give rise to the complex line-of-sight velocity maps that we observed.

  • 9.
    da Silva Santos, Joäo Manuel
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Temperature constraints from inversions of synthetic solar optical, UV, and radio spectra2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A124Article in journal (Refereed)
    Abstract [en]

    Context. High-resolution observations of the solar chromosphere at millimeter wavelengths are now possible with the Atacama Large Millimeter Array (ALMA), bringing with them the promise of tackling many open problems in solar physics. Observations from other ground and space-based telescopes will greatly benefit from coordinated endeavors with ALMA, yet the diagnostic potential of combined optical, ultraviolet and mm observations has remained mostly unassessed. Aims. In this paper we investigate whether mm-wavelengths could aid current inversion schemes to retrieve a more accurate representation of the temperature structure of the solar atmosphere. Methods. We performed several non-LTE inversion experiments of the emergent spectra from a snapshot of 3D radiation-MHD simulation. We included common line diagnostics such as Ca II K, 8542 angstrom and Mg II h and k, taking into account partial frequency redistribution effects, along with the continuum around 1.2 mm and 3 mm. Results. We find that including the mm-continuum in inversions allows a more accurate inference of temperature as function of optical depth. The addition of ALMA bands to other diagnostics should improve the accuracy of the inferred chromospheric temperatures between log tau similar to [-6, -4.5] where the Ca II and Mg II lines are weakly coupled to the local conditions. However, we find that simultaneous multiatom, non-LTE inversions of optical and UV lines present equally strong constraints in the lower chromosphere and thus are not greatly improved by the 1.2 mm band. Nonetheless, the 3 mm band is still needed to better constrain the mid-upper chromosphere.

  • 10. Dalda, Alberto Sainz
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Pontieu, Bart
    Gošić, Milan
    Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach2019In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 875, no 2, article id L18Article in journal (Refereed)
    Abstract [en]

    Inversion codes allow the reconstruction of a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modeling is computationally very expensive because a non-LTE evaluation of the radiation field is required. In this study, we combine the results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in an easy-to-use, much faster way. We have applied these new methods to Mg II h and k lines observed by the Interface Region Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, nonthermal velocities, electron density, etc.) in the chromosphere and upper photosphere of an area equivalent to an active region in a few CPU minutes, speeding up the process by a factor of 10(5) - 10(6). The opensource code accompanying this Letter will allow the community to use IRIS observations to open a new window to a host of solar phenomena.

  • 11.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hansteen, Viggo
    Bellot-Rubio, Luis
    Ortiz, Ada
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. II. NON-LTE CHROMOSPHERIC DIAGNOSTICS AND INVERSIONS2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 810, no 2, article id 145Article in journal (Refereed)
    Abstract [en]

    Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca II 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca II 8542 angstrom profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere.

  • 12.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, Andrés
    NON-LTE INVERSIONS OF THE Mg II h & k AND UV TRIPLET LINES2016In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 830, no 2, article id L30Article in journal (Refereed)
    Abstract [en]

    The Mg II h & k lines are powerful diagnostics for studying the solar chromosphere. They have become particularly popular with the launch of the Interface Region Imaging Spectrograph (IRIS) satellite, and a number of studies that include these lines have lead to great progress in understanding chromospheric heating, in many cases thanks to the support from 3D MHD simulations. In this study, we utilize another approach to analyze observations: non-LTE inversions of the Mg II h & k and UV triplet lines including the effects of partial redistribution. Our inversion code attempts to construct a model atmosphere that is compatible with the observed spectra. We have assessed the capabilities and limitations of the inversions using the FALC atmosphere and a snapshot from a 3D radiation-MHD simulation. We find that Mg II h & k allow reconstructing a model atmosphere from the middle photosphere to the transition region. We have also explored the capabilities of a multi-line/multi-atom setup, including the Mg IIh & k, the Ca II 854.2. nm, and the Fe I. 630.25 lines to recover the full stratification of physical parameters, including the magnetic field vector, from the photosphere to the chromosphere. Finally, we present the first inversions of observed IRIS spectra from quiet-Sun, plage, and sunspot, with very promising results.

  • 13.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Danilovic, Sanja
    Stockholm University, Faculty of Science, Department of Astronomy.
    Uitenbroek, H.
    STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A74Article in journal (Refereed)
    Abstract [en]

    The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.

  • 14.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Uppsala University, Sweden.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    van der Voort, L. Rouppe
    CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, article id A40Article in journal (Refereed)
    Abstract [en]

    The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines. We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of- the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data. It is implemented and operated in IDL, with time-consuming steps delegated to C. CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument.

  • 15.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Hector
    Instituto de Astrofisica de Canarias.
    Are solar chromospheric fibrils tracing the magnetic field?Article in journal (Refereed)
  • 16.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Hector
    Instituto de Astrofisica de Canarias.
    Carlsson, Mats
    University of Oslo.
    Leenaarts, Jorrit
    Utrecht University.
    Non-LTE inversions from a 3D MHD chromospheric modelManuscript (preprint) (Other academic)
  • 17.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    SST/CRISP observations of Ca II 854.2 nm2008In: 12th European Solar Physics Meeting, 2008Conference paper (Other academic)
  • 18.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Measuring the solar atmosphere2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The new CRISP filter at the Swedish 1-m Solar Telescope provides opportunities for observing the solar atmosphere with unprecedented spatial resolution and cadence. In order to benefit from the high quality of observational data from this instrument, we have developed methods for calibrating and restoring polarized Stokes images, obtained at optical and near infrared wavelengths, taking into account field-of-view variations of the filter properties.

    In order to facilitate velocity measurements, a time series from a 3D hydrodynamical granulation simulation is used to compute quiet Sun spectral line profiles at different heliocentric angles. The synthetic line profiles, with their convective blueshifts, can be used as absolute references for line-of-sight velocities.

    Observations of the Ca II 8542 Å line are used to study magnetic fields in chromospheric fibrils. The line wings show the granulation pattern at mid-photospheric heights whereas the overlying chromosphere is seen in the core of the line. Using full Stokes data, we have attempted to observationally verify the alignment of chromospheric fibrils with the magnetic field. Our results suggest that in most cases fibrils are aligned along the magnetic field direction, but we also find examples where this is not the case.

    Detailed interpretation of Stokes data from spectral lines formed in the chromospheric data can be made using non-LTE inversion codes. For the first time, we use a realistic 3D MHD chromospheric simulation of the quiet Sun to assess how well NLTE inversions recover physical quantities from spectropolarimetric observations of Ca II 8542 Å. We demonstrate that inversions provide realistic estimates of depth-averaged quantities in the chromosphere, although high spectral resolution and high sensitivity are needed to measure quiet Sun chromospheric magnetic fields.

  • 19.
    de la Cruz Rodríguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    University of Oslo.
    Solar velocity references from 3D HD photospheric modelsArticle in journal (Refereed)
  • 20.
    de la Cruz Rodríguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, M.
    Radiative Diagnostics in the Solar Photosphere and Chromosphere2017In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 210, no 1-4, p. 109-143Article, review/survey (Refereed)
    Abstract [en]

    Magnetic fields on the surface of the Sun and stars in general imprint or modify the polarization state of the electromagnetic radiation that is leaving from the star. The inference of solar/ stellar magnetic fields is performed by detecting, studying and modeling polarized light from the target star. In this review we present an overview of techniques that are used to study the atmosphere of the Sun, and particularly those that allow to infer magnetic fields. We have combined a small selection of theory on polarized radiative transfer, inversion techniques and we discuss a number of results from chromospheric inversions.

  • 21.
    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.

  • 22.
    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.

  • 23. Esteban Pozuelo, S.
    et al.
    Bellot Rubio, L. R.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    LATERAL DOWNFLOWS IN SUNSPOT PENUMBRAL FILAMENTS AND THEIR TEMPORAL EVOLUTION2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 803, no 2, article id 93Article in journal (Refereed)
    Abstract [en]

    We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited scans of the Fe I 617.3 nm line taken with the CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of granular convection in the quiet Sun. There is a good relation between brightness and velocity in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of quiet Sun convection. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells forming filaments where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles that are suggestive of opposite magnetic polarities in the pixel.

  • 24.
    Esteban Pozuelo, Sara
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. CSIC, Spain.
    Bellot Rubio, L. R.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    PROPERTIES OF SUPERSONIC EVERSHED DOWNFLOWS2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 832, no 2, article id 170Article in journal (Refereed)
    Abstract [en]

    We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe I. 617.3 nm line with the CRISP instrument at the Swedish 1 m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes. V zero-crossing wavelengths, and Stokes. V maps in the far red-wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the line-of-sight velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid-and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime; some of them are recurrent. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward-moving grains in the mid-and outer penumbra.

  • 25.
    Esteban Pozuelo, Sara
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Drews, A.
    van der Voort, L. Rouppe
    Scharmer, Göran B.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Observationally Based Models of Penumbral Microjets2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 870, no 2, article id 88Article in journal (Refereed)
    Abstract [en]

    We study the polarization signals and physical parameters of penumbral microjets (PMJs) by using high spatial resolution data taken in the Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP and CHROMIS instruments at the Swedish 1 m Solar Telescope. We infer their physical parameters, such as physical observables in the photosphere and chromospheric velocity diagnostics, by different methods, including inversions of the observed Stokes profiles with the STiC code. PMJs harbor overall brighter Ca II K line profiles and conspicuous polarization signals in Ca II 854.2 nm, specifically in circular polarization that often shows multiple lobes mainly due to the shape of Stokes I They usually overlap photospheric regions with a sheared magnetic field configuration, suggesting that magnetic reconnections could play an important role in the origin of PMJs. The discrepancy between their low LOS velocities and the high apparent speeds reported on earlier, as well as the existence of different vertical velocity gradients in the chromosphere, indicate that PMJs might not be entirely related to mass motions. Instead, PMJs could be due to perturbation fronts induced by magnetic reconnections occurring in the deep photosphere that propagate through the chromosphere. This reconnection may be associated with current heating that produces temperature enhancements from the temperature minimum region. Furthermore, enhanced collisions with electrons could also increase the coupling to the local conditions at higher layers during the PMJ phase, giving a possible explanation for the enhanced emission in the overall Ca II K profiles emerging from these transients.

  • 26. Gošić, M.
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Pontieu, B.
    Bellot Rubio, L. R.
    Carlsson, M.
    Esteban Pozuelo, Sara
    Stockholm University, Faculty of Science, Department of Astronomy. Instituto de Astrofísica de Andalucía, Spain.
    Ortiz, A.
    Polito, V.
    Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 857, no 1, article id 48Article in journal (Refereed)
    Abstract [en]

    The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for similar to 3. minutes in the photosphere and similar to 12. minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

  • 27. Henriques, V. M. J.
    et al.
    Mathioudakis, M.
    Socas-Navarro, H.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 845, no 2, article id 102Article in journal (Refereed)
    Abstract [en]

    We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II lambda 8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier sunspot plumes, which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.

  • 28. Jafarzadeh, S.
    et al.
    Van Der Voort, L. Rouppe
    Rodriguez, Jaime De La Cruz
    Stockholm University, Faculty of Science, Department of Astronomy.
    MAGNETIC UPFLOW EVENTS IN THE QUIET-SUN PHOTOSPHERE. I. OBSERVATIONS2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 810, no 1Article in journal (Refereed)
    Abstract [en]

    Rapid magnetic upflows in the quiet-Sun photosphere were recently uncovered from both SUNRISE/IMaX and Hinode/SOT observations. Here, we study magnetic upflow events (MUEs) from high-quality, high- (spatial, temporal, and spectral) resolution, and full Stokes observations in four photospheric magnetically sensitive Fe I lines centered at 5250.21, 6173.34, 6301.51, and 6302.50 angstrom acquired with the Swedish Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line Stokes V signals from those in the far blue wing whose signal-to-noise ratio (S/N) >= 7. We find a larger number of MUEs at any given time (2.0 x 10(-2) arcsec(-2)), larger by one to two orders of magnitude, than previously reported. The MUEs appear to fall into four classes presenting different shapes of Stokes V profiles with (I) asymmetric double lobes, (II) single lobes, (III) double-humped (two same-polarity lobes), and (IV) three lobes (an extra blueshifted bump in addition to double lobes), of which less than half are single-lobed. We also find that MUEs are almost equally distributed in network and internetwork areas and they appear in the interior or at the edge of granules in both regions. Distributions of physical properties, except for horizontal velocity, of the MUEs (namely, Stokes V signal, size, line-of-sight velocity, and lifetime) are almost identical for the different spectral lines in our data. A bisector analysis of our spectrally resolved observations shows that these events host modest upflows and do not show a direct indication of the presence of supersonic upflows reported earlier. Our findings reveal that the numbers, types (classes), and properties determined for MUEs can strongly depend on the detection techniques used and the properties of the employed data, namely, S/Ns, resolutions, and wavelengths.

  • 29.
    Joshi, Jayant
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. University of Oslo, Norway.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Magnetic field variations associated with umbral flashes and penumbral waves2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A63Article in journal (Refereed)
    Abstract [en]

    Context. Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Can 8542 angstrom line. Recent studies have focussed on also explaining the observed polarization profiles, which show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. These changes could be explained by opacity effects or by intrinsic changes in the magnetic field strength.

    Aims. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high-temporal-cadence observations acquired in the Can 8542 angstrom line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in the magnetic field, B.

    Methods. We have performed non local thermodynamical equilibrium (non-LTE) data inversions with the NICOLE code of a timeseries of very high spatio-temporal-resolution observations in the Can 8542 angstrom, Fei 6301.5, and Fei 6302.5 angstrom lines. We analyze in detail the variations of the different physical parameters of the model as a function of time.

    Results. Our results indicate that the Can 8542 angstrom line in sunspots is greatly sensitive to magnetic fields at log tau(500 )= -5 (hereafter log tau = -5) during UFs and quiescence. However this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the log tau = -5 is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (up to similar to 270 G) and in RPWs (similar to 100 G).

    Conclusions. Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.

  • 30. Kuridze, D.
    et al.
    Henriques, V. M. J.
    Mathioudakis, M.
    van der Voort, L. Rouppe
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Spectropolarimetric Inversions of the Ca II 8542 angstrom Line in an M-class Solar Flare2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 860, no 1, article id 10Article in journal (Refereed)
    Abstract [en]

    We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using high-resolution full Stokes imaging spectropolarimetry of the Ca II 8542 angstrom line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semiempirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca II 8542 angstrom line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions.

  • 31.
    Leenaarts, Jorrit
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. University of Oslo, Norway.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kochukhov, Oleg
    Carlsson, Mats
    THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE2014In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 784, no 1, p. L17-Article in journal (Refereed)
    Abstract [en]

    The Ca II 854.2 nm spectral line is a common diagnostic of the solar chromosphere. The average line profile shows an asymmetric core, and its bisector shows a characteristic inverse-C shape. The line actually consists of six components with slightly different wavelengths depending on the isotope of calcium. This isotopic splitting of the line has been taken into account in studies of non-solar stars, but never for the Sun. We performed non-LTE radiative transfer computations from three models of the solar atmosphere and show that the line-core asymmetry and inverse C-shape of the bisector of the 854.2 nm line can be explained by isotopic splitting. We confirm this finding by analyzing observations and showing that the line asymmetry is present irrespective of conditions in the solar atmosphere. Finally, we show that inversions based on the Ca II 854.2 nm line should take the isotopic splitting into account, otherwise the inferred atmospheres will contain erroneous velocity gradients and temperatures.

  • 32.
    Leenaarts, Jorrit
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Danilovic, Sanja
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    Chromospheric heating during flux emergence in the solar atmosphere2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A28Article in journal (Refereed)
    Abstract [en]

    Context. The radiative losses in the solar chromosphere vary from 4 kW m(-2) in the quiet Sun, to 20 kW m(-2) in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aims. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm, H alpha, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure. Results. We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log tau(500) = -3 to a more space-filling and time-persistent heating above log tau(500) = -4. The inferred gas temperature at log tau(500) = -3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.

  • 33.
    Libbrecht, Tine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Danilovic, Sanja
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pazira, Hiva
    Stockholm University, Faculty of Science, Department of Astronomy.
    Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D-3 spectro-polarimetry2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A35Article in journal (Refereed)
    Abstract [en]

    Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 angstrom and He I 10830 angstrom.

    Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D-3 line.

    Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector.

    Results. Strong He I D-3 emission at the flare footpoints, as well as strong He I D(3 )absorption profiles tracing the flaring loops are observed during the flare. The He I D-3 traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to similar to 60 km s(-1) at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D-3 profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to similar to 2500 G is measured in the flare footpoints, confirming that the He I D-3 line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D-3 inversions. We propose a fan-spine configuration as the flare magnetic field topology.

    Conclusions. The He I D-3 line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.

  • 34.
    Libbrecht, Tine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Joshi, Jayant
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, Andrés
    Observations of Ellerman bomb emission features in He I D-3 and He I 10 830 angstrom2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, article id A33Article in journal (Refereed)
    Abstract [en]

    Context. Ellerman bombs (EBs) are short-lived emission features, characterised by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10 830 angstrom line, and conclusive observations of EBs in He I D-3 have never been reported.

    Aims. We aim to study the signature of EBs in neutral helium triplet lines.

    Methods. The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the H beta, He I D-3 and He I 10 830 angstrom spectral regions. We also obtained raster scans with IRIS and made use of the SDO/AIA 1700 angstrom channel. We used HAZEL to invert the neutral helium triplet lines.

    Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D-3 line. The helium lines have two components: a broad and blueshifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T similar to 2 x 10(4) 10(5) K.

  • 35.
    Libbrecht, Tine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pires Bjørgen, Johan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hansteen, Viggo
    Joshi, Jayant
    Line formation of He I D3 and He I 10830 Å in a small-scale reconnection eventManuscript (preprint) (Other academic)
  • 36. Louis, Rohan E.
    et al.
    Bellot Rubio, Luis R.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Héctor
    Ortiz, Ada
    Small-scale magnetic flux emergence in a sunspot light bridge2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 584, article id A1Article in journal (Refereed)
    Abstract [en]

    Context: Light bridges are convective intrusions in sunspots that often show enhanced chromospheric activity.

    Aims: We seek to determine the nature of flux emergence in a light bridge and the processes related to its evolution in the solar atmosphere.

    Methods: We analyse a sequence of high-resolution spectropolarimetric observations of a sunspot taken at the Swedish 1-m Solar Telescope. The data consist of spectral scans of the photospheric Fe I line pair at 630 nm and the chromospheric Ca II 854.2 nm line. Bisectors were used to construct Dopplergrams from the Fe I 630.15 nm measurements. We employed LTE and non-LTE inversions to derive maps of physical parameters in the photosphere and chromosphere, respectively.

    Results: We observe the onset of blueshifts of about 2 km s(-1) near the entrance of a granular light bridge on the limbward side of the spot. The blueshifts lie immediately next to a strongly redshifted patch that appeared six minutes earlier. Both patches can be seen for 25 min until the end of the sequence. The blueshifts coincide with an elongated emerging granule, while the redshifts appear at the end of the granule. In the photosphere, the development of the blueshifts is accompanied by a simultaneous increase in field strength of about 400 G. The field inclination increases by some 25 degrees, becoming nearly horizontal. At the position of the redshifts, the magnetic field is equally horizontal but of opposite polarity. An intense brightening is seen in the Ca II filtergrams over the blueshifts and redshifts, about 17 min after their detection in the photosphere. The brightening is due to emission in the blue wing of the Ca II 854.2 nm line, close to its knee. Non-LTE inversions reveal that this kind of asymmetric emission is caused by a temperature enhancement of similar to 700 K between -5.0 <= log tau <= -3.0 and a blueshift of 3 km s(-1) at log tau = - 2.3 that decreases to zero at log tau = - 6.0

    Conclusions: The photospheric blueshifts and redshifts observed in a granular light bridge seem to be caused by the emergence of a small-scale, flat Omega-loop with highly inclined footpoints of opposite polarity that brings new magnetic field to the surface. The gas motions detected in the two footpoints are reminiscent of a siphon flow. The rising loop is probably confined to the lower atmosphere by the overlying sunspot magnetic field and the interaction between the two flux systems may be responsible for temperature enhancements in the upper photosphere/lower chromosphere. This is the first time that magnetic flux is observed to emerge in the strongly magnetised environment of sunspots, pushed upwards by the convective flows of a granular light bridge.

  • 37. Martinez Gonzalez, M. J.
    et al.
    Asensio Ramos, A.
    Arregui, I.
    Collados, M.
    Beck, C.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    ON THE MAGNETISM AND DYNAMICS OF PROMINENCE LEGS HOSTING TORNADOES2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 825, no 2, article id 119Article in journal (Refereed)
    Abstract [en]

    Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves).

  • 38. Martinez Gonzalez, M. J.
    et al.
    Sainz, R. Manso
    Ramos, A. Asensio
    Beck, C.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Diaz, A. J.
    SPECTRO-POLARIMETRIC IMAGING REVEALS HELICAL MAGNETIC FIELDS IN SOLAR PROMINENCE FEET2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 802, no 1, article id 3Article in journal (Refereed)
    Abstract [en]

    Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below.

  • 39. Noda, C. Quintero
    et al.
    Kato, Y.
    Katsukawa, Y.
    Oba, T.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Shimizu, T.
    Orozco Suarez, D.
    Ruiz Cobo, B.
    Kubo, M.
    Anan, T.
    Ichimoto, K.
    Suematsu, Y.
    Chromospheric polarimetry through multiline observations of the 850-nm spectral region - II. A magnetic flux tube scenario2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 1, p. 727-737Article in journal (Refereed)
    Abstract [en]

    In this publication, we continue the work started in Quintero Noda et al., examining this time a numerical simulation of a magnetic flux tube concentration. Our goal is to study if the physical phenomena that take place in it, in particular, the magnetic pumping, leaves a specific imprint on the examined spectral lines. We find that the profiles from the interior of the flux tube are periodically doppler shifted following an oscillation pattern that is also reflected in the amplitude of the circular polarization signals. In addition, we analyse the properties of the Stokes profiles at the edges of the flux tube discovering the presence of linear polarization signals for the Ca II lines, although they are weak with an amplitude around 0.5 per cent of the continuum intensity. Finally, we compute the response functions to perturbations in the longitudinal field, and we estimate the field strength using the weak-field approximation. Our results indicate that the height of formation of the spectral lines changes during the magnetic pumping process, which makes the interpretation of the inferred magnetic field strength and its evolution more difficult. These results complement those from previous works, demonstrating the capabilities and limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry in a very dynamic and complex atmosphere.

  • 40. Noda, C. Quintero
    et al.
    Shimizu, T.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Katsukawa, Y.
    Ichimoto, K.
    Anan, T.
    Suematsu, Y.
    Spectropolarimetric capabilities of Ca II 8542 angstrom line2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 459, no 3, p. 3363-3376Article in journal (Refereed)
    Abstract [en]

    The next generation of space-and ground-based solar missions aim to study the magnetic properties of the solar chromosphere using the infrared Ca II lines and the He I 10830 angstrom line. The former seem to be the best candidates to study the stratification of magnetic fields in the solar chromosphere and their relation to the other thermodynamical properties underlying the chromospheric plasma. The purpose of this work is to provide a detailed analysis of the diagnostic capabilities of the Ca II 8542 angstrom line, anticipating forthcoming observational facilities. We study the sensitivity of the Ca II 8542 angstrom line to perturbations applied to the physical parameters of reference semi-empirical 1D model atmospheres using response functions and we make use of 3D magnetohydrodynamics simulations to examine the expected polarization signals for moderate magnetic field strengths. Our results indicate that the Ca II 8542 angstrom line is mostly sensitive to the layers enclosed in the range log tau = [0, -5.5], under the physical conditions that are present in our model atmospheres. In addition, the simulated magnetic flux tube generates strong longitudinal signals in its centre and moderate transversal signals, due to the vertical expansion of magnetic field lines, in its edge. Thus, observing the Ca II 8542 angstrom line we will be able to infer the 3D geometry of moderate magnetic field regions.

  • 41. Noda, C. Quintero
    et al.
    Shimizu, T.
    Katsukawa, Y.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Anan, T.
    Oba, T.
    Ichimoto, K.
    Suematsu, Y.
    Chromospheric polarimetry through multiline observations of the 850-nm spectral region2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 464, no 4, p. 4534-4543Article in journal (Refereed)
    Abstract [en]

    Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. focused on the infrared Ca (II) 8542 angstrom line and we concluded that it is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth trying to improve the results produced by this line observing additional spectral lines. In that regard, we examined the neighbourhood solar spectrum looking for spectral lines which could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines which greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line which also belongs to the Ca (II) infrared triplet, i.e. the Ca (II) 8498 angstrom line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca (II) 8542 A line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry.

  • 42. Ortiz, Ada
    et al.
    Bellot Rubio, Luis R.
    Hansteen, Viggo H.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy. Uppsala University, Sweden.
    van der Voort, Luc Rouppe
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. I. SPECTROPOLARIMETRIC OBSERVATIONS AND SIMULATIONS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 781, no 2Article in journal (Refereed)
    Abstract [en]

    We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.14. Simultaneous full Stokes observations of the two photospheric Fe i lines at 630.2 nm and the chromospheric Ca ii 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca ii 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca ii 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 kms-1. In about 3.5minutes it travels some 1100 km to reach themid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6.6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90. in the magnetic bubble interior, along with temperature deficits of up to 250 K at log t = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes.

  • 43. Ortiz, Ada
    et al.
    Hansteen, Viggo H.
    Bellot Rubio, Luis Ramon
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Pontieu, Bart
    Carlsson, Mats
    van der Voort, Luc Rouppe
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. III. THE PATH TO THE TRANSITION REGION2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 825, no 2, article id 93Article in journal (Refereed)
    Abstract [en]

    We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Ha 656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0 ''.14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C II, transition region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far-and near-ultraviolet, of which we have used the Si IV 140.3 and the Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona.

  • 44. Quintero Noda, C.
    et al.
    Iijima, H.
    Katsukawa, Y.
    Shimizu, T.
    Carlsson, M.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ruiz Cobo, B.
    Orozco Suárez, D.
    Oba, T.
    Anan, T.
    Kubo, M.
    Kawabata, Y.
    Ichimoto, K.
    Suematsu, Y.
    Chromospheric polarimetry through multiline observations of the 850nm spectral region III: Chromospheric jets driven by twisted magnetic fields2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, no 3, p. 4203-4215Article in journal (Refereed)
    Abstract [en]

    We investigate the diagnostic potential of the spectral lines at 850 nm for understanding the magnetism of the lower atmosphere. For that purpose, we use a newly developed 3D simulation of a chromospheric jet to check the sensitivity of the spectral lines to this phenomenon as well as our ability to infer the atmospheric information through spectropolarimetric inversions of noisy synthetic data. We start comparing the benefits of inverting the entire spectrum at 850 nm versus only the Ca II 8542 angstrom spectral line. We found a better match of the input atmosphere for the former case, mainly at lower heights. However, the results at higher layers were not accurate. After several tests, we determined that we need to weight more the chromospheric lines than the photospheric ones in the computation of the goodness of the fit. The new inversion configuration allows us to obtain better fits and consequently more accurate physical parameters. Therefore, to extract the most from multiline inversions, a proper set of weights needs to be estimated. Besides that, we conclude again that the lines at 850 nm, or a similar arrangement with Ca II 8542 angstrom plus Zeeman-sensitive photospheric lines, pose the best-observing configuration for examining the thermal and magnetic properties of the lower solar atmosphere.

  • 45.
    Robustini, Carolina
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Esteban Pozuelo, Sara
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Chromospheric observations and magnetic configuration of a supergranular structure2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A1Article in journal (Refereed)
    Abstract [en]

    Context. Unipolar magnetic regions are often associated with supergranular cells. The chromosphere above these regions is regulated by the magnetic field, but the field structure is poorly known. In unipolar regions, the fibrillar arrangement does not always coincide with magnetic field lines, and polarimetric observations are needed to establish the chromospheric magnetic topology. Aims. In an active region close to the limb, we observed a unipolar annular network of supergranular size. This supergranular structure harbours a radial distribution of the fibrils converging towards its centre. We aim to improve the description of this structure by determining the magnetic field configuration and the line-of-sight velocity distribution in both the photosphere and the chromosphere. Methods. We observed the supergranular structure at different heights by taking data in the Fe I 6301-6302 angstrom, H alpha, Ca II 8542 angstrom, and the Ca II H&K spectral lines with the CRisp Imaging SpectroPolarimeter (CRISP) and CHROMospheric Imaging Spectrometer (CHROMIS) at the Swedish 1-m Solar Telescope. We performed Milne-Eddington inversions of the spectropolarimetric data of Fe I 6301-6302 angstrom and applied the weak field approximation to Ca II 8542 angstrom data to retrieve the magnetic field in the photosphere and chromosphere. We used photospheric magnetograms of CRISP, Hinode Solar Optical Telescope spectropolarimeter, and Helioseismic and Magnetic Imager to calculate the magnetic flux. We investigated the velocity distribution using the line-of-sight velocities computed from the Milne-Eddington inversion and from the Doppler shift of the K-3 feature in the Ca II K spectral line. To describe the typical spectral profiles characterising the chromosphere above the inner region of the supergranular structure, we performed a K-mean clustering of the spectra in Ca II K. Results. The photospheric magnetic flux shows that the supergranular boundary has an excess of positive polarity and the whole structure is not balanced. The magnetic field vector at chromospheric heights, retrieved by the weak field approximation, indicates that the field lines within the supergranular cell tend to point inwards, and might form a canopy above the unipolar region. In the centre of the supergranular cell hosting the unipolar region, we observe a persistent chromospheric brightening coinciding with a strong gradient in the line-of-sight velocity.

  • 46.
    Robustini, Carolina
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    van der Voort, Luc Rouppe
    Fan-shaped jets above the light bridge of a sunspot driven by reconnection2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 590, article id A57Article in journal (Refereed)
    Abstract [en]

    We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the H alpha line. We study the origin, dynamics, and thermal properties of the jets using high-resolution imaging spectroscopy in H alpha from the Swedish 1m Solar Telescope and data from the Solar Dynamics Observatory and Hinode. The H alpha jets have lengths of 7-38 Mm, are impulsively accelerated to a speed of similar to 100 km s(-1) close to photospheric footpoints in the LB, and exhibit a constant deceleration consistent with solar effective gravity. They are predominantly launched from one edge of the light bridge, and their footpoints appear bright in the H alpha wings. Atmospheric Imaging Assembly data indicates elongated brightenings that are nearly co-spatial with the H alpha jets. We interpret them as jets of transition region temperatures. The magnetic field in the light bridge has a strength of 0.8-2 kG and it is nearly horizontal. All jet properties are consistent with magnetic reconnection as the driver.

  • 47.
    Robustini, Carolina
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    The chromosphere above a delta-sunspot in the presence of fan-shaped jets2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, article id A14Article in journal (Refereed)
    Abstract [en]

    Context. Delta-sunspots are known to be favourable locations for fast and energetic events like flares and coronal mass ejections. The photosphere of this sunspot type has been thoroughly investigated in the past three decades. The atmospheric conditions in the chromosphere are not as well known, however. Aims. This study is focused on the chromosphere of a delta-sunspot that harbours a series of fan-shaped jets in its penumbra. The aim of this study is to establish the magnetic field topology and the temperature distribution in the presence of jets in the photosphere and the chromosphere. Methods. We use data from the Swedish 1m Solar Telescope (SST) and the Solar Dynamics Observatory. We invert the spectropolarimetric Fe I 6302 angstrom and Ca II 8542 angstrom data from the SST using the non-LTE inversion code NICOLE to estimate the magnetic field configuration, temperature, and velocity structure in the chromosphere. Results. A loop-like magnetic structure is observed to emerge in the penumbra of the sunspot. The jets are launched from this structure. Magnetic reconnection between this emerging field and the pre-existing vertical field is suggested by hot plasma patches on the interface between the two fields. The height at which the reconnection takes place is located between log tau(500) = 2 and log tau(500) = -3. The magnetic field vector and the atmospheric temperature maps show a stationary configuration during the whole observation.

  • 48.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Royal Swedish Academy of Sciences, Sweden.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sliepen, Guus
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Is the sky the limit? Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam reimaging systems2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A55Article in journal (Refereed)
    Abstract [en]

    We discuss the use of measurements of the solar granulation contrast as a measure of optical quality. We demonstrate that for data recorded with a telescope that uses adaptive optics and/or post-processing to compensate for many low- and high-order aberrations, the RMS granulation contrast is directly proportional to the Strehl ratio calculated from the residual (small-scale) wavefront error (static and/or from seeing). We demonstrate that the wings of the high-order compensated point spread function for the Swedish 1-m Solar Telescope (SST) are likely to extend to a radius of not more than about 2 '', which is consistent with earlier conclusions drawn from stray-light compensation of sunspot images. We report on simultaneous measurements of seeing and solar granulation contrast averaged over 2 s time intervals at several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS beam). These data were recorded with the SST, which has been revamped with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of which were polished to exceptionally high optical quality. Compared to similar data obtained with the previous 37-electrode adaptive mirror in 2009 and 2011, there is a significant improvement in image contrast. The highest 2 s average image contrasts measured in April 2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630 nm, and 853.5 nm with compensation only for the diffraction limited point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%, respectively. Similarly, the highest 2 s contrasts measured at 395 nm, 400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%, 16%, and 12.5%, respectively. The granulation contrast observed with SST compares favorably to measured values with SOT on Hinode and with Sunrise as well as major ground-based solar telescopes. Simultaneously with the above wideband red-beam data, we also recorded narrowband continuum images with the CRISP imaging spectropolarimeter. We find that contrasts measured with CRISP are entirely consistent with the corresponding wideband contrasts, demonstrating that any additional image degradation by the CRISP etalons and telecentric optical system is marginal or even insignificant. Finally, we discuss the origin of the 48 nm RMS wavefront error needed to bring consistency between the measured granulation contrast and that obtained from 3D simulations of convection.

  • 49.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Werkhoven, T. I. M.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    High-order aberration compensation with multi-frame blind deconvolution and phase diversity image restoration techniques2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 521, p. A68-Article in journal (Refereed)
    Abstract [en]

    Context. For accurately measuring intensities and determining magnetic field strengths of small-scale solar (magnetic) structure, knowledge of and compensation for the point spread function is crucial. For images recorded with the Swedish 1-meter Solar Telescope (SST), restoration with multi-frame blind deconvolution (MFBD) and joint phase diverse speckle (JPDS) methods lead to remarkable improvements in image quality but granulation contrasts that are too low, indicating additional stray light. Aims. We propose a method to compensate for stray light from high-order atmospheric aberrations not included in MFBD and JPDS processing. Methods. To compensate for uncorrected aberrations, a reformulation of the image restoration process is proposed that allows the average effect of hundreds of high-order modes to be compensated for by relying on Kolmogorov statistics for these modes. The applicability of the method requires simultaneous measurements of Fried's parameter r(0). The method is tested with simulations as well as real data and extended to include compensation for conventional stray light. Results. We find that only part of the reduction of granulation contrast in SST images is due to uncompensated high-order aberrations. The remainder is still unaccounted for and attributed to stray light from the atmosphere, the telescope with its re-imaging system and to various high-altitude seeing effects. Conclusions. We conclude that statistical compensation of high-order modes is a viable method to reduce the loss of contrast occurring when a limited number of aberrations is explicitly compensated for with MFBD and JPDS processing. We show that good such compensation is possible with only 10 recorded frames. The main limitation of the method is that already MFBD and JPDS processing introduces high-order compensation that, if not taken into account, can lead to over-compensation.

  • 50.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Narayan, Gautam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lagg, Andreas
    CRISP spectropolarimetric imaging of penumbral fine structure2008In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 689, no 1, p. L69-Article in journal (Refereed)
    Abstract [en]

    We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16 . Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows 0.3 wide spines, separated by 1.6 (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below.

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